Power System Protection [Vol 3 - Application] 2nd Ed (IEEE, 1995) WW

8/18/2019 Power System Protection [Vol 3 - Application] 2nd Ed (IEEE, 1995) WW

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o n t e n t s

Foreword

Chapter authors

Editor ial panel

Pro tec t ion sym bols used in c i rcui t d iagrams

12

xii

xiv

X

xvi

Pr otec t ion of genera tors t ransformers genera tor- transformer uni t sandt ransformer feeders J R u s h t o n a n d K G M M e w e s I

12 .1 In t roduc t ion 112 .2 Per fo rmance requ i rements 2

12.2.1 G en erato r faul ts 212.2 .2 Tran sform er faul ts 4

12 .3 Ge nera to r p ro tec t ion sys tems 7

12.3 .1 Unbiased di fferent ia l pro tec t io n 712.3 .2 Biased di fferent ia l pro tec t io n 912.3 .3 Back-up overcu rrent and ear th-faul t pro tec t ion 1212.3 .4 Negat ive phase-sequence pro tec t ion 1712 .3 .5 In te r tu rn fau l t p ro tec t ion 2112.3 .6 Loss of exc i ta t ion ( f ie ld fa ilure) pro tec t ion 2212.3. ' / Pr ote ct io n against pole-slipping 2412 .3 .8 Rotor ea r th - fau l t p ro tec t ion 2 /12.3 .9 Sensit ive pow er pro tec t ion 29

12 .3 .10 Low forward pow er in te r lock 2912.3 .11 Overspeed pro tec t ion 3012 .3 .12 Un derexc i t a t ion l imi t ing 3112.3 .13 M echanical and hyd raul ic t r ips 3 1

12.4 Ga s-turbine driven gen erators 3412.4.1 Direct co nn ecte d, gas-turbine sets 3412.4 .2 Tran sform er con nected , gas- turbine se ts 36

12 .5 Trans former p ro tec t ion 3912.5 .1 Unbiased di fferent ia l pro tec t io n 39

12.5 .2 Biased di fferent ia l pro tec t io n 4212.5.3 Rest r ic ted ear th-faul t pro tec t ion 4512 5 4 Overcur ren t p ro tec t ion 47

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ontents

12.6

12.7

12.8

12.9

12.5.5 Directional overcurrent protect ion 4812.5.6 Interlocked overcurrent protec tion 5012.5.7 Standby earth fault protect ion 5112.5.8 Tank earth fault protection 5212.5.9 Winding temperature protect ion 52

12.5.10 Gas generation and oil surge protec tion 53Protection schemes for typical transformers 5412.6.1 Distribution transformers 5412.6.2 Two winding transmission transformers 5512.6.3 Station transformers 5712.6.4 Autotransformers for transmission 57Protection system for generator transformer units 6012.7.1 Biased differential protection 6312.7.2 Stator earth fault protect ion 64

12.7.3 Tripping arrangements 6512.7.4 Generator transformer overfluxing protection 65Transformer feeder protection 6612.8.1 Overall protect ion for feeder and transformer 6712.8.2 Separate protection for feeder and transformer 6912.8.3 Intertripping 7412.8.4 Neutral displacement protect ion 7712.8.5 Directional overcurrent protect ion 7912.8.6 Typical protect ion arrangements for transformer

feeders 79Bibliography 80

13 Busbar protection L C W Frerk 81

13.1 History of the development of busbar protect ion 8113.2 General considerations 81

13.2.1 The basic philosophy of busbar protect ion 811 3 2 2 Earth fault protection versus phase and earth fault

protection 8313.3 The clearance of busbar faults by non unit circuit protect ion 83

13.3.1 Back up overcurrent and earth fault relays 8313.3.2 Distance protect ion 84

13.4 Unit systems of busbar protect ion for metalclad distributionswitchgear 8613.4.1 General considerations 8613.4.2 Frame earth systems 87

13.5 Unit systems of busbar protect ion for transmission substations 9313.5.1 General considerations 9313.5.2 Current balance using circulating current principle 9413.5.3 Connections for circulating current busbar

protection 9513.5.4 The influence of c.t. performance on through fault

stability 10313.5.5 Basic principles of high impedance circulating current

busbar protection: stability 10313.5.6 Basic principles of high impedance circulating current

busbar protection: operation 108

13.5.7 Extension of the basic principles to busbar protect ion 11113.5.8 Typos of high impedance relays 112

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14

13.6 Prac t ica l cons idera t ion s13.6 .1 Fac tors a ffec t ing the pos i t ion of c . t. s in busbars13.6.2 Effec t of c . t. loca t ion in outgoing c i rcuit s13.6.3 M ult iple che ck zones13.6 .4 Busbar se lec tor auxi l ia ry swi tches13.6.5 C.T. test l inks13.6 .6 Precaut ions against m alop era t io n of busbar

p r o t e c t i o n13.6 .7 Tr ipping and a larm c i rcui t a r rang em ents13.6.8 Back-tr ipping13.6.9 Te st faci l it ies13.6 .10 Fau l t se t t ings13.6.11 Stab i l i ty l imits

13 .7 Circui t breake r fai l pro tec t ion13.7.1 Principle of op era t ion13.7 .2 Precaut ions agains t m alop era t io n13.7.3 Cu rrent chec k re lay se t t ings13.7.4 Circui t brea ker fail t im er set t ings

13.8 Terminology13.9 Bibl iography

Pro tec t ion o f mo to r s r eac to r s boos te r s and capac i to r sP M Dolby14.1 In t roduct ion14.2 Motors

14.2.114.2.214.2.314.2.414.2.5

14.3 Reactors14.3.114.3.214.3.314.3.414.3.5

14.4 Boosters14.4.114.4.214.4.314.4.414.4.514.4.6

14.5

Charac ter i s t ics of d .c . and a .c . motorsAppl ica t ion of d .c . and a .c . motorsMotor con t ro lTypes o f f au l tA .C . and d . c . moto r p ro tec t ion

The p lace o f r eac to r s in a power sys temTy p e s o f r e a c t o rReac to r r a t ingReac to r app l i ca t ionR e a c t o r p r o t e c t i o n

The p lace o f boos te r s in a pow er sys temTran s fo rm er t ap -chang ingBoos te r t r ans fo rmersThe moving-coil regula to r

The induc t ion regu la to rPro tec t ion o f boos te r sCapaci tors14.5 .1 Capac i to r s in an in te rcon nec te d pow er sys tem14.5 .2 Ser ies-connected capac i tors14 .5 .3 Shun t -connec ted capac i to r s14.5 .4 Series or shun t con nect io n14.5.5 The capa ci tor uni t14 .5 .6 Pro tec t io n of capaci tors

14.5 .6.1 Series capa ci tor in terna l pro te c t io n14.5 .6 .2 Series capa ci tor ex tern a l pro tec t ion14 5 6 3 Shun t capac i to r in te rna l p ro tec t ion

ontents

118118120123127130

131132134139140142

142142144145145147148

149149149149162163

6 4

164181181182185186190199199200200205

210213215215216217219221222

223224230







































































































ontents

1 4 5 6 5

14.6 Bib l iography

P r o t e c t io n o f s y n c h r o n o u s s h u n tc o m p e n s a t o r s

15 T h e a p p l i c a t io n o f p r o t e c t i o n t o r ur al d i s t r i b u t i o n s y s t e m s J H a r r i s

15 .1 In t roduc t ion15.2 Fuses

1 5.2 .1 Ty p e s e m p l o y e d15 .2 .2 App l i ca t ion

15 .3 A u to m at i c c i rcu i t r ec losing15.3.1 Principle15 .3 .2 Re pea te r fu ses15 .3 .3 Po le m oun ted au to m at i c c i rcu i t r ec lose r s15 .3 .4 Sub s ta t ion c i rcu i t b reake r s

15 .4 Sens it ive ea r th f au l t p ro t ec t io n15.5 Arc suppress ion co il s1 5 .6 P e r f o r m a n c e / c o s t c o m p a r i s o n o f p r o t e c ti v e e q u i p m e n t f o r

ru ra l sys t ems15 .7 P r imary ne tw orks in ru ra l a r eas15 .8 Bib l iography

16 T h e a p p l i c a ti o n o f p r o t e c t i o n t o u r b a n a n d m e t r o p o l i t a n s y s t e m sK A J C o a t es

16 .1 In t roduc t ion16 .2 Charac t e r i st i c s o f u rban and m e t ro po l i t a n a reas16 .3 D i s t r ibu t ion sys t em p ro tec t ion r ad ia l l.v. sys t ems

16.3.1 Services16.3.2 L.V. cables16 .3 .3 Sub s ta t ion t r ans fo rm ers16 .3 .4 H.V. cables16 .3.5 P r imary subs t a t ions

16 .4 D i s t r ibu t ion sys t em p ro tec t io n in t e rc onn ec te d 1 .v. sys t ems16.4 .1 The l.v. ne tw ork16 .4 .2 Subs ta t ions16 .4 .3 The h .v. ne tw ork16 .4 .4 P ro tec t ion on a d i s t r ibu ted b lock l.v.

i n t e r c o n n e c t e d s y s te m16.4 .5 Sup ply to la rge po in t loads16 .4 .6 Supp ly to h .v. consu m ers

16 .5 Pr iva te gen era t ion1 6 .6 F u t u r e t r e n d s16 .7 Bib l iography

17 T h e a p p l i c a t i o n o f p r o t e c t i o n t o t r a n s m i s s i o n s y s t e m sJ C W h i t t a k e r

17.1 Ge nera l p r inc ip le s o f app l i ca t ion o f p ro tec t io n to t r ansmiss ions y s t e m s17.1.117 .1 .21 7 1 3

I n t r o d u c t i o nSys tem des ign cons ide ra t ionsF a c t o r s w h i c h i n fl u e n c e t h e c h o i c e o f p r o t e c t i o n17 .1.3 .1 P lan t t o be p ro tec t e d17 .1 .3 .2 P robab i l i t y o f va r ious types o f f au l t

2 3 7245

2472472 5 02 5 02 5 02552552572 5 72 6 9275278

2 8 02822 8 9

2912912 9 22 9 42 9 42 9 42952973 0 23033 0 43 0 4305

3 0 63133 1 63 1 63 1 93 1 9

321

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ontents

17.2

17.3

17.4

17.517.6

17.7

17.1.3.4 Voltage and current ratings ofprotected plant 323

17.1.3.5 Necessity or otherwise for high speedoperation 324

17.1.3.6 Importance of security of supply 32517.1.3.7 Compatibility with existing protection 32717.1.3.8 Availability of signalling channels 32717.1.3.9 Cost 328

Main and back up protection and location of currenttransformers 3281 7 2 1 Main and back up protection 328

17.2.1.1 Main protection 3291 7 2 1 2 Back up protection 330

17.2.2 Effect of location of current transformers indetermining protection to be provided 334

17.2.3 Two stage overcurrent protection 337Intertripping and protection signalling 33817.3.1 General 33817.3.2 D.C. signalling 34017.3.3 Post Office signalling 34217.3.4 Carrier signalling 34317.3.5 Fault throwing 343

Automatic switching 34417.4.1 Design and application of automatic switchingequipment 344

17.4.2 High speed automatic reclosing 34517.4.3 Delayed automatic reclosing 34517.4.4 Equipment design and programming 34817.4.5 Commissioning 356Economic considerations 357Typical protection applications in a major transmissionsystem17.6.1

17.6.217.6.317.6.417.6.5

17.6.617.6.717.6.8Typical protection applications in a minor transmissionsystem or major distribution system17.7.117.7.217.7.3

17.7.417.7.517.7.6

358Feeder protect ion 35817.6.1.1 Protection for a long overload feeder 35817.6.1.2 Protection for a short overhead feeder 36217.6.1.3 Protection for an underground feeder 363Protection for a transformer 365Protection for banked transformers 373Protection for a teed feeder 376Protection for a transformer feeder 383

Protection for a double busbar station 385Protection for mesh stations 387Protection for complex primary circuit configurations 387

Protection for a feederProtection for a transformerProtection for banked transformers and dualsecondary transformers

Protection for a teed feederProtection for a transformer feederProtection for a double busbar and mesh station

392392393

393

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ontents

17.7.7

18

Pro tec t ion fo r complex p r imary c i r cu i t conf igura t ions

17.8 Bibl iography

Tes t ing commiss ion ing and management o f p ro tec t ionE C Smith and

D Hay18.118.21 8 3

18.4

18.5

18.6

18.7

18.818.9

I n t r o d u c t i o nCon t rac tua l ob l iga t ionsThe men ta l approach to commiss ion ing t e s t sCom miss ion ing t e s ts18 .4 .1 Reasons for com miss ioning tes ts18 .4 .2 Planning of com m iss ioning tes ts18 .4 .3 Inspect ion pr ior to tes t ing18.4 .4 Th e tes ts

18.4.5 Phasing tests18.4.6 Closing up18.4.7 On-load tests18 .4 .8 M odif ica t ion to exis t ing subs ta t ion sR o u t i n e m a i n t e n a n c e t e st s18 .5 .1 Causes and effec ts of de ter io ra t ion18 .5 .2 F requ ency o f rou t ine m a in tenanc e18.5 .3 Inspect ion and tes ts18 .5 .4 M aintenance of busbar pro te c t ion back- t r ipping

and c i rcui t -breaker fa i l pro tec t ion a t double-busbartype subs ta t ions18.5 .5 M aintenance and tes t ing of in ter t r ippin g and

pro tec t ion s ignal ling equ ip m entFaul t inves t iga t ion18.6.1 Prim ary faul ts18 .6 .2 Fau l t s on the p ro tec t ive equ ip m ent18.6 .3 Faul t s on so l id-s ta te equ ipm entThe avoidance of e r rors when tes t ing

18.7.1 GeneralT h e t e st e q u i p m e n tRecords8.9.1 Relay set t ings8.9.2 Te st resul ts

397

398

3993993994 0 04 0 04 0 04014054 0 7

4334354354 3 7437438438439

441

4434464464464474 4 8

4 4 84 5 0463463

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C h a p t e r 1 2Pro tec t ion o f genera to rs t r ans formers

gene ra to r- t r ans fo rmer un i t s andt r n s f o r m e r f e e d e r s

b y J . R u s h t o n , re v is ed b y K .G . M . M e w e sII

1 2 . 1 I n t r o d u c t i o n

In the ear ly days of e lectr ic i ty, supply generators were operated by the SupCompany to supply a local load. Economical ly i t was desirable to generate a t distr ibu tion system voltage. As the size of local netw ork s grew, higher distr ibu t

vol tages became necessary, and by the t ime the 132 kV Nat ional Grid westablished m any of the larger supply co m panies were generating and dis t ribut ina voltage o f 33 kV .

With the development of the 132/275/400kV systems, the overal l pat tern generation changed result ing in larger stat ions si ted to take advantage of fuel cool ing water suppl ies , and operated over an interconnected t ransmission netwto give lowest cost and max imu m effic iency. Generators were conn ected to main t ransmission busbar via an associated t ransformer, and this arrangemperm it ted the addi t ional faci li ty of vol tage and pow er factor control by t ransfortappings.

Optimisat ion of machine design has resul ted in s tandard machine ra t ings ,w hich the following is a typical selection:

Genera tor ou tp ut Vol tage ofMW h.v. conn ect ion

• •

60 132 kV120 132 kV5 0 0 2 7 5 / 4 0 0 k V6 6 0 4 0 0 k V

i i 1 i

Gen erated vol tage

1 1 8 k V13.8 kV22 kV2 2 k V

Increase in outputs has been achieved without proport ional increase in f rame s

by employing more eff ic ient cool ing methods. The protect ive systems appl iedsmall directly connected generators are equally applicable to large transform



Protection of generators and transformers

to thei r smal ler f rame s ize) require more comprehensive schemes of high perforance protect ion.

12 .2 Per formance requi rements

The high costs associa ted wi th large generat ing and t ransforming plants accentuthe need for re liable , h igh speed schem es of protec t ion to :

a)

b )

c)

minimise faul t damage and so reduce the poss ible need to replace the placapi ta l out lay)

reduce repair outage t ime and so minimise the need to run lower m eri t lcost -eff ic ient) p lant in order to m eet the dem and revenue exp end i ture)assist in ma intaining system stabil i ty.

The degree of p ro tec t ion to be prov ided for the p lan t is de te rmined by pro tec tengineers in consul ta t ion wi th plant des igners and system operat ion engineethe ob jec tive be ing to prov ide a min im um of p ro tec t ion cons i s ten t w i th adequcoverage o f a l l cond i t ions liable to cause damage or affect the con t inu i ty o f suppBefore consider ing in deta i l the many forms of protect ion f i t ted to generators

transformers, i t is desirable to consider the origin and effects of faults and otsystem dis turbances so that the signif icance of the pro tect ion arrangem ents m ayapprecia ted.

12.2.1 e n e r a t o r f a u l t s

a) Sta to r faul ts:Stato r faul ts involve the m ain curren t carrying con duc tors anm ust there fore be c leared qu ick ly f rom the power sys tem by a com ple te shu td oof the generator. They m ay be faults to ear th , betw een phases or betw een turn s phase, s ingly or in combination. The great danger from all faults is the possibil i tdamage to the lam inat ions of the s ta tor core and s ta to r windings due to the hgenerated a t the p oin t o f faul t. I f the dam age so caused is o the r th an superfic ia l ,s ta tor would have to be dismant led, the damaged laminat ions and windings replaand the s ta tor reb ui l t , a ll of w hich is a lengthy and cost ly process.

Limita t ion of generator s ta tor ear th-faul t current by means of res is tance ear th

is no rm al practice see Ch apter 1) and serves, am on g oth er things, to m inimise cburning.

Phase-to-phase faul ts and inter tu rn faul ts are bo th less co m m on than ear th fauI t i s re la t ively easy to provide protect ion for phase- to-phase faul ts , but in ter tfaults are, on the othe r han d m ore di ff icul t to detect and pro tect io n is no t usuaprovided. General ly speaking, in ter tu rn faul ts qu ickly involve con tact wi th ear ththe s ta tor core and are then t r ipped by s ta tor ear th-faul t protect ion.

b) R oto r fau l ts :Rotor fau l t s may be e i ther to ea r th or be tween tu rns and mabe caused by the severe mechanical and thermal s t resses act ing upon the wind




The f ie ld sys tem is no t n orm al ly conn ec ted to ea r th so th a t a sing le ea r th fadoes no t g ive r ise to any fau l t cur ren t . H owever, a second fau l t to ea r th w ould shc i rcu i t pa r t o f the f ie ld wind ing and the reby p roduce an asymm et r ica l f ie ld sys tand unbalanced forces on the ro tor. Such forces wi l l cause excess pressure

bearings and shaf t d i s to r t ion , i f no t qu ick ly remov ed .Un der the genera l head ing o f ro to r fau lt s can be inc luded loss o f exc i ta t ion . T

may be caused by an open c i rcui t in the main f ie ld winding or a fa i lure e lsewherthe exc i ta t ion sys tem.

Loss o f ex c i ta t ion in a genera to r con nec ted to a l arge in te rco nn ec ted powsystem results in a loss of syn chro nism and s l ight ly increased ge nera to r speed, sithe pow er in pu t to the m achine is unchanged . The mac hine behaves as an indu c t

genera to r d rawing i ts exc it ing cur ren t f rom the rema inder o f the sys tem in the foof wa t t l ess cu r ren t whose m agni tude ap prox im ates to tha t o f the fu ll load ra t ingthe mach ine . Th is m ay cause overhea t ing o f the s ta to r wind ing and inc reased rolosses due to the cur ren t s induced in the ro to r body and damper wind ing . Tcondi t ion shou ld no t be a l lowed to pers i s t indef in i t e ly and cor rec t ive ac t ion e i tto res to re the f i e ld , o r to o ff -load and shu t do wn the ma chine shou ld be t ak en .

W i th ge nera to r ou tpu t s above ha l f r a ted load , po le -s lipp ing caused by w eak fcondi t ion, would cause severe vol tage var ia t ions which may, in turn , cause ope

t ion o f the u ndervo l tage p rote ct io n on th e boi ler auxi l iar ies . The resul taopera t ion o f loss o f bo i ler f tr ing p ro tec t ion wo uld then shu t d ow n the ge neraun i t . O ther genera to rs conn ec ted to the same busbar m ay a lso be caused to swiand sys tem ins tabi l i ty would resul t . Pole s l ipping may a lso resul t f rom insuff ic ienfas t c learance of a sys tem faul t and require the t r ipping of the uni t .

c) M echanical cond itions: Themechanica l cond i t ions requ i ring cons idera t ion a roverspeed due to sudden loss of load, loss of dr ive due to pr ime mover fa i lure aloss o f condenser vacuum .

The prob lem of overspeed l imi ta t ion , pa r t i cu la r ly in re la t ion to sudden lochanges , i s considered la ter. With modem large uni ts i t i s essent ia l to ant ic ipaoverspeed and take correct ive ac t ion. Mechanical overspeed devices which operon the s te am s top valves are invar iably f i t ted .

In the even t o f fa ilu re o f the p r ime m over, a genera to r wi ll con t inu e to r

synchronous ly d rawing power f rom the sys tem. This can somet imes l ead todangerous mechan ica l condi t ion i f a l lowed to pers i s t , a l though the condi t ionimm ed ia t e ly obv ious t o t he a t t end an t .

Se t s hav ing an in te rna l combus t ion p r ime mover mus t be p ro tec ted aga inengine fa i lu re , where , i f the a l t e rna to r con t inues to motor se r ious eng ine damam ay resu lt .

Vacuum fa i lu re (o r low vacuum) de tec t ion i s necessa ry to p reven t a r i se con den ser pressure w hich m ight lead to sh at ter ing o f 1 .p . cas ing and con densers .

d) Ex ternal faul ts :Turboa l te rna to rs mus t be pro tec ted aga ins t the effec t s o



Pro tection o f generators and transforme rs

cleared by the appropriate protection. The main condition of interest is that an unsymmetrical fault producing negative phase sequence currents in the statwinding. The effect o f these currents is to p roduce a field rotating in opposite sento the d.c. field system produc ing a flux wh ich cuts the r oto r at twice the ro tation

frequency thereby inducing double frequency currents in the field system and throtor bod y. These currents produce severe rotor heating and m odern machines haa limited negative phase sequence current capability.

Automatic tripping is therefore required for the higher negative phase sequenccurrent conditions.

This capabil ity l imit applies to all m ode m hydrogen-cooled machines and ma nair-cooled machines, but some of the older air-cooled machines are designed twithstan d full negative sequence curren ts continu ously. In large mo dern alternatorparticularly those employing direct cooling of the stator and rotor conductors, thtemp erature rise caused by abnormally high stator currents is mo re rapid than the less highly rated machines an d the capability limit is therefo re lower.

1 2 2 2 T r a n s f o r m e r f a u l t s

a) Faults within the transform er tank:Th ese ma y comprise phase-to-earth,phase-to-phase, or int ertu m faults on the w indings, interwinding faults, tap changfaults, insulator bushing failure and core overheating due to failure of core insultion. The possibility of damage is high for these faults as is the risk of fire, ansho rt fau lt clearance tim es are advantageous.

The connections of the power transformer and the method of earthing play aim po rtant part in determ ining curre nt m agnitude available for relay ope ration, aeach ca se requires separate consideration. Fi gs. 12.2.2A and B give the curredistribution under fault conditions for various transformer arrangements based

the performance of typical transformers.For a resistance earthed, star-connected winding, a winding-to-earth fault w

give rise to a current dependen t on the va lue of the earthing resistor and thdistance of the fault from the neutral end of the winding. The effective ratio transformation between the primary winding and the short circuited portion o f tsecondary w inding varies with the fault position. The c urrent flowing throug h thtransformer terminals is therefore, for all practical purposes, proportional to th

square of the percentage o f the winding short circuited. Th is is illustrated F i g 12.2.2A(a~.For a solidly earthed star winding, the fault current bears no simple relationsh

to the distance o f the fault from the neutral end since the effective reactance o f tfault path changes with fault position. F ig. 12 .2.2A (b)s ho w s that the minim uvalue of fault curren t occurs for a fault 30 to 40 from the neutral end.

For a delta connected winding the minimum voltage on the delta winding is the centre of one phase and is 50 norm al phase-to-earth voltag e,and an illustrati

of the appro xim ate m eth od of calculation is give n in Fig. 12.2.2B. The range values of fault cu rrent varies less than with the star c onn ected winding. The value





Pro tection o f generators an d transformers

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D i s t a n c e ~¢ o f f a u l t f r o m e n d o f w i n d i n g ( % )

h ) Ty p i c a l f a u l t current va lues

F i g , 1 2 . 2 . 2 B r a n s f o r m e r f a u l t c u r r e n t f o r d e l t a w i n d i n g

Fig . 12 .2 .2B b) show that the minimum value of faul t current occurs for a faul t athe cen t re o f one phase wind ing . The impedance o f the wind ing unde r succond i t ions wi ll be be tw een two and four t imes the no rmal va lue .

Phase- to-phase fault s rare ly oc cur on a pow er t ransform er. Clearly such faul twill give rise to large cu rren ts.

In ter turn faul ts a re m ore l ike ly to occur tha n phase- to-phase faul ts . The in te r turinsula t ion on a pow er t ransfo rm er is no t so grea t as the in terw inding insula t ion , an



Pro tection o f generators and transformers

A sh or t c i rcu i t o f a few tu rns o f the wind ing will p rodu ce a heavy cu r ren t in faul ted loop and a very smal l te rminal current . In th is respect i t has some s imi lato a neutra l end faul t on a sol id ly ear thed s tar winding.

Core fau l t s can occur due to l amina t ion insu la t ion becoming shor t c i rcu i tThis can cause ser ious overheat ing due to eddy current losses . Core c lamping bm us t a lways be insu la ted to p reven t th is t roub le . I f core insu la tion becomdefect ive due, poss ib ly, to the fa ilure o f core bo l t insula t ion o r debr is in the tani t mus t be de tec ted qu ick ly.

b) Fau l ts on t rans former conn ec t ions :These m ay compr i se any t ype o f no rma lsys tem fau l t on open cop perw ork conne c t ions o r f la shover o f co-ord ina ting ga

Damage due to such fau lt s is no t usua lly g rea t thou gh they m ay c ons t i tu te a se rihazard to pow er sy s tem s tab i li ty i f no t c lea red qu ick ly. Fau l t s be tw een the cur rt rans formers an d the assoc ia ted c i rcu it b reaker have to be inc luded in th is ca tego

c) Overheat ing:Fai lure of the cool ing sys tem wil l cause overheat ing and consque n t danger o f damage to the wind ings .

d) Fau l t s ex te rna l to the t rans forme r zone :These wi l l be of the usual range ofsys tem ear th and phase fau l t s to be c lea red by appro pr ia te ex te rna l p ro tec t isys tems . They wi l l a ffec t , the re fore , on ly the requ i rement o f t r ans former backp ro t ec t i on .

12 .3 Ge ne ra to r p ro t ec t i on sy s t ems

A genera tor i s invar iably protected agains t phase- to-phase faul ts by over

di fferent ia l protect ion us ing e i ther a b iased or unbiased re lay. The unbiased rei s usua l ly o f the a t t rac ted a rmature pa t t e rn and the b iased re lay uses e i the r induct ion -disc or rec t i f ier m oving coi l e lem ent . In a ll cases a h igh speed re layused.

12.3 .1 Unbiaseddifferent i l protect ion

The bas ic c i rcui t for ge nera to r d i fferent ia l pro tect io n sho wn in Fig . 12.3 .1Athe s imples t a r rangement o f a ba lanced cur ren t sys tem in which on ly two cur rt rans forme rs pe r phase a re requ i red to b a lance toge ther. The theory o f ba lancsystems is d iscussed in Chapter 4 which gives the des ign parameters for the corropera t ion o f an unb iased re lay sys tem under power sys tem t rans ien t condi t ioFor mos t app l ica t ions to the p ro tec t ion o f a d i rec t -connec ted genera to r, ins tantaneous unbiased re lay is used and th is g ives an adequate and predic tape r fo rmance .

The fau l t se t ting requ i red f rom the d i ffe ren tia l p ro tec t ion is de te rm ined by va lue o f the neu t ra l ea r th ing resi s to r and a lso by th e am ou nt o f wind ing to




I r

I

I '

, - k , . °I I v i - - - - 1

I G e n e r a t o r

I w o d .I " " ' - T ~ ' - "| -

, ~ ~ , I I

L _ _ _ j

F a u l t

Relay

Stabilisingresistors

R Y B

Bushars

F ig 1 2 3 1 A

F ig 1 2 3 1 B

Overa ll d i ffe re n t ia l c i rcu la ting cur ren t ) p ro tec t io n o f d i rec t -connec ted genera tor

/ : b / / :

/ // /

0.5 1.0 1.5 2.0

1.0C

0a_

0Q

0.9

e ~

~ 0 . 8

o u

Co.

0.7I1

0

~ o . 6<

Setting curre nt l R of neutr alearthing resistor as a multipleof generator ra ted cur rent

Protect ion set t ingas per cent ofgenerator ra ted current :

= I S

( a ) = s( b ) = ~ 0

( c) = 2 0

Rela t ion be tween neu t ra l res is to r se t ting , p ro tec t io n sens i t iv ity and am ou nt o fgene ra to r w ind ing p ro t ec t ed

IF - (1 - x)E/R w h i c h m u s t e q u a l t h e p r i m a r y f a u l t s e t ti n g o f th e d i f f e re n t i a lp r o t e c t i o n ,Is, f o r m i n i m u m o p e r a t i n g c u r r e n t .

T h u s ,

I s = 1 - x ) a n d l R R



Protection o f generators and transformers

whence x = (1 -I S / I R ) , I Rbein~ the ear thing res is tor curren t se t ting.The relat ionship is shown graphically in Fig. 12.3.1B.With a res is tor des igned to pass ra ted current of the machine on a faul t a t

m achine terminals , i t is c lear that a d i fferent ia l pro tect io n se tt ing of 10 wi ll pro

90 of the m achine winding , the unp ro tec ted por t ion be ing the 10 a t the neuend where fault r isk is reduced.

The differential pr ote ctio n w ill give greater than 90 cover for phase-to-phfaul ts s ince the faul t cu rrent i s l imi ted only by the machine impeda nce w hicsmal l wh en the faul ted po r t ion of the w inding is smal l.

1 2 . 3 . 2 B i a se d d i f fe r e n t ia l p r o t e c t i o n

Occasionally, there is some advantage in using a biased differential relay. The effof the bias feature is to enable the impedance of the re lay operat ing coi l c i rcuibe reduced for a g iven value of through faul t s tabi l i ty. The vol tage drop across current t ransformers a t se t t ing is correspondingly reduced and the magnet iscurrents have a negligible effect on the primary fault set t ing. The bias featurob ta ined by c i rcu la t ing the th rough fau l t cur ren t th rough an addi t iona l windwhich exerts a restraining force on the relay. The basic circuit connections shown in Fig . 12.3 .2A. Nominal ly no current f lows in the operat ing coi l unthrough fau l t condi t ions bu t , due to imper fec t m atch ing of the cur ren t t rans formsome spil l current m ay be present . T his spill current wi ll f low in the re lay operacircui t but wi l l not cause operat ion unless the re lay operat ing/bias se t t ing ra t ioexceeded. The magni tude of re lay operat ing coi l current to cause operat ion tincreases as the circulat ing cu rren t increases in a f'Lxed relat ion ship dete rm inedthe fundamental constants of t i le relay circuit . This is i l lustrated in Fig. 12.3.2B.

Referr ing to Fig . 12.3 .2A

relay op erat ing force= K ( I I - I2 ) N o

h +h)relay restraining force= K N r + S

2

where N o and N r are the operating and restraint coil turns, respectively, and S is thspring restraint force.

The e lect rical o perat ing and res t ra int forces are equal a t the balance po int o f re lay w hen the spr ing res t ra int is zero , whence

I1 I2 N r

~ I~ +I2 ) No

This equat ion shows that the character is t ic has a s lope determined by the ra



1 P r o t e c t io n o f g e n e ra to r s a n d t ra n s fo r m e rs

th m ean c i r cu la ting cu r ren t w hich i s fund am enta l ly a con s tan t r a tio fo r a ll cu r rem agn i tudes . Th i s i s p lo t t ed in F ig . 12 .3 .2C .

The p e rcen tage b ias fea tu re r edu ces c . t . r equ i rem ent s u nder t r ans ien t th rougfau l t cond i t ions . Wi th the co r rec t combina t ion o f b ia s and opera t ing c i r cu

res i s tance th rough fau l t st ab i li ty can be ob ta ined fo r any va lue o f th roug h facurren t . T he requi red va lue of stab i l is ing res is tance i s invar iably qui te low thg iv ing a low er r e lay opera t ing vo l t age than tha t r equ i red fo r an unb iased re l ay. T

M i i i iM

N r

I 2

N r = To t a l r e s t r a i n t c o i l t u r n s

N O = O p e r a t i n g c o i l t u r n s

F i g 1 2 3 2 A B a s ic c i r c u i t c o n n e c t i o n f o r b i a s e d d i f f e r e n t i a l p r o t e c t i o n

4 -

3 -

- . : .

=e ~

i .s_

¢ J

O¢ j

¢d

¢1

t _QJ

o

F i g . 1 2 . 3 . 2 B

I T r i p a r e a

• : P

I ~ _ ,~ /I ~ ~ S t a b l e a r e a I

. . . . R e l a y s e t t i n g

2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0

M e a n b i a s c o i l c u r r e n t ( p . u . ) . I 1 + 1 2

2

B i a s e d r e l a y c h a r a c t e r i s t i c i l lu s t r a t i n g p e r f o r m a n c e u n d e r in t e r n a l a n d e x t e r n



r o t ec t io n o f g e n e r a to r s a n d t r an s f o r m e rs

450

4 0 0

350-

3 0 0 -

2 5 0 -

2 0 0 -

150-

I 0 0 -

50-

l

~ + ,¢ ~ c q

. _ = ~

~

~ ° ~ ~

Trip a r e a

7.E 40 0 100 20 0 300 400 500 600 700 800 90 0~ 30 0 200 400 600 800 1000 1200

20 0 200 400 600 800 1000 1200 1400 1600 1800~ 10 0 400 800 1200 1600 20 00

M e a n b i a s c u r r e n t i n p e r c en t o f r a t e d cur ren t .I 1 + 12 f o r 2 w i n d i n g t r a n s f o r m e r s , I 1 + I 2 + I 3 f o r w i n d i n g t r a n s f o r m e r s

2 3

Z / , l / , / ,I / J / / / / / / i/ / 1 / / / / I /I 1 I/ / I / / / / /

1000

2 0 0 0

I 1 RC

F ig 1 2 3 2 C

12 Rt

13 R

O

T h r e e w i n d i n g t r a n s f o r m e r

R Aux. c. t . R e s t r a i n i n g c o i l s )O Aux. c. t . O p e r a t i n g c o i l )

e r c en t a g e d i f f e r e n t i a l r e l a y r e s t r a i n t c h a r a c t e r i s t ic

b i a s ed r e l a y i s t h u s m o s t s u i ta b l e f o r c a s e s w h e r e c t p e r f o r m a n c e i s l i m i t e d whe re l eads a r e l ong

C a re m u s t b e t a k e n t h a t t h e c t s d o n o t s a t u r a te t o o s e v e r e l y u n d e r h i g h c u r r eni n te r n a l f a u lt c o n d i t i o n s s i n c e a n y m a g n e t i s i n g c u r r e n t w i ll f l o w t h r o u g h t h e r e lb i a s c o i l s a n d m a y i n h i b i t o p e r a t i o n

H i g h s p e e d b i a s e d d i ff e r e n t ia l p r o t e c t i o n m a y b e p a r t i c u la r l y s u i ta b l e w h e n itnece s sa ry t o u se ex i s t i ng cu r r en t t r ans fo rmer s and the i r de s ign i s no t su i t ed t o a



2 Pro tection o f generators an d transformers

12.3 .3 Back up overcurrent and earth fault pr ote ction

Inverse , def ini te minimum t ime re lays are general ly f i t ted to provide back-up ocurrent and ear th-faul t prote ct ion of a generator, and i ts h .v. con nect ion s , pr ima

to provide last resor t t r ipping in the event of fa ilure o f the main pro tect iIdeal ly, i t should be se t to t r ip in the shor tes t poss ible t ime so as to minimise r isk o f loss o f sy s tem s tabi li ty.

The m in imu m permissib le re lay se tt ing is de te rmined by the requi rem ent tt r ipping must not occur for external h .v. sys tem faul ts which may be modiscr iminat ively c leared by other forms of protect ion. The re lay se t t ing should

F ig 1 2 3 3 A irect connection of overcurrent protection



Protec tion o f generators and transformers 3

• t - " - -i T , ~

_ I

SEQ/

Generator Gen. transformer500 MW 600 MVA

22kV ~

..ff

° .

30 MV transform er

[ '

F

H V

system

400 kV

Fig 12 3 3C Diagram of t rans former connec ted genera to r showing overcur ren t and n p srelays

chosen to provide adequate grading margins with negative phase sequence back-uprotection.

Ove rcurrent p rotec tion norm ally us es c.t.s in the h.v. circuit breaker which, ithe case of a transformer conn ected generator, is on the star-connected side of thgenerator transformer. Two types o f relay connection m ay be used, ei ther:

( a )

( b )

direct connection to star-connected current transformers (Fig. 12.3.3Awhere the relay is supplied w ith the phase currents, orconnection to star-delta interposing current transformers (ratio 1/0.578(Fig. 12.3.3B) where the relay is supplied with phase-difference currents s

avoiding any zero-sequence currents p resent in the prim ary fault curre nt.

Generally, the star-connected c.t. arrangement is used, but it may be seen fromFig. 12.3.3D th at the star-delta interposed conn ection appears to give a moracceptable range o f operating times for the mo re se ve re types of external h.vsystem fault. It also appears to provide better back-up to the negative phassequence protection described in Section 12.3.4.

The following analysis refers particularly to the g enerator-transforme r co nne cteunit illustrated in Fig . 12.3.3C

External system faults: Theovercurrent relay must n ot o perate for external-systemfaults for which the generato r w ill remain in synchronism , and settings chosen othe basis of 2.0 s minimum operating t ime will em bod y high safety margins. Threlay-fault-current curves for bo th cas es are shown in Fig. 12.3.3D. The highefault current for star-connected current transformers is obtained for the singlephase-to-earth fault, while, for delta-connected current transformers, the phaseto-phase fault giv es the w orst case. Fault settings for the tw o typ es of relaconnection are determ ined in Table 12.3.3A for a minimum operating time of 2



4

rotection o f generators an d transformers

T F X

0 . 0 4

5

4

. ~ - ' ~ ~ ~ -_ _ _

2

I li - - |

1 3

Ti m e S

a a n d gb a n d ec a n d df

P h a s e t o e a r t h f a u l tP h a s e t o p h a s e t o e a r t h f a u l tP h a s e t o p h a s e f a u l t3 p h a s e f a u l td e l t a c o n n e c t e d a u x i l i a ry c u r r e n t t r a n s f o r m e r ss t a r c o n n e c t e d c u r r e n t t r a n s f o r m e r s

F i g 1 2 3 3 D Va r i a t i o n o f r e l a y c u r r e n t f o r st ar a n d d e l t a c o n n e c t e d c u r r e n t t r a n s f o r m e r s





6 P r o t e c t i o n o f g e n e r a to r s a n d t ra n s f o r m e rs

I t is seen from Table 12.3.3 A tha t a low er t ime-m ultiplier sett ing is possible wthe del ta-connected current t ransformers because the effect of the del ta connectis to reduce the variat ions in relay-current level for different types of fault . Thwith del ta-connected current t ransformers , the maximum operat ing t ime for extern al fault , using con ven tional inverse-definite-minimum -time-lag i .d.m.relay characterist ics is 4.0 s for a single phase-to-earth fau lt) , while the m ax imoperat ing t ime w ith s tar-connected current t ransformers is 5 .7 for the 3-phase-fcase). Since longer fault-clearance t imes can be accepted for single-phase-to-efaults , the use of del ta-connected curren t t ransformers seems p referable .

Faults internal to generator The back-up protect ion should operate quickly fo

internal generator faults , s ince i t then provides discriminative tr ipping in the evof fa ilure of the m ain protec t ion. Faul t currents for an internal faul t w i ll usual lyhigher than those for external-system faults . Fig. 1 2.3.3E show s the relay-operatt im e curves for different typ es of faul t. The curves apply to a simple system w h

r ~

°

a lt~

O

2 -

1 -

i ) i i ) 5 1 0 1 5 20

Fault power x 103 MVA

a Star-connected current t ransformers: al l faul ts except phase- to-phaseb Star-con nected current t ransformers: phase- to-phase faul tc Del ta-connected current t ransforme rs: phase- to-ear th faul td Del ta-connected current t ransformers: al l faul ts except

phase- to-ear the Ext rem ely inverse relay characteristi c for cond iti on af Extr eme ly inverse relay characteris tic for cond iti on d

Note: Curves a- d apply for a conven tiona l i .d.m.t. l , relay toBS 142 :1966

i) Maximum external-faul t power for del ta-connected currentt rans formers

i i) Maximu m external-faul t power for s tar-connected currentt rans formers



P r o t ec t io n o f g e n e ra to r s a n d t ra n s f o rm e rs 7

neglec ts the in feed o f the faul ted genera tor and assum es an equ ivalent sys temhaving equal pos i tive- negat ive and zero-sequence infeed a t the busbar. For ths tar-connected current transformers the infeed f rom the phase- to-phase faul t wi lbe lower than for o ther faul t s w hi le for de l ta-con nected current t ransformers th

phase-to-earth faul t wil l give the lower infeed.I t i s seen f rom the curves tha t the t r ipping t imes obta ined f rom the use of de l ta

con ne cted current t ransformers are lower becau se of the lower t .m .s se t t ing needefor ex ternal faul t grading.

Fig . 12 .3 .3E a lso show s the characteri st ics of typica l ext rem ely inverovercurrent relays. This type of character is t ic gives great ly reduced faul t-clearanct imes a t the h igher current levels and where de l ta-conn ected current t ransformerare used the back-up-c learance t imes for faul ts w i th in th e genera tor cou ld grea t lassis t in m eet ing the sys tem -s tabi li ty requi remen ts .

12 .3 .4 Negat ive phase-sequen ce pro tec t ion

As previous ly m en t ione d there is a Limit to the d egree o f s ta tor-current unbalanca turboal ternator can withstand before i t suffers ser ious rotor overheat ing. Faults

O

. 3

o _

oL .

= 2¢ J

zP 4

u

C o o l i n g M e d i u m

A i r

H y d r o g e n 0 . 5 p . s . i . g ._ _

H y d r o g e n 1 5 p . s .i .g .

H y d r o g e n 3 0 p .s . i. g .

H y d r o g e n 4 5 - 6 0 p . s. i. g.

i i

P e r m i s s i b l e 1 2 2 t

Ty p e a

1 2 C = o . 1 5i

2O

2O

15

1 2i

2 3

t P e r m i s s i b l e ti m e in s e c o n d s )

b y 1 22t f a c t o r f r o m t a b l eu l t i p l y

Ty p e a ). M a c h in e s w i t h o u t d i r e c tl y c o o l e d r o t o r c o p p e r .Ty p e b ) . M a c h in e s w i t h d i r e c tl y c o o l e d r o t o r c o p p e r .

Ty p e b

1 2 C = 0 . I 0

1 3

1 0

7

_ _ - _ _

~.s/2.s



8 Protection of generators and transformers

ex te rna l to the genera to r a re usua l ly c lea red qu ick ly by c i rcu i t p ro tec t ion , f a ilu re o f rem ote p r o t ec t io n to opera te , o r i ts a ssoc ia ted c i rcu i t b reaker to trwould l eave the fau l ted c i rcu i t connec ted to the genera to r. Al l f au l t s o ther thone involving a l l three phases o f the pr im ary sys te m give r ise to a sys te m o f

ba lanced cur ren t s which may be reso lved in to i t s pos i t ive - , nega t ive -and zesequence components . By def in i t ion , the nega t ive sequence components ro ta te id i rec t ion con t ra ry to the d .c . f ie ld sys tem o f the genera to r. The s ta to r f lux thproduced the re fore cu t s the ro to r a ttwi e i ts ro ta t iona l ve loc i ty, the re by induc ingdouble - f requency cur ren t s in the f i e ld sys tem and ro to r body. The resu l tan t e

RI i i _ - - ~ .

' o r

Y>

B lb~

l

S e q u e n c e n e t w o r k , r e la y an d c . t . c o n n e c t i o n s

S' ~ I r Z

z R

. m

T

I r

P o s i ti v e s e q u e n c e v a l u e o f I x = i r RibZ iy

- 0 I rR

I b z

I r Z

r bJ /l l b R

I r R . lr z P o s i ti v e s e q u e n c e c u r r e n t s

l y

l b RN e g a t i v e s e q u e n c e v a l u e

o r I X : i r R i bz iy = I

I X = ly 1~ Z

N e g a t i v e s e q u e n c e c u r r e n t s . R e l a y i m p e d a n c e X = 0 )



Protection of generators and transformers 9

currents m ay be very large so causing severe overheating of the rotor at thopoints where the circulating eddy current is concentrated by winding slots. Tdisconnect the machine before damaging tem perature s are attaine d a negati

I RR . - - - . . . =

I y°y .-.. .---

I BB m

: N

•

f Z ~ fZ r - , J

Z b . - - ~ /i I

I - ~ 11

2N

I R, I r and V Z r

P o s i t i v e seq uence

I B and I b I Iy

V z b V z + Vr Zb

I R , I r a n d V Z r / 4A

o ~ . ~ N egative s eq u en c e

I y J ~ ' ~ l B a n d I bPhasor d iagram for

t r ip uni t (T)

R a n d I r

V/ f , Zr

I yI B, I b and VZ b

I y

I R and I r

t V z r ,o ~ ~ ' , ~ Vz r + VZb

' ~ ~ ~ I B , I b and V Z bPhasor diagram foralarm u n i t A )



2 Protection o f generators and transformers

phase sequence n .p . s ) cu r ren t m easur ing re lay is app l i ed wh ose n .p .s . cut ime character is t ic matches that o f the m achine. The re lay is so des igned as to g iwa rn ing w hen the m axim um cont inuous n .p .s , cur ren t ra t ing o f the genera to

2 . 0

1.5

0

1,1

u

de ~

~ 1 . 0

..~

:g

F ig 1 2 3 4 D

0 . 5

a

bc a n d de a n d gf a n d h

m m m m , , , m m ~ m

T F

\ . . -0

i x, \ a

•\

b \

\ \\ \

'b L ~. . , ~. . \ , , ' > : '_ . ~

d ~ ~ ~_ . ~ . . ~ . . - . ~ ~ ~qt

~ i i a ~ I I Ib.~ II

• - - - ' - * , B

• , ~ ,

xS

h- -. u - - . . . . - - - -- - -- J - - - '-

iI

2

T i m e , S

C u r r e n t a g a i n s t t i m e c u r v e f o r 1 2 t =3.O

C u r r e n t a g a i n s t t i m e c u r v e f o r 1 2 t =2 . 0P h a s e - t o - p h a s e f a u l tP h a s e - t o - e a r t h f a u l tP h a s e - t o - p h a s e - t o - e a r t h f a u l tu n l o a d e d m a c h i n el o a d e d m a c h i n e

G e n e r a t o r n p s c u r re n t /t ir n e c u rv e s f o r 5 0 0 M W g e n e r a to r lo a d e d a n d u n l o a d e d




exc eed ed (12 c) and to t r ip w hen the value of (12)2 t , where t < 100 s excee ds capab i l i ty o f the genera to r. F ig . 12 .3 .4A shows the capab i l it ie s o f mod ern Br itu rboa l te rna to r se t s .Ty pical n .p .s , ne tw ork s are sho w n in Figs . 12 .3 .4 B, C and D.

The ou tpu t o f the ne tw ork is p rop or t iona l to the genera to r n .p .s , cur ren t in ecase and is fed in to a re lay wi th an inverse square law character is t ic . The ovear rangement i s matched to the genera to r n .p . s , cur ren t capab i l i ty curve such tt r ipping occurs when the curve value is reached. An a larm uni t may a lso be pvided w i th a range o f se t t ings to co ver the required range of I2c. The requ iinverse square law character is t ic i s obta ined by use of an appropr ia te ly des ig

( . . T °

input

egativesequencene twork

C o m p a r a t o r

In tegra tor

r i p

Star terefinite [

t ime ~ _ A l a r m

F i g 1 2 3 4 E Static n.p.s, relay Type CTN, GEC Measurements)

ind uc t ion disc re lay, a therm al repl ica re lay (Fig . 12.3 .4C) or a s ta t ic re laywhich the character is t ic i s shaped by a res is tor /Zener d iode c i rcui t . (Fig . 12.3 .4E

The se t t ings o f the n .p . s , r e lay a re de te rmined by the s ta to r n .p . s , cur recapabi l i ty of the genera tor. F ig . 12 .3 .4D shows the n .p .s , currents for a typical 5MW m achine fo r d i ffe ren t types o f fau l t con d i t ion . The un load ed case ( shodo t ted ) g ives low er n .p .s , cur ren t s than the loaded case , exce p t fo r a phase- to -eaf au lt , and t he m ax im um cu r r en t , and t he r e fo re t he min imum ope ra t ing t ime , g iven for a phase- to-phase faul t o n a loaded m achin e . This g ives a requ ired t r ipptim e o f 1.3 s for ~ t = 2-0 and 1.9 s for I~ t = 3.0.

12 3 5 In ter turn fau l t p ro tec t ion

The d i ffe ren t ia l cur ren t p ro tec t ion desc r ibed in Sec t ion 12 .3 .1 cannot de tec t in tturn faul ts which remain c lear of ear th , s ince there i s a balance of the currenenter ing and leaving the winding de spi te the presence of a large curre nt c i rcula tthe shor ted tu rns . In te r tu rn fau l t s a re no t normal ly p ro tec ted aga ins t because o f t echn ica l d i ff i cu l ty o f so do ing . I f in te r tum fau l t s occur in the s ta to r s lo t s , thqu ick ly deve lop in to fau lt s to ea r th an d a re c lea red by the s ta to r ea r th fau l t p ro tt ion . There is , how ever, the poss ib il ity tha t they m ay occu r a t the wind ing ends aso cause extensive damage to the genera tor before the faul t evolves to one detecta




12 .3 .6 Loss o f exc i t a t ion f i e ld f a ilu re ) p r o tec t io n

Loss o f exc i ta t ion resu l t s in a genera to r los ing synchron ism and runn ing absynch ronous speed . Opera t ing as an induc t ion g enera to r i t wo uld p rodu ce i ts m

f lux from wat t less s t a to r cur ren t d rawn f rom the p ow er sys tem to wh ich i t was conn ec ted . Exc i ta t ion under these condi t ions requ i res com pon ents o f reac

~ u r r e n t wh ich may we ll exceed the r a t ing o f t he gen e ra to r and so ove r load t he s t

R o t o r e a r t hf a u l t r e l a y

G e n e r a t o rf ie ld

S h u n t

M a i ne x c i t e r

T | I

I

A l a r m ~ = ~ ~

F i g . 1 2 . 3 . 6 A Generator field failure protection using sensitive relayDiagram also shows simp le rot or E /F scheme)

G e n e r a t o rf ie ld

S h u n t

M a in [ ~ 1 ~ " ~ 1 1ex c i t e r 1 I

P - U . , U - - I

T I Ie ~- -=~:::¢~ i

I t J

la rm 5 s e e s " -= - - - - - , , - -0 I C . . . . .

r ip 60 sees ~ ~ ~



P r o t e c t io n o f g e n e r a to r s a n d t ra n s f o r m e rs 3

rA l t e r n a t i v e | ~ /position IX ~f o r v. t .

III

L . . . . . . . . ~ I - --~--~1

( a) R e l ay c o n n e c t i o n s _ _

M h o r e l a y

Typicalpower wing ocus S y s t e m f a u l t l o c u s

- R

1A 1B

/ IM e a n l o c u s o fi m p e d a n c e v e t o r

o n l o ss o f f i e l d

- X

R e l a y c h a r a c t e r i s t i c

F ig 1 2 3 6 C

( b ) R e l a y c h a r a c t e r i s t i c s h o w i n g m e a n l o c u s o f i n p u ti m p e d a n c e a t g e n e r a t o r t e r m i n a l s u n d e r l o ss o f e x c i t a t i o nc o n d i t i o n s . N . B . E f f e c ts o f r o t o r s a l i e n c y n o t i n c l u d e d .

I A - L a g g i n g c u r r e n t ( h e a v y l o a d )I B - L a g g i n g c u r r e n t ( l i g h t . l o a d )

oss o f ex c i ta t ion p ro tec t ion us ing off se t impedance measur ing re lay a tgenerator terminals

winding. Addi t ional ly the s l ip f requency currents induced in the damper windinof the roto r wo uld cause abnorm al heat ing o f the rotor.

Op erat ion as an indu ct ion generator does no t therefore damage the se t immedate ly but as the higher ra tings of m ode rn machines are obta ined b y advancecool ing techniques ra ther than increased f rame s ize the sho r t therm al t ime constan

require the machine to be deloaded an d t r ipped in a mat ter o f seconds .Fig . 12.3 .6A i l lus t ra tes a method commonly used to provide f ie ld fa i lure an




com bination operates when the f ield current is zero (or cyclically passing thro ug hzero during asynchronous running) and tr ips or alarms after a predetermined t imdelay set o n relay ? 2.

An alternative method employed a d.c. underpower relay in the f ield circuit ashow n in Fig. 12.3.6B . This relay is responsive to pow er flow an d is held ino perativby the normal f low of current from the main exciter to the f ield windings. Shouthe power fall below a certain level, or reverse its direction (due to induced a.c. slip freq uen cy) the relay co ntac ts w ould close after a time lag to g ive an alarm.

As m od ern large generators m ay be required to operate with very low values oexcitat ion, both of the above schemes would be unsuitable. When a generator lossynchronism, the quanti ty which changes most is i ts impedance as measured at thstator terminals. Loss of field will cause the term inal voltage o f the gene rator tbegin to fall , while the current begins to increase. The apparent impedance of thm achin e will therefo re be seen to decrease and its pow er facto r to change. A reladesigned to detect the change of impedance from the normal load value matherefore be used to provide p rotec tion against asyn chro nou s ope ration resultinfrom the loss of excitat ion (Fig. 12.3.6C).

12 .3.7 Pr otec t ion against pole s l ipping

Generally speaking, pole-slipping protection was not provided on earlier, smalmachines b ut on larger, m od ern sets , the sev ere mechanical torq ue osciUatiomaccompanying pole sl ipping condit ions require prompt action to be taken if damagto the generat ing plant is to bep rev ente d and pow er-system dis turbance minimisePole-sl ipping condit ions do not normally occur for system faults correctly cleareby high speed m ain p rotect ion

Pole slipping, ei ther between generators and the sy stem, or betw een two sectionof the system , results in a f low of synchronising pow er, which reverses in directiotwice every slip cycle. When displayed in the com plex imp edance plane, this f low osynchronising power is characterised by a cyclic change in the effective load impedance V as measured a t the terminals of the two asynchronous systems and theload impedance locus passes between the +R and - R quad rants as the real pow eflow reverses in direction. The transit ion point on the/X axis corresponds approxm ately to the instant w hen the two systems are 180 ° ou t of phase (assuming thload impedances are largely reactive). At this instant only reactive power is flowinand the system voltage collapses to zero at the electrical m idp oin t o f the twosystems. This m idp oin t is the centre a bo ut w hich pole-sl ipping can be com idered tbe taking place and i ts location, with respect to the relaying point , can be determ ined from the app arent load imped ance to the po int w here the locus crosses thX axis. The load imp edances seen from the terminals o f individual generators ei the

side of the pole-sl ipping centre will depen d on their respective contrib ution s to th

total synchronising power f lowing at any instant . Measurement of the magnitudedirection and rate o f change o f load imped ance relative to a generator s terminal



Pro tec t i on o f gene ra to r s and t rans fo rmer s 25

I _ = S t a g e 1 v I S t a g e 2 _ _ 1I I I

S y s t e m

I iI I

II I

I - - - -I F - C _ _ _ _ _ _ ]d e t e c t i o n u n i t C o u n t e r s I

I ~ I

[I ~ S t a g e 1 a n d 2 t r i p I

L_ _ _ _ _ _ _ _ _ J

S i m p l i f i e d b l o c k d i a g r a m

O f f s e t m h o s t a g e 2l i m i t o f r e a c h \

H . V. d i r e c t i o n a lc o n t r o l r e l a y

O h m 1 jX

¢ - S y s t e m i m p e d a n c ea n g l e

Z I,, - G e n e r a t o r r e a c hZ R 1 " O f f s e t m h o r e ac hZ R 2 - S y s t e m r e a c h

O h m 2R e s t r a i n

7O p e r a t e

L o c u s o f p o l es l i p p i n g c e n t r e( S t a g e 2 )

R e l a y c h a r a c t e r i s t i c s

F o r o p e r a t i o n :

t 2 t I < [ a p p r o x 4 0 m s .

ZI.,

'~K L o c u s o f p o l es l i p p i n g c e n t r e(S t age )

O f f s e t m h o s ta g e 1l i m i t o f r e a c h



26 Protec t ion o f genera tors and t ransformers

I . . . . . . S t age 1 _ I _ S t age 2 __1. . . . . . . . . . . .

I ~ - I

S y s t e m

r T _ . Z I _ . . ]

] F P o l e . s l i p p i n g [I ~ t i o n u n i t I

I S tag e 1 and 2 t r ip [

w . . . . . . . . . . . . . . . . . . A

S i m p l i f i e d b l o c k d i a g r a m

\\

Z R 2

- S y s t e m im p e d a n c eang le

6 - Lens ang leZ F - G e n e r a t o r r e a c hZ R 2 - S y s t e m r e a chZ R I R e a c t a n c e

e l e m e n t r e a c h

R e a c t a n c e e l e m e n t

H . V. d i r e c t i o n a lr e l ay - - - -

L o c u s o f p o l e- ' - - - s li p p in g c e n t r e

( S t a g e 2 )

. . ~ n l / ~

O h mline

? ~ ~ Loc us o f po l es l i p p i n g c e n t r e(S tage 1 )

R e l a ycharac te r i s t ics

F 'o r o p e r a t i o n l o c u s m u s t e n t e r l e n sf r o m l e f t o r r i g h t , t r a v e r s e w h o l el en s a n d r e m a i n w i t h i n t h e l e n s f o ra m i n i m u m o f 5 0 m s .



Protection o f generators and transforme rs 7

s l ipping is taking place . Individual gen era tors , g enera l ly those on the s ide of l a rger sys tem, a l thoug h con t r ibu t ing synchron is ing po we r wi ll no t see a ps lipp ing cen t re ( i. e. the i r im pedan ce loc i wi ll no t c ross th e / X ax i s ) i f the i r power ou tpu t remains pos i t ive , bu t ca re needs to be exerc i sed in des ign ing se t ting po le - sl ipp ing pro tec t ion based on load impe dance m easure m ent i f sa t i s facd i sc r imina t ion is to be ob ta ined .

The pole-s l ipping detect ion re lays are connected to c . t . s and v. t . s on the ls ide o f the genera to r t r ans form er and the d i rec t iona l con t ro l r e lay to c .t .s and vf i t t ed on the h .v. s ide o f the t ran s form er. Schem at ic d iagrams of two m anu fac tu rschemes are i l lus t ra ted in Fig . 12.3 .7A and B.

In bo th cases, the p ro tec t ion is o f the im pedanc e-measur ing typ e , hav ing t

stages:

Stage reach ing th rough the genera to r to the neu t ra l po in t and in to the t rafo rmer to cover the en t i re wind ing as fa r as , bu t no t beyond , the h .v. t e rmin( the d i rec t iona l con t ro l r e lay inh ib i ting op era t ion i f the sys tem cen t re occbeyond the t rans former h .v. t e rmina l s ) . ; andStage 2 opera t ing a f te r an ad jus tab le number o f po le s l ips , whe ther the sys tcen t re i s wi th in the genera to r t r ans former un i t o r ou t on the sys tem.

The pole-s l ipping re lays are capable of detect ing the f i rs t pole-s l ip condi tw hen a s lip , corresp ond ing to th e speed of pole-s lipping is in the range +0 .1 +10 on a 50 Hz basis . The p ro tec t ion m us t , o f course , r emain uno pera ted s teady s ta te load ing , power swing ing and cor rec t ly c lea red sys tem fau l t cond i t ioa l though some o f it s com pon en t r e lays m ay op e ra t e .

1 2 . 3 . 8 R o t o r e a r th f a u lt p r o t e c t i o n

Tw o m etho ds a re ava ilab le fo r the de tec t ion o f ea r th fau l ts in the ro to r c i rcu i t. m e th od u t il ises a h igh res i stance conn ec ted ac ross the ro to r c i rcu it , the cen t re pof w hich i s con nec ted to ea r th th rou gh the co il o f a sens it ive re lay (F ig . 12 .3 .6This re lay w i ll d e tec t ea r th fau l ts over m os t o f the ro to r c i rcu i t . There is , how ea b l ind spo t a t the cen t re po in t o f the f ie ld w ind ing wh ich is a t equ ip o ten t ia l wthe m idpo in t o f the res is to r under ea r th - fau l t cond i t ions . Th is b l ind spo t can

examined by a r rang ing a t app ing swi tch which , when opera ted , sh i f t s the recon nec t ion f rom the cen t re o f the res is to r to a po in t a l it t le to one s ide . Al te rt ively, one hal f of the res is tor can be replaced by a nonl inear res is tor which, s incwill change i ts value for d i ffe rent values of ro tor vol tage , wi l l con t inu ou sly vthe effec t ive res is tor tap ping vol tage as the f ie ld con di t ions chan ge. This m ethhas been used in the USA .

A second method u t i l i ses a smal l power pack connec ted to the pos i t ive po lethe f ie ld c i rcui t , in ser ies wi th which is connected the faul t de tec t ing re lay anhigh res is tance (Fig . 12.3 .8A). A faul t a t any point in the f ie ld sys tem wil l pascdr re n t of suff ic ien t m agni tude throug h the re lay to cause ope ra t ion . Th e f i



8 P r o t ec t io n o f g e n e r a t o r s a n d t ra n s f o r m e r s

F i e l dc i r c u i tb r e a k e rII

R o t o r

x c i t e r

DischargeR e s i s t o r

F i e l d

w i n d i n

S e n s i t i v em o v i n g c o i lr e l a y

A u x i l i a r y Q ~a . c . s u p p l y

_

. . . . . I ~ A l a r m

C u r r e n t l i m i t i n g r e s i s t o r

3 0 V d . c .

¢

F i g . 1 2 . 3 . 8 A R o t o r e a r t h f a u l t d e t e c t io n u s in g n e g a t i v e ~ p o t e n t ia l b ia s i n g d e v i c e

for a la rm o nly as there i s no im m ed ia te danger to the se t . T his device i s s imi larpr inc ip le to the ba t te ry negat ive b ias ing device refer red to in Sect ion 7 .2 .

I t w i l l be seen tha t w i th bo th o f these sch em es the ea r th re tu rn pa th i s th rougthe b od y o f the ro to r. S ince , how ever, t he con ta c t b e tw een ro to r and s t a to r th rough the bea r ing o i l f i lm w hich i s non- (o r in t e rm i t t en t ly ) cond uc t ing , i t essent ia l to ear th the ro tor shaf t b y an add i t ional ear thed brush . This a l so servthe essen t ia l pu rpose o f d ischarging s ta t ic e lec t r ic i ty in du ced in the turb ine ro tby s t eam f r i c t ion , the reby p reven t ing the bea t ing su r faces f rom p i t t ing .

In some modern mach ines the t r ad i t iona l d i r ec t cu r ren t exc i t e r i s r ep laced by aa l t e rna to r wi th a ro ta t ing a rmatu re , the f i e ld wind ing o f the ma in genera to r be insupp l ied throu gh rec t if iers carded o n the ro tor, and a brushless des ign i s there bach ieved . The inaccess ib i l i ty o f the ma in d . c . f i e ld c i r cu i t makes d i rec t de tec t io f ro to r ea r th - fau l ts im poss ib le .

The s ing le ea r th - fau l t (o r d iod e open c i r cmt ) w ou ld p rod uce li t t le change in tl evel o f exc i t a t ion . A m ore severe in t e r tu rn f au l t (o r d iode shor t c i r c u i t ) w ou ld ,




i s som et imes em ploy ed to in i t ia te deloading and t r ipping. T he diodes are themsepro tected by fuses and a fuse fa i lure pro tect io n re lay is no rm al ly included .

12.3.9 e n s it iv e p o w e r p r o t e c t i o n

Reverse pow er pro tect io n has in the past been f i t ted to d etect fa i lure of the prmover ( that i s the engine or turbine) which i t does by measur ing the power draf rom the sys tem by the genera tor w hen i t is m otor ing .

The pow er taken by the genera tor under such condi t ions is very low, be ing f rabo ut 10 for eng ine dr iven se ts to 2 for tu rbo a l tem ators . The pow er fac torthe cur ren t depends , o f course , u pon the exc i ta t ion level and m ay thus be e it

leading or lagging. The wat tmetr ic re lay must , therefore , be highly sensi t ive have an accurate q uad rature adju stm ent . I t m ust a lso be conn ected as a ' t rwat tmeter and no t be f i t t ed wi th phase angle or low vol tage compensa t ion , as di rect ional re lays for faul t po w er ap pl ications .

The wat tmetr ic re lay must be associa ted wi th a t ime lag re lay to preveoperat ion due to power swings .

12 .3 .10 Lowf o rw a r d p o w e r in ter lock

A more recent appl icat ion of a sensi t ive power re lay is on large modem al ternawhere i t i s des i rable to delay t r ipping of the e lect r ical load on the generator uaf ter the s team has been t r ipped, so that the s team entra ined in the turbine is uin supplying e lect r ical power ra ther than causing the se t to overspeed, that i s generator acts as a 'brake ' . This feature is only provided for 'non-urgent ' t r ippcon di t ions . The co ntac t of the w at tm etr ic re lay is associa ted w i th a time lag rewhose contact i s connected in ser ies wi th the 'non-urgent ' t r ipping contacts , wi th th is a rrangem ent the genera tor is no t a l lowed to t r ip un t il the pow er ou tputhe gen erator falls be low a small preset value. (Fig. 12.3.13 A).

Opera t ion of the re lay a t low leve ls o f fo rward power permi t s fu l l p ro tec tdur ing run up and pr ior to synchron is ing, s ince the re lay contacts wil l be c lodur ing this per iod.

To provide the highest degree of secur i ty und er a ll con di t ions the re lay conta

are arrangedt

close when the generated pow er fal ls to 0 .5 of full load outp uSuch an accurate se t t ing requires the cu rrent and vol tage transform ers to be of haccuracy. Two re lays are normal ly used and have thei r contacts connected parallel.

The generator in ternal faul t t r ipping devices(for example di fferent ia l protect ie tc . ) are arranged to t r ip indep end ent ly of th is sensit ive pow er in ter lock.

An al ternat ive method of prevent ing the e lect r ical t r ipping of a generator unaf ter i t s s team supp ly h ad been cut off was to use a pressure operated swiconn ec ted in to a su itab le po in t on the tu rb ine . The se t ting of such a swi tch hadbe not appreciably higher than the no-load steam pressure and the contacts w




Tw o pressure sw i tches w i th para ll e l conn ec ted con tac t s were som et imes usedgive addi t ional safeguard agains t mechanical fa i lure . A t ime delay re lay wassoc ia ted w i th the p ressure sw i tch so tha t t r ans ien t overshoot o f the swi tch wono t pe rmi t t r ipp ing of the gen era to r c i rcu i t b reaker und er unsafe condi t ions .

The m e thod was r ecom m ended on ly fo r non - r ehea t mach ines i n v iew o f add i t iona l com pl ica t ions where rehea t is used . In the case o f rehea t machines thposs ible cont ingencies must be guarded agains t :

a )b )c )

fa i lure to c lose of the main s team supply valvesfa i l u re to c lose o f the valves a f te r the rehea te r andfai lure of both sets of valves.

Tw o ser ies-conn ected pressure swi tches were requ ired to cover these con tgencies; one op era ted by the pressure d i fference across the h .p . cyl inder and o ther b y the p ressure a t some p o in t in the tu rb ine dow n-s t ream of the rehea t va lIn v iew of these add i t iona l compl ica t ions the use o f p ressure swi tches i s no t nr ecom m ended fo r r ehea t mach ines .

12 3 11 Overspeed protec t ion

I t is e ssen tia l fo r the govern ing sys tem of a tu rb ine g enera to r to incorpo ra te sa fdevices to p reven t dang erous overspeeding. Th e im po rtan ce of these devices is stha t rou t ine t es ting fac il it ie s shou ld be inc luded to check the co r rec t func t ion ingthe va lve gear on n orm al load and norm al opera t ing p roced ure shou ld ver ify opera t ion o f a ll sa fe ty dev ices p r io r to go ing on load and a f te r remova l o f lbe fore shu t down.

M ost large turbine s are now f i t ted w i th o verspeed l im i t ing gear des ignedde tec t sudden loss o f load an d to c lose emergency valves imm edia te ly to l imi t magni tude o f the t emporary speed r i se .

In a typ ica l scheme th i s i s ach ieved by moni to r ing the e lec t r i ca l ou tpu t o f genera to r us ing a wa t tmet r ic re lay. Th is re lay wi l l de tec t a sudden loss o f ou tand opera te ins tan tane ous ly to c lose it s con tac t s . A second re lay m oni to rs s t eam inpu t to the tu rb ines a t a chosen s tage and the con tac t s a re he ld dosed wthe s team pressure is in the fu ll load region.

A sudden loss o f load wi ll g ive ins tan tane ous ope ra t ion o f the ou tpu t re lay bthe s team inpu t re lay does no t opera te immedia te ly because s team i s be iadm i t t ed t o and expand ing i n t he t u rb ine . Unde r t hi s cond i t i on t he emergenvalve solenoids are energised giving ins tan tane ou s co ntro l o f s team adm iss ion. Tem ergen cy valves rem ain c losed u nt i l fa ll ing pressure or res to ra t ion of load res tothe machines to normal con t ro l .

The ac t ion o f th i s equ ipm ent is c lea rly m uch fas te r than th a t ob ta inab le f r

the govern ing sys tem which requ i res an ac tua l overspeed to p roduce a response t ake cor rect ive ac t ion . I t is fo r th i s reason tha t overspeed l imi t ing equ ipm ent o f



Protect~on o f generators an d transformers 3

s team pipes of re la t ively large bore in terconnect ing each reheater and the rehsect ions of the associa ted boi ler p lant present specia l problems due to the lavo lume of s team en t ra ined .

The overspeed l imi t ing equ ipm ent then opera tes addi t iona l ly in to the in te rcep

emergency s top valves associa ted wi th each interceptor s team chest to give instaneou s con trol o f the s team en ter ing the turbine a t a ll s tages.

In the ul t ima te , ov erspeeding of the m achine be yo nd the safe l imi t (10 ) wcause op erat ion of the overspeed bo l ts , and sh ut the s top valves .

12 .3 .12 Un derexc i ta t ion l imi t ing

The operat ing characteris tics of a cylindr ical roto r a .c . gen erator feeding an inconnec ted sys tem are such tha t a def 'mi te pos i t ive min imum of ro tor exc i ta tm us t be main ta ined to ensure syn chrono us s tab il ity.

Fro m the design cr iteria th is l imi t can be d escr ibed and is usual ly term ed ' theo retica l s tab il i ty l im it ' . To g uaran tee sta bil i ty in service a safety m arginadded to the theoret ical l imi t to make a 'pract ical s tabi l i ty l imi t ' .

I f the co nt ro l o f exc i ta t ion is by m anual ad jus tm ent the op era t ing po in t o f generator i s usual ly mainta ined by observat ion and adjustment wi thin an a

bounded by the pract ical s tabi l i ty l imi t in the leading (underexci ted) reactzone, the s ta tor and rotor heat ing l imi ts in the lagging (overexci ted) zone, the l imi t being set by the turbine capaci ty or s team condi t ions .

When an au tom at ic vo ltage regula tor is em ploye d to cont ro l exc i ta t ion , measum ust be taken to ensure that ex ci ta t ion is no t reduce d b elow a safe l imi t . This l iwi ll depen d up on the form o f vol tage regulator and will corres pon d to the practs tab i l i ty l imi t fo r mos t e lec t romechanica l regula tors , bu t may approach ttheoret ical s tabi l i ty l im i t for regulators having no dead ban d and a high speedresponse. I f the level of exci ta t ion approaches the l imi t a subsidiary control loopem ploy ed to feed a m odi fy ing signal in to the au tom at ic vo l tage regula tor con tloo p to p revent fu r ther low ering, and in cer ta in cases, to ra ise exc i ta t ion in addi tto giving an alarm.

The ini t ia t ing signal for the u nd er ex ci ta t ion l imi t ing device ma y be take n f ro mmeasure of ro to r ang le , o r o f ro tor exc i ta t ion cur ren t , o r o f a der iva tion of tgenerator operat ing point f rom a measure of wat ts , revs and terminal vol tage.

1 2 3 1 3 M e c h a n i c a l a n d h y d r a u l i c t r ip s

In addit ion to the electr ical protection discussed earl ier in the chapter, variomechanical and hydraul ic devices are employed to detect abnormal i t ies in tturbine and i ts boi ler. S om e of the devices in t ia te a larms to the opera tor, and theare no t deal t w i th here , b ut o thers t r ip the un i t autom at ical ly an d so, to i l lus t rcom ple te p ro tec t ion of a m od ern , la rge , tu rbo-genera tor, they have been inc lud

in Fig . 12.3 .13A.



3 P r o t e c t i o n f g e n e r at o r s a n d t r an s f o r m e r s

L O C A L T U R B I N FT R IP L E V E R ~ "'

OVF.RSPEEITR P t

: ~ L O W S T E A M I N L E T P r E s s u r E t ~ ~L O SS O V L U B R I C AT I N G O I L -

- R l . . . . .

._~ - I L O S S0 ~ s ~ .e ~ :o ( ;O V ~ , N O , _ T, ~ , ~ I

, ,t " ' L O S S O I .' B O I L E R , WAT E R , j _

EM . . . . . . . . . . REMOTE:E R G E N C Y P U S HB U T T I ) N & LOCAL

• - i N E G A T I V EH A S E S E Q U E N C E ' ,, ,,

G E N . S T AT O R E /V I N S T A N T A N E : O U S

GEN. TRANS.W D G . TEM: 'ERA'rURI , :~ . T a ^ N ~ . O ~ ~ ^ ~ C ~ O ~ : :

G E N . T A N s . . u ( . 'H H O L Z s u r~ ; ,. :

G E N . T R A N S . H . V, O V E R C U R R E N T

G E N .T R A N S . H . V. R E I " .

E'iRST MAIN ~E N .E'EEI~L:R PROT .

- - - { .[

, . - GI L .

G I : (

" - " G I ,

- - - G I

- - - I . 'i L . (

~= " " S E C O N D M A I N G E N . F E E D E R P R O 3

~1 " " I N T E R L O C K E D O V E : R C U R R E N T

I BREA KER E:AIL

~ ~ , Um T TR^N; '. OV~:R^LL'~OT.

1 - - I U N ,T T ~ ^ N S . H .V. O V ~ : ~ C ~ k H ~ T

r .

J H ' V " B U S B A RPROTECTION . . . . . . IJ

a.

0 -

E l e c t r i c a ls o l e n o i d o n

II t u r b i n e t o

r e l e a s e f l u i d - J& t r i p t u r b i n es t e a m v a lv e s T U R B I N E "

i S T E A M I= , / ~ [ VA LV E

R e l a yfluid / I

s w i t c h [ BOILER

" - - ~ i I . ' iR I N ( . ;TRIP

• - ' - E l e c t r i c a l s i g n al

- - ' - - M e c h a n l c a l / h y d r a u l i cs igna l

A - C o n t a c t w h i c h c lo s e s o no p e r a t i o n o f s e n si ti v e p o w e rr e l a y f o r d e t e c t i n g c u t - o f f

o f s te a m s u p p l y t o t u r b i n e

U N I TT R A N S .L . V. C . B .

E ' I E L D~ SWITCHES

. V. C . B .




at ten da nt to shu t-down the set local ly should the need arise .

b) Overspeedt r tp: a centrifugal device init iat ing tr ipping if the turbine speerises 10 above the syn chro no us speed.

c) L o w steam inlet pressure:a device sensing loss o f turbine steam pressure dueto loss o f boiler firing.

d) Loss o f lubricating oil:an off-pressure device monitoring the supply olubricating oil to the turbine bearings.

e) Loss o f boiler water:a water-level detector init iat ing boiler f ir ing tr ippinto prevent extensive damage being done to the boi ler tubing due to ov erheat ing.

f) Vacuum trip:independent vacuum-opera ted unloading cont ro l and t r ipp indevices. The control progressively reduces the steam flow into the turbine as vacuum falls over a predetermined range, but if the vacuum falls below a pdeterm ined m inim um value a vacuum ope rated switch init iates t ripping.

g) Em ergency push button:opera t ion of an emergency push but ton in i t ia tet r ipping of the turbine s team valves . The rem aining m ain plant will t rip via

low pow er in te r lock contac t A.To enchance the in tegr i ty of modem turbogenera tor pro tec t ion t r ipp ing , ttr ip-init iat ing devices are divided be tw een several , ind ep en de nt d.c. tr ipping supptypically, as follows:

H .V. switchgear, first su pplyBusbar protect ion

G enerator feeder, f i rs t m ain protect io n re lay 1Inter locked overcurrent re layG enerator t ransform er h .v. overcurrent re lay

H. V. switchgear, secon d sup plyBreaker fa il pro tect ionG enerator feeder, second main pro tect ion re lay 1

Pow er station, p rs t sup plyG enerator feeder, f irs t main p rotect ion re lay 2Unit t ransformer overal l protect ion

Unit t ransformer Buchholz surgeG enerator s ta tor E lF re lay ( instantaneou s, low resis tance)




Negative phase-sequence pro tectionGenerator transformer Buchholz surgeGenerator t ransform er winding temp erature t r ipEmergency push bu t ton

Ge nerator s tator E /F relay 1 st i .d.m.t . , high resistance)Loss of exci ta t ion protect ionLoss of vacuu m tripLow steam inlet pressure tr ipUnit t ransformer 1.v. s tandby E/FPole-slipping protectionLoss of s ta tor w ater f lowLoss of lubricating oilLoss of speed governor tr ipLoss of boiler w ater

Pow er station second supplyGenerator feeder, second main protection relay 2

Un it transform er 1.v. restricted E /FUnit transformer h.v. overcurrentGe nerator stator E/F relay 2n d i .d.m.t . , high resistance)Ge nerator stator E/F relay i .d.m.t . , low resistance)Ge nerator t ransformer, h .v. restr ic ted E/FNegative phase-sequence pro tection

12 .4 Gas-turbine driven genera tors

12.4.1 Direct con ne cted gas-turbine sets

At al l modem power s ta t ions in the United Kingdom, gas- turbine powegenerators have been installed to provide:

a)

b)

c)

d)

an inde pen de nt supp ly to th e auxiliaries o f the m ain steam driven units in event of unaccep tably low frequency on the gr id system

additional generation into the grid system if required by system conditions

start-up supplies to a station detached from the grid system

independent supply to ensure operation of essential drives, e.g. main bearlubricating oil in the event o f loss of norm al supplied. This du ty is , in effec




1 . 6 M VA

i

E -E iE t . . .

4 1 5 V

~~

1 k V u n i t b o a r d

:

- E , - ~- E [

1

i Ii i

IIII

. IIiI

3 5 M W IIII _

1s[2.L~ t

F ig 1 2 4 1 A

Et E

S t a g e 1 t r i p s h . v .c . b , o n l yS t a g e 2 t r i p s g a s t u r b i n e

Protective relay a.c. circuit s for po we r station emergency gas turbine set

Fig. 12.4 .1A shows the p rote ct io n of a 3 5 MW, 11 kV g enerator and i ts con nect iobut not the gas- turbine) . I t comprises :

d i ffe ren t ia l cur ren t p ro tec t ion covering the genera tor and i ts ou tgo ing connec t iloss o f exc i ta t ion pro tec t ions tandby ear th - fau l t p ro tec t ion 2 s tages)

- stage 1 tr ips the h.v. circuit breaker on ly- stage 2 tr ips gas-turbine

vol tage-contro l led o vercur ren t p ro tec t ion

negat ive phase-sequence protect ionThe teed uni t t ransformer and i t s h v connec t ions are pro tec ted by overcur re




12 .4.2 Transformer co nn ecte d gas-turbine sets

The fu nctions o f main gas-turbine generating plant are to provide:a) No rmal, econom ic generation close to load centres, giving added security

load demand by reducing i ts dependence on remote generat ion and associated transmission system

b) Em ergency generation at t imes whe n severe system disturbances cause a lgeneration deficit

c) Peak-lopping capacity for system freque ncy regulation purposes, therereducing the am ou nt o f national spinning reserve required

d) Reactive compen sat ion w hils t running on load, or synchron ous com pensatw ith clutc h disengaged).

Typically gas-turbine sets are each of 70 MN capacity powered by either t35 M N or four 17.5 MW pow er turbines Fig. 12.4.2.4,) . The pro tection arrange mand tripping logic are i llustrated in Fig s. 12.4.2B and 12.4.2C respectively.particular point of interest in Fig. 12.4.2B is the absence of biased differential cin the uni t t ransformer tee-off connect ions. In this ca se the low rat ing himpedance) of the uni t t ransformer l imits through faul t currents on the 415 vsystem to a value well below the differential pro tection fault setting.

G a sg e n . I A 2

1P o w e r

t u r b i n e l A

, ,G a s

g e n . I A

415V sw .bd.

I

7 2 2 . Ji ° . , ,

E l e c t r i c a lg e n e r a t o r

G a sg e n . I B I

r

tup:;:~rl

7 1G a s

g e n . 1 B 2



P r o t e c t i o n o f g e n e ra to rs a n d t ra n s f o rm e r s 3 7

l - S

3 2 k V

I

~ ~ ~

I

I

m

F . - - _. ..

E

IIII

7 5 M VA1 1 1 3 2 k V

g , i 5 0 0 k VA

4 1 5 V

II 7 o M W



8 P r o t ec t io n o f g e n e r a to r s a n d t ra n s f o r m e r s

G A S G E N E R AT O R I A I T R IP S

G A S G E N E R AT O R I A 2 T R IP S

P O W E R T U R B I N E A T R I P S

P O W E R T U R B I N E I A O V E R S P E E D

E M E R G E N C Y P U SH B U T T O N

G A S G E N E R AT O R I B T R I P S

G A S G E N E R AT O R I B 2 T R I P S

P O W E R T U R B I N E I B T R I P S

P O W ER T U R B I N E I B O V E R S P E E I )

F U E L VA L V ES Y S T E M

I A I & I A 2

F U E L VA L V ES Y S T E M

I B I & I B 2

E L E C T R I C A L G E N I ' R A T O R

L U B . O I L P R E S S U R E L OW

T H R U S T B E A R I N G D R A I N T E M P. E X C E S S I V E

A IR IN L E T D I F F E R E N T I A L P R E S S . E X C E S S I V E

O U T L E T C O O L I N G A I R T E M P. E X C E S S I V E

B E A R I N G T E M P. E X C E S S I V E

B E A R I N G V I B R AT I O N E X C E S S IV E

L OW F R E Q U E N C Y ( 40 H z ) T R I P

N E G AT I V E P H A S E S E Q U E N C E T R I P

S T AT O R E A R T H F A U LT I D M T ( F I R S T )

S T AT O R E A R T H F A U LT I D M T ( S E C O N D )

L OS S O 1 E X C I T AT I O N

A . V. R . F A I L

I A & I B

F U E L S Y S T E M SB O T H T R I P P E D

U N I T T R A N S F . .

L . V. C . B .

H . V. C . B .i 3 k V

S U P P R E S S V O LT SB Y A . V. R .

G E N E R A T O R T R A N S F O R M E R

B U C H H O L Z S U R G E

O V E R A L L P R O T E C T I O N

W I N D I N G T E M P E R A T U R E

H .V. R E S T R I C T E D E A R T H F A U LT

O V E R F L U X I N G P R O T E C T I O N

U N I T T R A N S F O R M E R

B U C H H O L Z S U RG E ;

H .V. O V E R C U R R E N T

L .V. R E S T R I C T E D E A R T H F A U LT

G E N E R AT O R F E E D E R P R O T E C T I O N

B U S B A R P R O T E C T I O N

G E N E R A T O R H .V. O V E R C U R R E N T

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12 5 Transformer pro tect ion

The pro tec t ion provided for a pow er t ransformer depends to some ex ten t up onsize and ra t ing, and will com prise a nu m be r ofsyst ms each designed to provide the

requis ite degree o f pro tect io n for the di fferent faul t con di t ions out l ined in Sect12.2.2. For large units high speed protection is essential .

12.5.1 Unbiaseddifferential protect ion

The unbiased different ia l overall t ran sform er p rote ct ion system is s imilar pr inciple to that for a generator. In th is case , however, the di fferent ia l protectsystem com pares h .v. and 1 .v. currents , wh ich are in a kn ow n re la t ionship unhea l thy condi t ions , ra ther than the same cur ren t en te r ing and leaving the pro tecapparatus , as for generator pro tect ion . I t i s for th is reason tha t the t ransformdifferent ia l protect ion system is capable of detect ing in ter turn shor t c i rcui ts s ithese change the effect ive overal l t ransformat ion ra t io of the power t ransformers

The c . t. conne ct ions m ust be arranged to give a thro ug h faul t balance taking iaccount the t ransformer vector group reference wi th respect to windings , conntions, and turns rat io. The guiding principles in establishing c. t . connections

tha t a ) ze ro-sequence cur ren ts should be e l im ina ted f rom or cor rec tly com pensain the re lay c ircui ts s ince , a lmo st invar iably, the t ran sform er c onn ect ions wi ll perm it t ran sform at ion of zero-sequence currents , and b) the phase shif t due to through t ransformat ion of posi t ive and negat ive-sequence currents must cor rec tly com pensa ted .

C . T . r a t i o

I p / ( . . ~ )

Ydll ¢r~1R K

. ~ . ~ N

a ) T r a n s f o r m e r c o n n e c t i o n s

- - L I I )~ x p ( Y- R_ _ _ j o -_ _Z _ >

~ ~ V ¢ 3 Ip ( IB - IY )

- - L - ( I R - I B )Ip . . . .

q

A q

b ) R e l a y a n d c . t . c o n n e c t i o n s - -

I r

I b

r

Y

b

C .T. ratioI S / I

l r / l S

l y / l SJ- ~ Y

lh/l s-~ b

Previous Page





4 Protec tion o f generators and transformers

The e l im inat ion of zero-sequence c urrents f ro m the re lay c i rcui ts is usuaach ieved by assoc ia t ing a de l t a c . t . connec t ion wi th a s t a r connec ted t rans formwinding.

Cons ider ing the typ ica l exam ple o f a s t a r-de l ta t r ans form er F ig . 12 .5 .1

hav ing a s t a r /de l t a vo l tage t rans fo rm at ion ra t io o f K : 1 , then the l ine cur ren t s o n de lta side I~, ly an d Ib a re

K

I ,= - ~ ( - )

KI b = - ~ ( I B - R )

w h e n I R I y and IB are the l ine currents on the s tar s ide . Balance is thereforob ta ine d us ing s ta r co nne c ted c .t .s on the de l t a s ide o f the t rans form er and deco nn ecte d c . t. s on th e s tar s ide of the t ransfo rm er, as sho w n in Fig . 12.5 .1A , s iw i th tu rn s r at io K : ~ /3 andIs KIp

m ly - IR ) e tc .

I f s imi la r reason ing i s app l ied to the t rans former and ea r th ing t rans formcom bina t ion o f F ig . 12 .5 .1B, the fo llowing re la t ions a re ob ta ined

K ~

• - - -- - -- . .

C ~ ~B - I R ) - / ~ 3

wh ich shows tha t th rou gh fau l t ba lance is ob ta ined by the add i t ion o f neu t ra l cto the del ta s ide of the t ransfo rm er, as sho w n in Fig . 12.5 .1B. I t wi ll be no ted tthe add i t iona l neu t ra l c . t . s have a ra t io which i s th ree t imes tha t o f the s

con nec ted c .t .s in the de l t a side o f the p ow er t rans form er.In Figs. 12 .5 .1A a nd B, Ip a ndI s are , r espec t ive ly, the p r imary and secondary



I R~ r

Y d l lul

( a ) T r a n s f o r m e r c o n n e c t i o n s


|~--.--r1

i b~ - - b

" ~ n

C . T r a t io _ 1I p / ~

R

2 _

( b ) R e l a y Ld c . t . co nn ec t ions

( . T. r a t i o I S / I

t

N e u t r a lc . t . r a t i o

l s / - - ~ - )

III=

F i g . 1 2 . 5 . 1 B O v e r a l l d i f f e r e n t i a l p r o t e c t i o n o f s t a r / d e l ta t r a n s f o r m e r w i t h I .v. e a r t h i n gt r a n s f o r m e r

The e ffec t o f t ap changing equipment upon the overa l l t rans former ra t io malso be born e in mind in the app l icat ion o f a d ifferentia l pro tect io n sy stem. Thiseffect , changes the ra t io of the t ransformer according to tap posi t ion so tha

differentia l p rote ct ion system using a f ixed c . t . ra t io cann ot give adeq uate b alanThe unba lance cur ren t resu l t ing f rom inaccura te match ing of t rans former and ra t io wi l l increase in magni tude as the through current increases , so that , exa m ple, a rat io difference pro du cing 15 spill cu rren t at rated load will give to a spill curre nt of 150 for a throu gh faul t of 10 t imes ra ted load. An unb iahigh speed relay would require a fault set t ing in excess of this if s tabil i ty is to ach ieved unde r th rou gh fau l t condi t ions . In p rac t ice , fo r op t im um per forman ce , c . t . s ra t ios are based on the t ransformer turns ra t io a t the mid- tap posi t ion.

An inverse t ime lag re lay is used for unbiased different ia l protect ion systemoften in co njun ct ion w ith a high set ins tantan eou s relay operat ing at a higher le



4 P r o t ec t io n o f g e n e r a t o r s a n d t ra n s f o r m e r s

ad jus ted so tha t it w i l l g rade co r rec tly w i th back-up p ro tec t ion under m ax im uunba lance cond i t ions . W here b o th the inver se and h igh se t r e lays are em plo yetypica l cur rent se t tings are 50 for the inverse t im e lag re lay and 600 for the h iset relay.

S o u r c e Z = 5 % o n i 0 0 M VA

1 0 0 / 0 . 5 7 7 1 3 2 / 3 3 k V, 3 0 M V A , 1 0 % 6 0 0 / 1

5 7 0 A 3 9 5 0 A

J,_ i2

I 1 4 0 A

9 . 8 7 A 6 . 5 8 A

I . I ) . M . T . r e l a y sI A = C u r r e n t s e t t i n g

0 . 5 = T i m e m u l t i p l i e r

9 . 8 7 A~ r

0

6 . 5 8 As

0

A B I C

,l

A [ B C

O n i n t e r n a l f a u l t ( I , ' 1 ) , A = B = 9 . 8 7 A ( N o I .v .i n f e e dO n e x t e r n a l f a u l t ( I . ' 2 ) , I A = B = 3 . 2 9 A

F ig 1 2 5 1 C R o u g h b a l an c e s y s te m o f t r a n s f o r m e r d i f f e r e n t i a l p r o t e c t i o n s h o w i n g r e la yc u r r e n t s u n d e r i n t e r n a l a n d e x t e r n a l f a u l t c o n d i t i o n s

An in te res t ing ex ten s ion o f th i s sys tem i s the ' rough ba lance ' p ro tec t ion , wh ich the h .v. and 1 .v . c . t . r a tios a re de l ibe ra te ly mism atche d . The indu c t iopa t t e rn r e l ay thus p rov ides low speed d i ff e ren t i a l p ro tec t ion and a l so a morpos i t ive measure o f back-up p ro tec t ion fo r ex te rna l f au l t cond i t ions .

A typ ica l app l i ca t ion o f the schem e i s sho w n in F ig . 12 .5 .1C .W here h igh-speed p ro tec t ion i s r equ ired w i th a low fau l t s e t t ing it i s e s sen t ia l u se a pe rcen tage b ias r e l ay to ove rcome the unba lance cu r ren t e ffec t s due to t achanging.

12 .5 .2 B iased d i ffe ren t i a l p ro tec t ion

In p ro tec t ing pow er t r ansfo rmers equ ipped w i th o n load t ap chang ing fac il it i e s th

overal l d i ff e ren ti a l p ro tec t ion m us t incorpora te a b ia s f ea tu re i f a low fau l t s e t t inand h igh opera t ing spee d a re to be ob ta ined . For sm a l le r t r ansfo rmers the




. - - - 0 - - - - - -

a) h)

F ig . 1 2 . 5 . 2 AInd uc t ion - typ e b iased -d i ffe ren t i a l . re lay fo r ove ra l l t r ans fo rm er gene ra to r o rg e ne ra to r / t r a n s f o r m e r p r o t ec t io n

some improvemen t onthe unbiased schemes described inthe previous Sect ion bu tis s low in op erat ion. W ith the larger and m ore im po rtan t t ransform ers the recurren t and t ime set tings necessary to ensure s tabi l i ty o n the mag net ising inrcurrents pro du ced by sw itching in the t ransform er are inadeq uate to provide hspeed p ro tec t ion .

A high speed biased different ia l re lay inco rpora t ing a harm on ic res tra int featwill prevent re lay o pera t ion und er m agnet is ing inrush current con di t ions . A typosci llogram of t ransform er m agnet is ing inrush currents i s show n in Fig . 12.5 .2B ini t ia l peak value of the inrush current in any phase depending on such factorsthe ins tan t o f swi tch ing and the magnet ic cond i t ion of the core . M aximum pvalues equal to 6-8 t im es the ra ted curren t of the t ransform er can occur. Insofainrush current affects the opera t ion o f t ransfo rm er di fferent ia l protect io n re ltwo aspects are of s ignificance. Firs t that th is curren t f lows in one winding on ly

Red phase

C' lc le

nstant of switching

Ye l l o wphase

B lu e p h a se _



Protec tion o f generators and transformers

B i a scoil

t ' 2

L2

Ins tan taneou s ~ H.S.high-set ~e l e m e n t

Stabil is ing resistor

L I C I = F u n d a m e n t a l f r e q u e n c y a c c e p t o r c ir c ui t

L2C 2 = Seco nd harm onic acce p to r c i rcu i t

L3C 3 = Th i rd ha rm onic acce p to r c i rcu it

+

~ O p e r a t i n gcoil

Fig 12 5 2C GEC biased ~i ffe ren t ia l t rans former pro te c t ion wi th ha rmon ic res t ra in t featureon ly one phase show n)

the pow er t rans form er ( the w ind ing being energ i sed) and the re fore app ears to p ro tec t ion as an in te rna l fau l t con d i t ion . Second , tha t th is cur ren t d i ffe r s f rom internal faul t current insofar as i t s wave form comprises a h igh percentage harmonics . Of these , the second harmonic has par t i cu la r p rominence under sw i tching in con di t ions , as Fig . 12.5 .2B shows.

I t thus fo ll ows t ha t a r e lay de signed t o de t ec t t he s econd ha rm on ic com pon ein the magnet is ing inrush current can be made to u t i l i se th is as a means d i sc r imina t ion be tw een in te rna l fau l t cur ren t s and magne t i s ing in rush condi t ioThis i s achieved by the use o f a seco nd-h arm onic f i l te r wh ich is ar range d to in jan add i t iona l b ias cur ren t in the re lay c i rcu i t p ropor t iona l to the second-harmon

component . Typica l a r rangements o f p rac t i ca l schemes a re shown in F igs . 12 .5 .and 12 .5 .2D.




m

L I L Z

I ~ I l . - . . i . - J

L C S e c o n d h a r m o n i c r e s t r a i n t c i r c u i t

T Tr a n s d u c t o r

R Re lay

Fig 12 5 2D R e y r o l l e D u o b ia s b i as e d d i f f e r e n t i a / tr a n s f o r m e r p r o t e c t i o n w i t h h a r m o n i cr e s t r a i n t f e a t u r e

as sho w n, w hereas in Fig . 12 .5 .2D a t rans du ctor re lay is used with the harm onrestraint derived from all three phases and used to bias al l three transductors.

A sensi t ive re lay e lement is essent ia l to reduce c . t . burdens to a minimum, an

modern harmonic res t ra ined re lays ut i l i se a sensi t ive moving coi l or t ransducttype re lay as the basic comparator. Typical current se t t ings for b iased different it ransform er prote ct ion re lays are 50-100 for the low-speed induct ion-d isc re land 20-30 for the harm on ic res tra int re lays . In bo th cases a range f rom 10-40 usual ly provided for the through current b ias .

12 .5 .3 Res t r i c ted ea r th fau lt p ro tec t ion

The difficult ies in here nt in the provis ion of an adequ ate ear th faul t sensi tiv i ty the overal l d i fferent ia l protect ion system of ten require that res t r ic ted ear th-fau




F i g 1 2 5 3 A

t R E F

, _ L _ L J _ . ,I Ii F T IL . . 1 J

t L - - -~

i

J _g

Overcurren t and restricted ea rth faul t pro tec tion

H . V. L . V.

i.J

y

. F r .

. r r - ~ .

~ L .

,I

v

I .T.

m . _ .

V

A u x c t B i a sed d i f f e r en t i a l

r e l ay

A u x c t

Fig 12 5 3B C T cir c i t diagram for combin ed biased diffe renti al and restricted earth fa lt



Protection o f generators and transformers 7

be used for th is purpo se , as show n in Fig . 12.5 .3A, or the res t r ic ted ear th-faprotect ion may be opera ted f rom the c . t . s associa ted wi th the overa l l d i fferentpro tec t ion . A typ ica l a r rangem ent i s show n in F ig . 12 .5 .3 B .

I t wil l be seen that com pen sat ion for the p ow er t ransfo rm ers h .v. /l .v, s tar deconnec t ion i s ach ieved by a s imi la r s t a r /de l t a connec t ion o f the aux i l i a ry cassocia ted wi th the h .v. , c . t . s . The s tar /del ta /s tar auxi l iary c . t . associa ted wi th tl .v., c . t .s provides the four-wire co nn ect io n of the line c . t. s (necessary to co nn ew i th the 1 .v. neu t ra l c .t . to g ive l .v.. r e s t ri c ted ea r th - fau l t p ro te c t io n )w h i le l imina t ing the ze ro sequence co m po nen ts o f 1 .v. ea r th - fau l t cur ren t f rom tthree-wire connect ion required by the phase faul t d i fferent ia l re lay. Ratcor rec t ion fo r a mism atch o f h .v. and 1 .v. l ine c . t . r a tios m ay be acc om m od ated

e i ther, or b oth , se ts of aux i l iary c . t. s. Fo r sol id ly ear th ed , s tar-con nectet ransfo rm er windings , an effec tive se t t ing of greater than 50 bu t less than 100 ra ted cur ren t i s usua l ly spec i f i ed . For t r ans former de l t a wind ings connec ted res is tance ear the d sys tems, the se t ting speci f ied shou ld l ie betw een 2 0 and 25 the neutra l res is tor current se t t ing .

12 .5 .4 O vereur ren t p ro tec t ion

The degree o f p ro tec t ion a ffo rded by an overcur ren t re lay o f the i.d .m. t , typ esom ew hat l imi ted wh en app l ied to a t r ans former. S ince the re lay m us t no t operaund er em ergency load ing condi t ions i t requ i res a h igh cur ren t se t t ing (o f ten abo200 ra t ing) . Also , the t ime se t t ing may have to be h igh in ord er to grade w iother overcurrent re lays on the sys tem. Clear ly overcurrent re lays provide negl ig ibpro tec t ion fo r fau l ts ins ide the t rans form er t ank , and m ay be very s low even fterminal faul ts where h igh faul t currents are involved.

On la rge t rans formers , the re fore , overcu r ren t re lays a re usua l ly em ploy ed on ly

back -up pro tect io n for term inal faul ts , or uncleared 1 .v. sys te m faul ts. In such cathe overcur ren t re lays may be ins ta ll ed on one o r bo th s ides o f the t rans form eaccord ing to requ i remen ts . M oreover the re lays ma y t r ip on ly the s ide o f the t ranformer wi th which they a re assoc ia ted , o r they may t r ip bo th .

Two-s tage ov ercur ren t p ro tec t ion is som et imes em ploy ed ins tead o f separai .d .m . t, overcur ren t o n b o th s ides o f a t r ans former. The two-s tage scheme compr i sone i .d .m. t , ove rcurren t re lay an d one t ime- lag re lay. The i .d .m. t , re lay is usualenerg i sed f rom cur ren t t r ans forme rs on the h .v. s ide o f the t rans former, the sourcof infeed. O perat io n of th is re lay t r ips the 1 .v. brea ker and s tar ts the t ime- lag re laThe la t ter requires a t ime se t t ing, say 0-3 sec . , so that i t does not opera te before t1 .v. break er t r ips . I f the faul t pers is ts the t ime- lag re lay com pletes i t s op era t io n ant r ips the h .v. brea ker. Since the co ntacts of m ost i .d .m. t , re lays have a long rest ime i t is usua l ly necessa ry to con nec t in ser ies wi th them the con tac t o f ains tan taneous overcu r ren t e lem ent w hich has a h igh speed of rese t , so tha t the t imlag re lay is de-energised as soon as the faul t i s c leared. The ins tantaneous e lemen

w ou ld have the same c urren t se t t ing range, and p rescr ibed se t t ing as the i .d .m.re lay. The advan tages o f two-s tage overcur ren t p ro tec t ion a re main ly econom ic , th




A disadvantage of th i s a r rangement i s , however, tha t i t does no t p rovidecr im ina tive back up pro tec t ion for fau l ts on the t ransfo rm er 1 .v. conn ec tFrequent ly, supergr id t ransformers have the i r l .v. connec t ions separa te ly pro tby d i ffe ren t ia l p ro tec t ion F ig . 12 .6 .4B) bu t i f th is p ro tec t ion should fa il to op

cor rec t ly, the two s tage , h .v. overcur ren t p ro tec t ion on the t ransformers feethe l.v. sys tem w ould opera te and ran do m ly t r ip the i r l.v. b reakers . I f how evert r ans fo rm er is equ ipped wi th l.v. ove rcu r ren t p ro t ec t ion , b reake r t ri pp ing woudiscr iminat ive s ince individual l .v. system infeeds would aggregate in overcur ren t p ro tec t ion of the fau l ted t ransformer to g ive i t fas tes t opera t ion .

1 2 5 5 i r e c ti o n a l o v e r c u r r e n t p r o t e c t i o n

Direct ional overcurrent relays m ay be used to provide discr im inat ive phase faupro tec t ion for tw o paral lel t ransform ers where there is no 1.v. source infeed. Whean l.v. sou rce infeed exis ts care m ust be taken to ensure correct d iscr im inat ion fofaults on the h.v. system externa l to the transforme rs.

An analysis of the various faul t cond i tions on t ransformer c i rcui ts shows that th90 ° -4 5 °) ar rangement obta ined w i th a 90 ° conn ect ion o f a 45 ° re lay gives be

discrim ination . Fi_g. 12 .5.5A shows the basic connections fo r the d irect iona l ove

current relay, and Fig. 12.5.5B i l lustrates the performance of the relay undephase-fault condit ions. The circuit diagram i l lustrates the current f low in th

Bu.4hars

~

. . . . . . : - - -

I,¢ Y B

( u r r e n tc i r c u i t s

V ~ l t a g ec i r c u i t s



P r o t e c t i o n o f g e n e r a t o r s a n d t ra n s f o r m e r s 9

Y d l

q 4 1 m~ b T _ _

h v °

h usha ri .v.b u s b a r

I i e a l t h yc , , n d i t i , , n s J ~ h

B ~ I~ y Y

I a u l t ~ x . YI Jc~ nd i t i<~ns h

I ~ ' \ ~ . , '¢ Y

v ~ I I /

Iy P k~ IY i y

P l~as~J rd iagram for re lay a t A

V Ih

II~ V ry ~ ,,¢ , V I

v h ~ " ~ ' - y n

B x

h

/

VY I h a s~ J r d i a g r a mf o r r e l a y at B

V R V y V B = S t a r s i d e v o l t a g e s

V ~ V ~ V ~ = S t a r s i d e r e l a y c o i l v o l t a g e s ( n o t t o s c a l e )R Y B

I R Y = S t a r s id e f a u l t ( a n d r e l a y ) c u r r e n t s

V r V y V h ~ De l ta S ide vo l t age s

V t V t V u ~ l ) e l t a s i d e r e l a y c e c il v o l t a g e s ( n o t t o s c a l e )r y h

r l y i h = D e l t a s i de f a u l t ( a n d r e l a y ) c u r r e n t s

A s s u m e d f a u l t i m p e d a n c e a n g l e ~- 7 0 °

F ig 1 2 5 5 B U s e o f 9 ° c o n n e c t i o n o f 4 5 ° r e la y f o r p a r a l l e l t r a n s f o r m e r s s h o w i n g f a u l t




para l le l tr ans formers and the pha sor d iagrams fo r the p r imary sys tem vo ltages unboth the hea l thy and the fau l ty condi t ions a re shown separa te ly.

Fro m a s tu dy of the ph asor d iagrams fo r the re lays a t A and B, i t is seen tha t use o f the (90 ° -4 5 ° ) a r rang em ent g ives pos it ive op era t ing to rqu e fo r the re lays

al l three phases of the faul ty c i rcui t ; i t be ing assumed that pos i t ive torque occwh en the angle be tw een the re lay co il vo l tage and the re lay cur ren t is wi th in range o f +90 ° t o - 9 0 ° . On the hea l thy c i rcu i t the cur ren t s a re in the oppo sdirec t ion , and i t is clear th at al l thre e relays will restrain in this case. If a s imis tudy is made fo r re lays using the 30 ° co nne c t ion ( see C hapte r 8 ) it is foun d tone o f the en d B re lays on the he a l thy c i rcu i t will opera te inco r rec t ly, the recaus ing t r ipp ing of the hea l thy c i rcu i t , and one o f the re lays on the fau l ty c i rcwi l l res t ra in when i t should in fac t opera te .

Direct ional ov ercurre nt pro tec t ion is a lso f i t ted to the 1 .v. s ide of t ransfo rmfeeders to detect reverse f low of current through the t ransformer, such as woccur for h .v. feeder faul ts . Typical appl ica t ions are d iscussed under t ransformfeeder p ro tec t ion (Sec t ion 12 .9 ) .

Di rec t iona l overcur ren t p ro tec t ion cannot normal ly be app l ied where genera texis ts on the 1 .v. s ide of the pow er t ran sform er s ince reverse load f low is poss iund er h ea l thy sys tem condi t ions . In such cases, how ever, the d i rec t iona l e lemof the re lay can be ar ranged to have an addi t ional vol tage res t ra in t fea ture such tre lay opera t ion only occurs for reverse current f low associa ted wi th a vol tacol lapse denot ing a faul t condi t ion.

1 2 5 6 I n t e r l o c k e d o v e r c u r r en t p r o t e c t i o n

Where for economic reasons i t i s necessary to locate the protect ive curret rans formers on one s ide o f the c i rcu i t b reaker on ly, a fau l t occur r ing be tw een

c i rcu i t b reaker and i t s a ssoc ia ted cur ren t t r ans former may no t be adequa tgua rded agains t.

In Fig . 12.5 .6A , for exam ple , a faul t occ urr ing a t po int F is dete c ted by tbusbar protect ion which t r ips the c i rcui t breaker. I t i s c lear, therefore , tha t tf au l t wi l l no t be d i sconnec ted , excep t poss ib ly by back-up pro tec t ion invo lv inlong c lea rance t ime . One so lu t ion would be to a r range the busbar p ro tec t iont r ip bo th s ides o f the t rans forme r. Th is wo uld no t be accep tab le i f the o the r s idethe t rans form er were conn ec ted , fo r examp le , to a s ing le- swi tch o r mesh subs ta t io r i f the t rans former was banked wi th ano ther. In the case o f the genu ine busbfau l t i t wo uld cause unnecessa ry d i sconnec t ion o f hea l thy eq u ipm ent .

To d i scr imina te be twee n a busbar fau l t and the fau l t a t F, busbar p ro te c t ion tre lay c on tacts are used to s tar t an ind uct io n disc re lay. I f faul t infeed pers is ts more than i t s opera t ing t ime (about 0-5s) , the re lay in i t ia tes t r ipping to the backbreakers.

A la te r deve lop m ent o f the in te r locked overcur ren t p r inc ip le is no w be i

ins ta lled on the supergr id sys tem . K no w n as c i rcu i t -breaker fai l pr ot ec t io n i t s ris to ensure selec tive au to m at ic t r ipp ing o f the b reakers back ing a c i rcu i t b rea




F ig 1 2 5 6 A

B B ~ ~pro t

Busbar

Busbarprotection

Fault ~ I

~ ~ Unitprotection

~

? : - i

I~ L ~

hi

App lic ati on o f in terlocked o vercurren t pro tec t ion

an in te rva l m arg ina lly longer than des igned t r ipp ing t ime of the b reaker, i t mbe assumed i t has e i ther,a) fa iled to t r ip corre ct ly, or th a tb ) fau l t cur ren t con t inue s to be fed in to the p ro tec ted zone f rom a rem ote e

In e i the r case, a m in im um nu m ber o f add i t iona l b reakers have to be t r ipp

to c lear the faul t f rom the sy s tem and th is is achieved by the breake r fp ro tec t ion . The scheme i s dea l t wi th a t g rea te r l eng th in Cha pte r 13).

12.5.7 Standb y earth fault protect ion

Back-up pro tec t ion aga ins t l .v. ea r th fau l ts can be p rov ided b y an indu c t ion pa t tre lay energ i sed f rom a cur ren t t r ans former connec ted in the power t rans former 1neu tra l . Fo r res is tance ear th ed sys tem s the re lay sho uld have a t ime se t t ing henou gh to d i sc r imina te w i th the 1.v. ne tw ork p ro tec t ion , and be a r ranged to t r ip t ransfo rm er in the ev ent of a sus ta ined l .v. faul t . Th e re lay also serves to p rote ct




Du plicate relays m ay be used, the coils being conn ecte d in series. The f irst St1) relay should have the shorter t ime set t ing, say 5 s , and should tr ip tt rans form er 1.v. circuit break er. The Stage 2 relay, say 7 s sho uld tr ip the h.v. cirbreakers e i ther d i rect ly or, where appropr ia te , v ia in ter t r ipping equipment .

Where the transformer has an individual h.v. circuit breaker, as for example, adouble busbar substa t ion, there is no necess i ty to provide discr iminat ive t r ippbetween h.v. and l .v. circuit breakers and only one stage is required. On mcon nected substa t ions , ban ked t ransformers , and t ransform er feeders , i t is usuaprovide two stages and discriminative tr ipping.

12.5.8 Tank earth fault pr otect ion

Where ban ked t ransforme rs are p rotec ted by a s ingle overal l d i fferent ia l pro tectsystem there is no ind icat ion given by the pr otec t ion regarding wh ich t ransform efaul ty. This indicat ion can conv enient ly be provided by means o f a tank ea r th-frelay.

The t ransformer tank is l ight ly insula ted f rom ear th and a l l ear thed casheaths , and then bonded to ear th via a s ingle copper s t rap, over which is mouna current t ransform er c onn ected to a relay. Any ear th fault wi thin the t ransform

tank wil l produce a current in the ear thing s t rap which operates the re lay.Faul ts external to the t ransformer and i ts bushings are not covered by t

device , bu t these can usual ly be located by inspec t ion.This is the pr inciple of f rame-leakage protect ion which has been appl ied in

past for the ear th-faul t pr otec t ion of d is t r ibut ion t ransforme rs , as an a l ternat iveres t r ic ted ear th-faul t protect ion.

12 .5 .9 Wind ingtemperature protect ion

Large t ransform ers wi th forced cool ing are usual ly f i t ted w i th winding tem pera tdevices to detect over loading of the t ransfo rm er or fai lure o f the cool ing equip m e

The winding temperature device is of the type descr ibed in Chapter 6 . The bis s ituated in a special po cke t loca ted in the f low o f ho t o il and is , in add i t iheated by a small heater energised f rom a curren t t ransform er conne cted

measure the t ransformer winding current . The device thus indicates the top temp era ture of the t ransformer p lus an increm ent p ropor t iona l to the load on t ransformer, th is increment being arranged to match the di fference between top and winding ho t spot tem peratures . FuU use is m ade of the t ransform er over lcapabil ity by a r ranging the thermal t ime c ons tan t o f the equipm ent to be s imi latha t o f the t ransformer.

Two winding tempera ture ins t ruments a re genera l ly f i t t ed to each t ransformeach ins t rum ent is f i t t ed wi th tw o m ercury sw i tch contac ts . Opera t ion of oins t rument is ar ranged to s tar t cool ing fans and pumps, and to give an a larm. Tother ins t rument is arranged to give the same alarm and to tr ip the l .v. c i rc




Typical set tings em ploy ed are:

Inst rument 1Coolers In: 90°C Alarm

Out: 65°CAlarm 110 °C Trip

Ins t rument 2l l 0 ° C

125°C

At one t ime, transformers were f it ted with separate oil and winding tem peraturinstrum ents. The form er m easured the oil temp erature on ly which did no t givereliable indication of hot spot temperatures because of the long thermal t imcons tant of the oil app roxim ately 10 t imes that o f the winding).

In the case of t ransformers w ithou t forced cooling, an oi l temperature a larmonly is sometimes fi t ted.

12 .5 .1 0 Gas genera t ion and o il surge pro tec t ion

All faults within the transform er tank give r ise to generation of gas, which may bslow for m ino r or incipien t faults or violent in th e ca se o f heavy fau lts.

Buchholz re lay

\

ransformer

t ank

View of r e laymounted in pos i t ion

Secondary wi r ingto t r ip and a larm ci rcui ts




The genera t ion of gas i s used as a means of faul t de tec t ion in the gas and opera ted re lay which comprises one or two hinged f loats , buckets , or s imbuoyan t masse s wh ich a r e i n se r t ed i n t he p ipework (F ig . 12 .5 .10A)be twet rans former t ank and conserva tor and which a re normal ly he ld in equ i l ib r ium

the oi l ( see Ch apter 6) .The r is ing bubb les p rod uced by the s low genera t ion o f gas, due to a m inor fa

pass upwards towards the conserva tor bu t a re t r apped in the re lay chamber causa fall in oi l level inside i t . T his disturb s the equil ib rium o f the gas f loat , the rc los ing it s con tac t s w hich w ould norm al ly be co nne c ted to g ive an a la rm.

A heavy faul t wi l l produce a rapid genera t ion of gas causing violent d isplacemof the o i l which moves the surge f loat sys tem of the re lay in pass ing to

conserv ator. This w ill resul t in c losure of the surge f loat conta cts w hich a r ranged to t r ip the t rans former.To re l ieve the v iolent surging of o i l which may cause spl i t t ing of the t ransform

tan k wal l and the e jec t ion of i t s bushings , t ransform ers of smal l ra t ing are f i twi th exp los ion ven ts tak ing the fo rm of a ch im ne y capped by a burs ting-dd iaphragm. F or t r ans formers o f l a rger ra t ing it becomes imposs ib le to ac com m odthe exp los ion vents and m ainta in thei r ear th faul t c learance f rom the lconductors . The solut ion to the problem is to f i t a spr ing- loaded pressure red iaphragm in the t ank wal l th rough which surg ing o i l may escape in to the busur rounding the t rans former. Qu a l i tro l dev ices as they a re ca ll ed a re equ ipped wcontac t s which m ay be used fo r a la rm or t r ip in i t i a tion .

The gas- and off -opera ted re lay gives the bes t poss ib le pro tect io n agains t sucondi t ions as defect ive coi l bol t insula t ion and shor t -c i rcui ted laminat ions , inc ipient fa i lure of the main insula t ion. The a larm element wi l l a lso opera te for o il con di t ions e tc . , as wi ll the t r ip e lem ent i f the con di t ion deter iora tes .

Ana lys i s o f the gas co l lec ted in the Buchholz re lay chamber may f requenass is t in the d iagnosis of the type of faul t , and the ra te of gas genera t ion givesindicat ion of the sever i ty.

W here the t ap changer se lec to r swi tches a re in a separa te o il co m pa r tm en t f rthe m ain t rans form er, p ro tec t ion can be p rov ided by e i the r a separa te gas -a nd opera ted re lay, o r by a r rang ing the o i l p ipew ork so tha t the t ap change co m pa r tmis connec ted to the t rans former conserva tor v ia the main o i l -and gas -operarelays.

12 .6 Pro tec t ion schemes fo r typ ica lt r a n s f o r m e r s

The degree o f p ro tec t ion p rov ided d epends to a g rea t ex ten t up on the s ize afunc t iona l im por tan ce o f the un i t . A fu r ther imp or tan t fac to r is the econo maspec t . The cos t o f p ro tec t ion fo r power t rans formers t ends to be p ropor t ionah igher than the cos t o f p ro te c t ion fo r o ther i t ems of p lan t.

1 2 6 1 D i s t r i b u t i o n t r a n s f o r m e r s




the minimum protection is usually provided consistent with acceptable overaperforma nce. H igh speed prote ction is not always necessary particularly for phasfaults as pow er system stability problems have rarely to be considered.

The larger siz es o f distribution transforme rs say abo ut 5 or 10 M V A arequipped with on-load tap changing and may have forced cooling. The smaller sizgenerally h a v e neither and are o ften equipp ed with fu se s rather than circubreakers.

Larger distr ibution transformers are protected by overcurrent and earth-fauprotection. Where fault current can be fed from h.v. and l.v. sides the overcurrenprotection is usually fitted to both sides of the transformer. Fig. 12.6.1A shows typical arrangement.

Larger distr ibution transformers m ay also have overall differential p rotection which the restricted earth fault protection is incorporated. The protection of suctransformers in practice m ay differ li tt le from transmission transformers d escribein Section 12.6.2.

_ _

2

f au l tre lay

F ig 1 2 6 1 A

1 2 6 2

h v I v

h v

overcurrentrelay

Restr ic tede a r t h

fault relay

o v e r c u r r e n tre lay

O v e r c u r r e n t a n d e a r t h f a u l t p r o t e c t i o n f o r a d e lt a / s ta r d i s t r ib u t i o n t r a n s f o rm e r

Two winding transmission transformers

Economic considerations in the protection of transformers connected to the132 kV 275 kV and 400 kV grid system tend to be outweighed by the need fohigh speed fault clearance necessary for system stability reasons and the need tminimise fault damage.

The two-winding transmission transform er norm ally has the h.v. winding solidlyearthed and the 1.v. winding resistance earthe d thro ugh an interconn ected-staearthing transformer. On-load tap changing and forced cooling are always provided

To achieve high-speed discriminative fault clearance for both phase faults and

earth-fau lts differential p rote ctio n w ith h.v. and 1.v. restricted earth fault is fittedThe differential pro tectio n usually incorpo rates load b ias and harm onic restrain



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Ga s- and off-actuated relays are fitted to both the m ain trans form er and itassociated earthing auxiliary) transformer. Duplicate winding tem peratu reinstruments are fitted for starting the cooling, and for alarm and trip.

Back-up prote ction is provided by overcurrent relays and 1.v. stand by earth-faulrelays one or tw o stages).

A typical 132 kV g ri d transformer protection arrangement is shown inFig. 12.6.2A.

On a single transform er installation the h.v. connec tions are normally included ithe zone of the overall differential and balanced earth-fault protections. Whertransformers are banked it is preferable to have separate overall differential anbalanced earth-fault protection for each transformer, and a separate circulatin

curren t system for the pro tection of the h.v. interconnec tions. Th is requires currentransformers in the transformer h.v. bushings.Banked transformers will invariably be fitted with two stage stand by earth faul

protection.

1 2 6 3 S t a t i o n t r a n sf o r m e r s

At most major generating stations transformers are installed to provide a supply tthe station auxiliaries from the grid system. Though their MVA rating m ay becomparatively low, these station transformers are important units, and theirprotection is governed by technical rather than economic considerations.

Station transformers n orm ally hav e on-load tap changing; but forced cooling ino t e mp loyed . The w indin gs are usually star/delta/star, the h.v. winding beingsolidly e arth ed and the 1.v. wind ing resistance earth ed .

Since these transforme rs are connected to the grid system, internal fault current

can reach very high multiples of transform er rating for example 300 times c.trating), and the p rotectio n arrangem ents m ust be capable of correct operation athese high currents.

The p rotectio n fitted to a station transform er is similar to that app lied to a gridtransformer exce pt that winding tem perature indicators m ay be om itted.

1 2 6 4 A u t o t r a n s f o r m e r s f o r t r a n s m i s si o n

The use of an autotransformer rather than a two winding transformer hasecono m ical advantages where the h.v./1.v, voltage ra tio is low , since in thesecircumstances the auto transfo rm er w ill be smaller for a given MV A rating. Auto-transformers are used to interconnect the British 400 kV, 275 kV and 132 kVsystems, thus mak ing possible the installation o f units o f high rating in situationswhere the use of a two winding transformer would introduce transporta~tiondifficulties. Bo th h.v. and l.v. systems have com m on neutral earthing arrang em entssolid earthing in thecase of the 400 kV, 275 kV and 132 kVs y s t e m s .

Most autotransform ers are fitted with delta-conne cted tertiary windings voltage



8 Pro tection o f generators and transformers

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synch ronous com pensa to r. The p ro t ec t i on o f synch ronous com pensa to rs descr ibed in Chapter 14.

Large au to t rans formers a re f i t t ed wi th on- load t ap changing and fo rced coo lequipment . The tap changing is genera l ly carr ied out a t the lower vol tage terminand in con seque nce the tap changer has to be fu l ly insula ted and the d iverswi tches are the re fore m ou nte d on to p o f the lower vo ltage t e rmina l bush ings .

The pro tec t ion a r rangements fo r l a rge au to t rans formers a re s imi la r in morespects to the protect ion of two winding t ransformers . A s impler d i fferentpro tec t ion scheme, how ever, can usua l ly be em ploye d . Typ ica l o f the d i ffe ren tpro tec t ion fo r an au to t ran s form er is the c i rcu la ting cur ren t sys tem shown F ig . 12 .6 .4A. Each phase w ind ing fo rms a th ree -ended pro tec t ion zone , and c . t . s in the l .v. , h .v. and neutra l ends are connected in para l le l to form a c i rcula tcur ren t sys tem.

All c . t . s have the same ra t io and a s imple ins tantaneous re lay is used s ince p ro tec t ion i s no t a ffec ted by e i the r magne t i s ing in rush cur ren t s o r t ap changiThe ea r th con nec t ion o f the t e r t i a ry wind ing is norm al ly t aken f rom inside the zof the c i rcu la t ing cur ren t p ro tec t ion so tha t opera t ion occurs fo r ea r th fau l ts on ter t iary winding.

W ith bank ed t ransform ers i t is usual to provide separa te c i rcula t ing currepro tec t ion fo r each t rans former us ing a fu r the r c i rcu la t ing cur ren t scheme to cofau l t s on the connec t ions be tween the h .v. c i rcu i t b reaker and t rans former h

bushings . (Fig . 12.6 .4B) . This ar rangement has the advantage of d is t inguishco-ordinat ing gap f lash-overs an d h .v. co nn ect io n faul ts and provides bas ic logic



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6 P r o t ec t io n o f g e n e r a to r s a n d t ra n s f o r m e r s

12 .7 P ro t ec t ion sys t em fo r gene ra to r t r ans fo rm er un i t s

Al l l arge m ode rn gen e ra to r s a re d i r ec tly con ne c ted to an ind iv idua l pow et r a n sf o r m e r a n d t h e c o m b i n a t i o n is s w i t c h e d o n t h e h . v. s id e o f t h e tr a n s fo r m e r a

the m a in t r ansm is s ion vo l tage . A un i t t r ans fo rmer wh ich supp l i e s t he gene ra toaux i li a ry sys t em is co nn ec te d a t t he gene ra to r te rm ina ls .

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Alte rna t ive methods o f connec t ing genera to rs a re shown in F ig . 12 .7A bu t each o f the a r rangem ents show n, the p r inc ip les o f p ro tec t ion a re s imi lar. In fo l lowing d i scuss ion the conven t iona l a r rangement shown in F ig . 12 .7A(aassumed.

The genera to r wind ing and t rans former l .v. wind ings fo rm an i so la ted sys twhich the re fore requ ires it s own neu t ra l ea r th . Co m m on m ethod s o f ea r th ing ulow res is tance , a h igh res is tance (poss ibly us ing a d is t r ibut ion t ransformer seco nda ry loading res is tor) or a res istance- loaded vol tage t ransfo rm er. The earfaul t current i s usual ly l imi ted to 200-300 A in the case of low res is tanear th ing, to 1-10 A in the case of h igh res is tance ear th ing, and to a neglig ible vin the case of v. t . s ear th ing. Care must be taken wi th v. t . ear th ing to avofe r ro resonance e ffec t s be tween the vo l tage t rans former and the t rans formwinding capac i tances. The v i r tue c la imed fo r th i s m eth od is tha t the genera to r be lef t in service wi th an ear th faul t on the winding unt i l i t i s convenient to t ranthe load , and a r range a p lanned shu t dow n.

M odern p rac ti ce in the U ni ted Kingdom tends tow ard the use o f h igh resi staea r th ing se t to l im i t ea r th - fau l t cur ren t to 10A.

The pro tec t ion sys tems fo r the genera to r t r ans former un i t a re genera l ly s imiin pr inciple to those appl ied individual ly to genera tors and t ransformers . Since

genera to r s t a to r and the t rans forme r 1 .v. (de l t a ) wind ings fo rm an i so la ted zonepro tec t ion fo r ea r th fau l t s , however, a s imple s ta to r ea r th - fau l t p ro tec t ion schecan be used . The d i ffe ren t ia l p ro tec t ion sys tem , mo reover, can be a r ranged to coboth the genera to r and main t rans former. (F igs . 12 .3 .13A and 12 .7B) .

12 7 1 Biased different ia l pro tect io n

When ge nera to r and t rans form er a re so l id ly con nec ted the re a re no p rob lem s duemagne t i s ing cur ren t in rush , s ince bo th the genera to r and i ts t rans form er a re exc igradual ly up to thei r fu l l opera t ional values .

Thro ugh fau l t co nd i t ions can resu l t in co llapse o f vo l tage , bu t the subsequeinrush due to the recovery of the vol tage is far less onerous than the swi tchin

inrush. Inrush current condi t ions being re la t ively l ight , no specia l harmonic res t rafeature need be appl ied to the o vera ll d i fferent ia l pro tect io n as long as the fase t t ing is not too low.

A biased di fferent ia l re lay w i th a se t t ing o f 20 and a b ias of 20 is genera lsa t is fac tory; the b ias i s required, of course , to cover t ransformer on load tchanging.

The genera to r- t rans former overa l l d i ffe ren t ia l p ro tec t ion does no t inc lude tun i t t ra nsfo rm er for w hich, because of i t s low ra t ing, a separa te overa ll d i fferent

pro tec t ion is p rov ided (F ig . 12 .7A) . T he fau l t se tt ings o f the un i t t r ans formdi ffe ren t ia l p ro te c t ion can thus be re la ted to i ts own ra ting ra ther tha n the ra t ing







6 Protec tion o f generators and transformers

12.7 .2 Sta torearth fault protection

A simple ear th-faul t p rotec t ion can be appl ied us ing a re lay op erated f rom a currt ransformer in the generator neutra l connect ion, or in the secondary winding of

dis t r ibut ion t ransformer, or f rom the secondary winding of the vol tage t ransformin the case o f v. t . earthing.Where a low value earthing resistance is used to l imit earth-fault current

200 to 3 00 A, high-speed t r ipping f ro m ins tantane ous re lays is required to minimdamage . A se tt ing of 10 of m axim um ear th- fau lt cur ren t is the m in im upermiss ible i f the poss ibi l i ty of re lay operat ion, due to the zero sequence curret ransmit ted by generator- t ransformer in ter winding capaci tance dur ing external hear th-faul t condi t ions is to be avoided. The protect ion arrangement , therefousual ly co m prises two re lays , an ins tantane ous re lay having a se t t ing of 10 andi.d.m .t , ind uc tion disc relay, having a set t ing o f 5 .

W hen the g enerator i s ear thed throu gh a high res is tance ( l imi t ing the m ax imstator ear th-faul t curre nt to app rox im ately 10 A )a longer t ime delay is permiss iD uplicated i .d.m .t , relays, having 5 set tings, are used in such cases.

Both of the above systems of protect ion leave the neutra l ends of the s tawindings un pro tected against ear th faults ( typical ly the b ot to m 5.10 of t

windings) and, a l thoug h ear th faults near the neutra l ends are unl ikely to be cauby electr ical s tressing, faults due to mechanical stressing cannot be ruled out 100 winding pro tect ion is required and two system s are available.

One m etho d in jec ts coded a .c . cur ren t in to the genera tor neu t ra l connec t ion amoni tors i t s ampl i tude, as drawn by the to ta l sys tem to ear th capaci tance ( i .e . tof the generator s ta tor, generator t ransform er 1 .v. and uni t t ransfo rm er hwindings and co nnec t ions) . The effect o f an ear th faul t on the windings wo uldto reduce the system impe dance a nd so increase the level of in jected curren t t r ip the se t if the d eviat ion f rom the datum level exceeded a pred eterm ined value

A second m etho d makes use o f the th i rd harm onic vo ltages norm al ly presen tthe neu t ra l connec t ion of the genera tor. A fau lt near the neu t ra l end of the m achwo uld reduce th is voltage to n ear zero and so ident i fy faults in those sect ions ofwindings not protected by the associa ted 50 Hz vol tage sensing protect ion, se tcover the upper 90-95 of the windings . A system of f il ters re ject un w anfrequencies f rom the complementary systems, so as to provide substant ia l over

in the zones covered by the tw o re lays .The f irs t of the tw o descr ibed m etho ds has the advantage o f enabl ing a checklow resistance to be m ade on the m achine b efore i t is run u p - a significant facipar t icular ly when re turning a se t to service af ter a long shutdown or maintenaoutage.

W here v. t . earthing is used, earth -fault cu rren t is negligible and an i .d.m .t , rem ay be used to give a larm only.

I t should be no ted tha t ne i ther the d i rec t nor the ind i rec t connec ted genera t

a re p rov ided wi th in te r tu rn fau l t p ro tec t ion . Such fau lt s occur ra rely in m odgenerators but when they do, they rapidly involve ear th and are c leared by




12 7 3 Tr ipp ingarrangements

Tripping arrangements for large generator units require particularly carefuconsideration. With modern high capacity generating units both turbines an

generators are designed to a com paratively smaller frame size per MVA tha n earlieunits , and in consequence, the relat ive inert ia of the principal components is mucsmaller. This m ay result in greater r isk of damage and introdu ces add it ional controproblem s under cond it ions of heavy load rejection.

For these reasons the com plete tr ipping of a m ode rn generator un it is carded ouonly for internal fault condit ions, and for al l external faults the generator back-upr ote ctio n devices are arranged to trip the h .v. circuit breake r o nly, any speed risbeing dealt with by the governor equipm ent.

For very large units it is considered necessary to minimise the risk of a serunning away in the event of the turbine emergency stop and governing valvfail ing to close w hen the pro tection operates. An interlock is therefore included tprevent the generator circuit breaker from being opened unti l the steam supply tthe turbine has been cut off . This interlock is applied only to those protectiocircuits for w hich a significant increase in the dam age caused by the fau lt is unliketo result from delaying the opening of the generator circuit breaker unti l cut of f o

the steam has been detected.

12.7.4 Gen erator transform er overfluxingprotect ion

It is necessary to safeguard generator step-up transformers against the risk odamage t ha t m ay b e caused if they are opera ted at flux density levels significantgreater than their design m axim um value typically 1.9 T). Such cond it ions m a

occur when a u nit is on load, bu t are more l ikely to arise when i t is on o pen circuund er auto m atic voltage regulator a.v.r.) or m anual excitat ion con trol , part icularlwhen running up to , or dow n from , synchron ous speed.

I t fol lows from the fundamental t ransformer equat ion E = 444( B A ) . t TthatB = k . E / f f l ,that is f lux density B is directly prop ortiona l to indu ced voltage E andinversely pro por tional to frequency f and turns T, so tha t disp ropo rtional variationin these quan ti ties m ay giv e r is e to core overfluxing. If the core f lux densit

increases to a po int above saturation level , the f lux w ill no longer be contain ewi th in the core and so will ex tend in to o ther un lam ina ted )p ar t s o f thtransform er and give r ise to ed dy current circulation.

Depending on the qua nt i ty of f lux, the dimensions of the metal path and i tphysical propert ies, losses would be generated which may be manifested in fouw a y s

a )

b )c )

a large increase in m agnetising curre nt

an increase in winding temperaturean increase in transfo rm er noise and vibration




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5 t o 3 0 s e c o n d s .( I n h i b i t e d w h e n h . v.c i r c u i t b r e a k e r c l o s e d )

F i g . 1 2 . 7 C ene rator ransformer overfluxing protection

Since other protect ion devices do not afford complete protect ion against a l l these c ond i t ions , overf luxing pro tect ion is provided e i ther as an in tegral featurethe a.v.r, equ ipm en t i tself , or as separate relay system Fig. 12.7C). In bo th systethe act ion of the overf luxing protect ion is to in i t ia te a reduct ion of exci ta t ionnormal exci ta t ion control and, i f that should fa i l to produce the necessacorrect ion, to pro ceed af ter a sho r t time d elay typical ly 5-7s) to t r ip the generat

12 .8 Transformer feeder p ro tec t ion

The t ransformer feeder resul ts f rom the need for economy in switchgear, tt ransformer being connected di rect ly to the feeder wi thout an in tervening switI t i s used m ainly to provide a bulk supp ly f rom a m ajor switching s ta t ion. A seccase is that of in tercon nect ing tw o low vol tage systems by m eans of an h .v. l ieach end o f wh ich is d i rect ly conn ected to a transform er.

The problems of t ransformer feeder protect ion are usual ly s imilar to thoencountered in the protect ion of t ransformers and feeders as separate uni ts . Sosimpl i f icat ion in the feeder p rotec t ion is of ten poss ible by vi rtue of the imped an

of the t ransformer windings , and a high-speed protect ion is therefore obta inaover the whole feeder length using distance or high set overcurrent relay princip



Protection o f generators an d transformers 7

m inim um fau l t levels , f eeder and t rans former im pedan ce , e tc . and m us t be ca re fuconsidered a t the des ign s tage .

The com ple te i so la t ion o f the t rans form er and feeder fo r a fau l t on e i the r a lminvar iably requires the use of in ter t r ipp ing or faul t throwin g.

12.8.1 Overallp r o t e c t i o n f o r f e e d e r n d t r n s fo r m e r

I t is poss ib le to p ro tec t the feeder and t rans form er in one zone o f p ro te c t ion usa d i fferent ia l sys tem. This necess i ta tes p i lo t wires , but has the advantage thcur ren t t r ans formers a re n o t requ i red in the t rans form er bush ings . The fau l t se t to f such schemes makes adequa te p ro tec t ion o f the t rans former wind ings vediff icul t , and th is together wi th the long opera t ing t ime (due to the need to s tabi lthe re lay aga ins t magne t i s ing in rush condi t ions by means o f t ime de lay) l imi t s appl ica t ion of th is type of scheme to the smal ler t ransformer c i rcui ts . In a l l casesis des i rable to u t il ise separa te h igh-speed e ar th fau l t p rotec t ion .

The g enera l pr inciple of overa ll d i fferent ia l pro tect i on providing pro tect iaga ins t phase fau l ts is show n in F ig. 12 .8 .1A. T he sum m at ion t rans formers do nincorpo ra te an ea r th -fau l t w ind ing since ea r th -fau l t ba lance is no t requ i red . A n eafaul t o n the del ta s ide wi ll no t resul t in the appearanc e o f zero-sequence c urre

on the s tar s ide and vice versa . In order to avoid bl ind spots for phase faul ts on s tar s ide (which gives a 2 :1:1 cu rrent d is t r ibut io n on the del ta s ide) the twsec t ions o f the summat ion t rans former have an unequa l number o f tu rns .

J m ....

J L _ _ ~

J L.._

Ear thf a u l t

0 ~ 0

F ig 1 2 8 1 A Voltage balance system for del ta s tar t ransformer

A separa te re lay sys tem is required to g ive sa t is fac tory ear th-faul t protect ioand a typ ica l a r rangem ent o f phase fau l t d i ffe ren tia l p ro tec t ion wi th ins tan taneores t r ic ted ear th-faul t p rotec t ion is show n in Fig . 12.8 .1B. O n the ear th ed s tar s i

the re lay is energised f rom a core balance t ran sform er wh ich has pr im ary windinenergised f rom l ine and neutra l c . t . s . In the case of the del ta winding the ear th-fau



8 P r o t e c t io n o f g e n e r a to r s a n d t ra n s f o r m e rs

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F i g 1 2 8 1 C Translay phase and ear th-fau l t differen t ia l prote ct ion for a del ta /s tar t ransform er




Fig. 12.8.1C shows a typical overall differential prote ction scheme giving bophase and e arth-fault pro tection to a transform er feeder. In this cas e two separarelay elements are used at each end of the line. One relay element is energised froR and B current and the second from the Y phase current and this arrangeme

avoids a blind spo t u nder 2:1:1 fault con ditions (se e Cha pter 8). The interconnetion between the relays at the two ends requires the use of three pilot wires shown.

Both types of scheme are suitable for use with 7/.029 pilot circuits up to abou15 m iles in leng th and have fault settings in the range o f 60-200 .

The separate ea rth-fault pro tection arrangem ent o f Fig . 12.8.1B can have a lofault setting with high speed of operation, and it is often possible to protect thwhole of the d elta winding and a considerable po rtion , say 90 , of the star windiunder solid earthing conditions. Some difficulty may be experienced if the eartfault setting on the delta side of the pow er transforme r has to be high to preveoperation by residual capacitance current effects under external fault conditionthis is described m ore fully in the general consideration of earth-fault p rotectio n.

12.8.2 Separate pro tectio n for feeder and transform er

Schemes employing separate protection for feeder and transformer are frequentused.

Unit feed er protect ion systemsThe choice is between pilot wire protection systems using privately owned orented GPO pilot circuits, and carrier current protection usually of the phas

com parison type . In either c ase the transforme r protec tion is conven tional and wem bo dy all the features discussed in Section 12.5.

Current transformers are required in the transformer bushings for the separatzones of feeder and transformer protection. Intertripping is also required. The usof separate unit schemes of feeder protection permits high speed tripping at botfeeder ends, and such me thods will often be used over transformer interconnectocircuits, where fault power infeed may occur at both ends.

(a) P~ lotwire feed er protec tion with separate transformer protection:Pilot wiresmay be either rented or privately owned and the protection scheme employed wicomprise any o f the stand ard arrangeme nts described in Ch apter 8. Severe magnetiing inrush suppressors, or equ ivalent stabilising m eans, m ay be nece ssary.

Post Office pilot wire systems are applicable only to two ended feeders and noto teed (three-ended) feeders for which distance or high-set overcurrent protectiowou ld normally be used.

(b) C arr ier current protect ion with sepa rate t ransformer protect ion:Phase



7 Prote ction o f generators and transformers

circuits , this being identical in every way to the standard schemes describedChapter 8 . Teed ci rcui ts cannot usual ly be protected by this method.

Non -unit feeder protect ion systems

The high impeda nce of the t ransforme r w il l of ten permit the appl ication of a hspeed non-uni t scheme of feeder protect ion. O f these types two are in general d istance protect ion and high set ov ercurrent protect ion. The la t ter is the meconomical form of protection possible for feeder transformer circuits and hasthe at t en da nt advantages o f simplici ty etc. I t requires careful application and mbe thoroughly investigated at the design stage.

Dis tance protec t ion is par t icular ly suitable for the protec t ion of tetransformer feeders. In general , however, i t tends to be more expensive than pwire systems for the shorter transformer feeder circuits .

c) Distance protection with separa te transformer protection:A single zonedistance pro tectio n will give high speed phase fault protec tion to the whole feethe set t ing reach normal ly being chosen to extend to the 'middle ' of the t raforme r, that is feeder impedanc e +50 of t ransforme r imped ance; ear th-faprotect ion can invar iably be provided by means of an ins tantaneous re lay wh

m ay in certa in instances be required to e m bo dy a directional feature .The distance relay m ust be carefully chosen to restr ict the effec t of the 'bl

spot ' on close up faults inherent in al l directional relays. This blind spot canavoided com plete ly by the use of an imped ance m easur ing re lay with overcurrstart ing feature , Fig. 12.8.2A; this arrang em ent being applicable as long as remote l .v. faul t infeed to a faul t behind the re lay does not exceed the overcurrstarting relay setting.

Where a high remote infeed may occur, a directional distance scheme is requiand the use of the polarised M ho relay offers the simplest solution for phase fprotect ion s ince i t combines the funct ions of di rect ion and dis tance measurem(see Fig. 12.8.2B). A memory action feature is invariably provided which reduthe effects of 'dose up ' faul ts bu t is no t p roo f against ' swi tched o n ' faul ts.

A simple nond irectional-e arth fault relay can be arranged to provide high spprotection in many cases, part icularly where a phase current check featureinco rpor ated as described in the n ex t section. The use o f a directional earth-f

relay with a definite current set t ing will provide high-speed earth-fault protectcovering al l exigencies, and this is usually provided with the distance protectscheme.

d) High-set instantaneous overcurrent and instantaneous earth-fault pro tectioOvercurrent protect ion of the i .d .m. t , type wil l normal ly be f i t ted to one or bends of the t ransformer feeders , e i ther as a main or back-up protect ion feature

m any instances an addi tional overcurrent re lay of the high set ins tantaneous tmay be f i t ted, i t s current se t t ing being chosen such that the re lay operates



Pro tec t ion o f genera tors an d t ransformers 71

__L~ l l

| L - -

f i

S = Ove rcu rre nt start ing relay con tro ls ,~peration of Z).Z = Impe danc e measuri ng relay. Operat ing coils norma lly

short-circuited by contacts on S)E = Earth fault relay con tac ts in series with s to trip).R.O. = Impedance relay operating and restraint coils

F i g 12 8 2A in g le z o n e im p e d a n c e p r o t e c t i o n f o r t r a n s f o r m e r f e e d e r w i t h o v e r c u r r e n ts t a r t ing

The s imple arrangement of Fig . 12.8 .2C is sui table for appl icat ion to re la t ivlow MVA ra t ing (h igh impedance) t rans formers connec ted to a h igh MVA ( limpedance) source . The range of appl icat ion of the s imple scheme can be extendby using the modif ied re lay connect ions of Fig . 12.8 .2D using del ta connectauxi l iary current t ransformers so that the re lay currents are s imilar under thrphase and two-phase faul t condi t ions .

Fu nda m ental ly the pro blem is tha t the re lay m ust operate for a faul t a t the hte rmina ls o f the t ransformer under m in im um plan t con di t ions ( fau lt a t F l Figs. 12.8.2C and D), and yet remain inoperative for a three-phase fault on the s ide of the t ransformer under condi t ions of maximum offse t ( fau l t a t F2) . For re lay connect ions shown in Fig . 12.8 .2D three phase faul t condi t ions need only considered, and the effect of the d .c . co m po ne nt in the faul t current can neglected if a transient free relay is used; such a relay is unresponsive to currlevels than 90 of i ts set t ing (see C hap ter 6) .

The ear th-faul t re lay must remain inoperat ive for a l l external faul ts , e i ther the local busbar (a t F3 ) or on the rem ote s ide of the t ransform er a t F2. This la t



7 P r o t e c t io n o f g e n e ra to r s a n d t ra n s fo r m e rs

|

) = D i r e c t i o n a l e a r t h f a u l t r e l a yE = E a rt h f a u lt r e la y c o n t r o l l e d b y c o n t a c t s o n D . )

M = I ' o l a r i s e d M h or e l a y

Fig 12 8 2B S in g l e z o n e M h o p r o t e c t i o n f o r t r a n s f o r m e r f e e d e r w i t h d i r e c t io n a l e a r t h f a u l tr e l a y

I r f n

i . 3 - , J . ~ .

P = P h a s e f a u l t p r o t e c t i o n r e l a yE = E a r t h f a u l t p r o t e c t i o n r e l a y

Fig 12 8 2C H i g h s e t o v e r c u r r e n t a n d e a r t h f a u l t p r o t e c t i o n f o r t r a n s f o r m e r f e e d e r

o n o n e s id e a n d d o n o t , t h e r e f o r e , tr a n s fo r m z e r o s e q u e n c e c u r r en t s. T h e z esequence in feed f rom the t r ans fo rmer s t a r w ind ing fo r a busba r f au l t a t F3 wi l l t ent o o p e r a t e th e r e si d u a ll y c o n n e c t e d re la y E ~ F i g . 1 2 . 8 . 2 D ) , b u t w i l l n o t o p e r ar e la y E 2 b e c a u s e o f t h e e f f e c t o f t h e d e l ta w i n d i n g in e l im i n a t in g z e r o s e q u e n

cur ren t s f rom the r e l ay. The com bin a t ion o f r e lays E~ and E2 wi l l t he re fo re ensutha t t he r e l ay is i nope ra t ive fo r a ll ex t e rna l f au l t con d i t ions a s sum ing no r em o




The res idua l ly connec ted ea r th - fau l t r e lays mus t be p rov ided wi th the cor rvalue of s tabi l i s ing res is tor to ensure that re lay opera t ion does not occur due to sa tu ra t ion under t r ans ien t condi t ions .

This type o f scheme is use fu l fo r the p ro tec t ion o f im po r tan t c ircu it s f eed iindus t r i a l loads where s impl ic i ty and economy of equ ipment i s a l l impor tan t .

As a genera l ru le th is scheme can be appl ied successful ly to t ransfo rm er feedo f up t o 45 MV A ra ti ng and f r equen t ly t o 60 and 90 MV A t rans fo rmer s wcond i t ions a re favourab le . T he low er o hm ic im pedanc es o f the l a rger unsomet imes precludes a successful des ign where large var ia t ions in source infeed possible.

In genera t ing s ta t ions a lso where s ta t ion s t ran sform ers are ba nk ed i t is usual

apply separa te d i fferent ia l protect ion to each t ransformer us ing c . t . s in tt ransfo rm er h .v. bushings . The ins ta ntan eou s ov ercurren t fea ture is ar ranged pro tec t the h .v. connec t ions be tween c i rcu i t b reaker and t rans former.

Ins tan taneo us ea r th - fau l t p ro te c t ion fo r the t ran s form er l.v. wind ing is o f couconvent iona l p rac t i ce in power t rans former p ro tec t ion .

In the case of ins tan tane ou s feeder ea r th-faul t protec t ion som e care is necessain the cho ice o f re lay se t ting . For ex am ple wh en the feeder is con nec ted to tune ar thed wind ing o f a pow er t rans former a l arge residua l capac i tance cur ren t m

, 0 - - -

T r i pP = P ha se f a u l t p r o t e c t i o n r e l a yE l - R e s i d u a l e a r t h f a u l t r e l a y

E 2 = E a r t h f a u l t c h e c k r e l a y

F i g 1 2 8 2 D H i gh s e t o v e r c u r r e n t a n d e a r t h f a u l t p r o t e c t i o n f o r t r a n s f o r m e r f e e d e r u s in g d e lt a




R Y B

I y c, . 2q

IBC

I C

I C = IBC Iy c = 3 x norm al charging cur rent /ph ase

[

I y c

~ IC

B Y

Fig 12 8 2E Showing the effect of residual capacitance current on an earth fault relay underexternal earth fault conditions

f low in the feeder und er external earth-fault con dit ions. Fig. 12.8.2E show s thway in which this current appears in the relay circuit , and i t is usual for the relasetting to be at least twice the residual capacitance current value.

12.8.3 Intertr ipping

lntertr ipp ing ( the tr ipping of rem ote circuit b reakers) is an essential requ irem enfor transformer feeders to ensure complete isolat ion under internal fault condit ionThe mo re co m m on condi t ions requiring in ter t r ipping are:

(a) Buchholz re lay operat ion for a t ransformer winding faul t which may produ cinsufficient current to op erate the rem ote p rote ct ion . I t will t r ip the local breakedirect ly but the remote breaker wil l require intertr ipping.

(b) Earth faults on a feeder con necte d to a delta or un earth ed star winding whic



r o t e c t i o n o f g e n e r a to r s a n d t ra n s f o r m e rs 7

o

_ i

|.

(a) Unearthed systema f t e r c l e a r a n c e fr~m ~nee n d

O ,,

(h) Lt~x~f a u l t c u r r e n t infee d I.v.e a r t h f a u l t x ~ i t hr e s i s t a n c e e a r t h i n g

L a r g e ] ~ / ¢ ~g e __~_ i ~ I 1 l Su~'~

i i

(c) t.o~f a u l t c u r r e n t infeed (duetc~ parallel l ineb e i n g

c ~u t o f s e r v i c e

F ig 1 2 8 3 A o n d i t io n s r eq u i r i n g in e r t r i p p i n g

the l .v. s ide of the t ransformer and the condi t ion wi l l g ive r i se to t rans ient ovvo l tage due to in t e rm i t t en t a rcing a t the po in t o f fau l t F ig . 12 .8 .3A ) . In te r t r ipp ii s r equ ired he re to d i scon nec t the f au l t f rom the r emain ing end .

c) W here the faul t cur rent infeed i s l im i ted d ue to res istance ear th ing or h igs o u rc e i m p e d a n c e F i g . 1 2 . 8 . 3 A b ) a n d c ) ). T h e f au l t c u rr e nt m a g n i t u d e m a y n




The various m ethods co m m only used for in ter tr ipping are described in C hapteThese may be summarised as:a) The use of receive relays over private pilot circuits. Surge -proofing o f

in ter tr ipping m ay be required.

b) The use of code d or frequ enc y shift s ignall ing over rente d GP O or carrchannels.

c) Short-circuiting the stabil ising signal of a unit prote ction schem e fromtransform er pro tectio n; this is applicable for higher fault levels.

A fourth method of achieving remote tr ipping, part icularly applicable transformer feeders, is the use of fault throwing switches. This is suitable for clearance o f transfo rm er faults a nd comprises an autom atic sw itch of special deconnec ted between one phase and ear th on the feeder s ide of the transformOp erat ion of the t ransformer protect ion w il l t r ip the local c i rcui t breaker aopera te the fault throwing switch, which applies a single phase-to-earth faultthe trans form er terminals. This fault is detec ted by the l ine prote ction at remote end, which tr ips i ts circuit breaker. The arrangement is shown Fig. 12.8 .3 B.

, j , O A

F i g 1 2 8 3 B pplication of fault throwing switches showing automatic disconnectors

Where intertr ipping is required only for transformer faults , ei ther fault throwior pilot , or carder systems are applicable. Fault throwing has advantages whautomatic isolat ion followed by autoreclosing is envisaged, because i t permposit ive interlocking betw een the faul t throw ing switch and autom at ic isola tor, reacts directly upon the power system without placing rel iance on pilot circuits .

W he re intertr ippin g is required to cover low fault infeed condit ions, a pischeme is essential i f high speed ope ration is required. The use of directiooverc urrent relays on the l .v. side of the pow er transfo rm er will often provselective clearance b ut m ore slow ly. As previously me ntion ed, how ever, a biadirect ional re lay is required where the t ransformer may be required to import

exp ort under di fficult system cond i tions .Pi lot or carder in ter tr ipping is invariably used for im po rtant in terconn ecto




1 2 . 8 . 4 N e u t r a l d i s p la c e m e n t p r o t e c t i o n

here a de l t a o r unear thed s ta r wind ing i s connec ted to a t r ans former feeder i te ssen tia l to ensure tha t th i s unea r thed wind ing canno t rem ain energ ised und

system ear th-faul t condi t ions . This condi t ion wi l l resul t in danger to l i fe aposs ib le hazard to the sound phases due to in te rmi t t en t a rc ing v ia sys tem eacapaci tance .

In i t ia t ion o f t r ipp ing m ay be by means o f a neu t ra l d i sp lacem ent re lay a r rangto detect res idual vol tage to ear th a t the t ransformer us ing e i ther vol tage t raform er o r coupl ing capac i to rs . Ty pica l a r rangem ents a re shown in F ig. 12 .8 .4A. Trelay wi l l opera te for external as wel l as in ternal feeder faul ts and must therefobe provided wi th a t ime delay to ensure d iscr iminat ion.

. J L . . . L C o up lin g± c ~ °

. I .

Relay

B_

Voltagetransformer

r

I

iii iii i

i

Voltaget ransformer



8 P ro t ec t ion o f gene ra to rs and t rans fo rmer s

i l L L ~3 0 C

~ t ~ s ]( a ) S epara te un it sys te m s

w i th in t e r t r ipp in g

i i , ~ , l q ~ l - - - -0 ( . 30(.I i ~ I i I

S DI

(h) N {}n-unit protection systems ( ~

~ i a - . . . . . . . i

L . _ - 3

(C) (.)verall u nit pr~}tection system ( ~ W(W~

~ . o i ~ o < . ~ . . ~ o cI R E S [ i

IN iT I N r ~

S

(d) I)istanc e protec tionNPI) - Ne utral po int displacem ent protection




12 .8 .5 Di rec t iona l overcu r ren tprotection

The app l ica t ion o f d i rec t iona l ove rcur ren t p ro tec t io n to t ran s form er c i rcu i ts desc r ibed in Sec tion 12 .5 .5 which il lus t ra ted the use o f the 90 0-4 5 ° ) a r rangem

for the re lay v ol tage c i rcui ts . This, of course , appl ies equ al ly to t ran sform er feeas well as to t ransform ers .

Di rec t iona l o vercur ren t p ro tec t ion is app l ied ex tens ive ly to pa ral le l t r ans forfeeder c i rcui ts wh ere the re i s no no rm al infeed f rom the l .v. busba rs , bu t w hereinfeed of faul t curre nt can oc cur thro ug h the l .v. s ides o f the t ransfo rm ers fofau l t on the feeder. The d i rec tiona l overcur ren t p ro tec t io n i s o f ten used as backto the in te r t r ipp ing equ ipment , s ince i t i s the on ly means whereby the t rans for1 .v. c i rcui t break er can be t r ipped in the even t o f a fa i lure o f in ter t r ip pin g eq uip m

12 8 6 Ty pical pro tection arrangements for transformer feeders

M any v ar ia tions are poss ible in the overa l l pro tect io n o f t rans form er feeddepending upon the t rans former and i t s func t iona l impor tance . Some typ iarrange m ents are show n in Figs. 12 .8 .6A an d 12.8 .6B. As w i th a ll forms pro tec t ion app l ica t ion , the bes t scheme for a pa r ti cu la r sys tem ar rangem ent is t

g iv ing the requ is it e pe r fo rm ance fo r min im um cos t . Trans forme r feeder app l ica t idepend t o a g rea t ex t en t upon t he power sy s t em pa rame te r s , t ha t is m ax im um m in im um fau l t l eve ls , l ine and t rans fo rme r imped ances , e tc ., and perhaps in m




than any other instance require a very careful investigation to ensure optimdesign. It should be no ted that in Figs. 12.8.6A and 12.8.6B directional overcur(OCD) and neutral displacement (ND) protect ion would only be employed whthere are tw o or m ore tran sform er feeders in parallel.

12 9 Bibliography

Books

Protection relays application guide(GEC Measurements Ltd. , 2nd edn. 1975)

Articles

Dev elopmen ts in electrical prote ction of large generators by J H Naylor and Nu nney (Journ6es Internationales d etud e des centrales electriques m ode mAIM, Lidge, O ctober 1974)

Some aspects of generator back-up protection in relation to synchronous-machperformance during h.v. system faults by V Cook and J Rush tonProc. lEE,July 1972)

Protecting transformer circuits by J R ushton and K T YeatesElectr. Times,January 1962)

Pole-slipping pro tectio n by A Stalewski; J L H Go ody-a nd J A Dow ries (SecInternational conference on developments in power system protection, IELondon, June 1980)



"' C h a ' p t e r 1 3

Busbar protection

by L.C.W.Frerk

13.1 His toryof the developm ent of busbar protect ion

Up to th e m id 1930s , no widesca le e ffo r t s had been mad e to p ro tec t busbars ouni t bas is . Also there was re luctance in ar ranging one protect ive equipment to cas imul taneo us t r ipp ing of a la rge num ber o f c i rcu it s .

Before the Br i t i sh Gr id Sys tem was bu i l t in the ea r ly 1930s , many under tak iran isola ted f rom adjacent ones , and so the power avai lable for busbar faul ts of ten re la t ively smal l , and damage due to these faul ts was genera l ly not extens ive

By the l a te 1930s , the Br i ti sh Power Sys tem s were ex tens ive ly in te rcon nec tw i th a conse quen t increase in fau lt po wer.

A num ber o f busba r fau l t s occur red abo ut th i s t ime , bu t du e to the ir r e la t ivs low c lea rance f rom the sy s tem by overcu r ren t and ea r th - fau l t r e lays , cons ideradamage resul ted , especia l ly in indoor s ta t ions .

These fau l ts led to e ffo r t s be ing mad e to p rodu ce busbar p ro tec t ion in sucform tha t i t cou ld be w ide ly app l ied wi tho u t i tse l f be ing a fu r the r hazard to sys tem.

Co ns t ruc t ion o f the Br it ish 275 k V supergr id sys tem began in abo ut 1953 ,wh ich t ime s t anda rd p r inc ip l es o f busba r p ro t ec t i on had been adop ted fo r ou tdswitchgear a t the h igher vol tages . At th is t ime the emphasis was placed on avo idance o f unw an ted ope ra t i ons in o rde r t o g ive m ax im um secu r i t y o f suppW ith the in t rod uc t io n o f 400 kV subs ta t ions in the 19 60s , the t rans ien t s t ab iof genera to rs became the more impor tan t cons idera t ion and th i s l ed to a changeemphas i s so tha t f ast opera t ing t imes and re li able ope ra t ion wo uld be o b ta ined ffau l t occur r ing wi th in the p ro tec ted zone , which in th is case wo uld be the busband swi tchgear.

13 .2 Genera l cons idera t ions

13 .2 .1 The bas ic ph i losop hy of busbarprotection





Busbar protection 83

13 .2 .2 Ea r th -f au lt p r o t ec t ion ve r sus phase and ea r th -f au l t p r o t ec t ion

In some substa t ions dividing wal ls , which may be par t of the bui lding s t ructureindoor switchgear, or s teel p la tes separate the three phases one f rom another.

oth er designs of sw i tchgear, each phase con du cto r is sup po rted w i thin an ear ttubular chamber. Such ins ta l la t ions are commonly referred to as 'phase-segregaswitchgear ' and provided th at the segregation is m ainta ined th rou gh ou t the buszone, i t fo l lows that a l l natural ly occurr ing busbar faul ts must be between onem ore phases and ear th . This being the case , the n, for lower vol tage systems wh ole ins ta lla t ion need on ly be pro tected against ear th faul ts . Three-phase faul tsphase-segregated switchgear can , ho w eve r, s ti ll occu r if the bu sbars are energise

er ror wi th sa fe ty ear ths connec ted . In these c i rcumstances the sum of the facurren t in the three phases (which is the net current f lowing to ear th) wo uldsmal l and unl ikely to operate the re lay(s) of an ear th-faul t protect ive system. Ifor th is reason that phase and ear th-faul t protect ion is appl ied to phase-segregam etalc lad switchgear on the supergr id system . A t lower vol tages the choice wi linf luenced by the addi t ional cost of the phase and ear th-faul t protect ive systems

With open terminal swi tchgear i t was or iginal ly considered adequate to protagainst ear th faul ts only, because exper ience has shown that most faul ts on busband switchgear had involved ear th . The consequ ences o f re lying on back -up ptect ion to c lear the few phase faul ts that d id occur were , however, suff ic iendras t ic to cause a change of po l icy and for many years now busbar p ro tec trespond ing to phase- to .phase faul ts as wel l as ear th-faults has been regardedessent ia l . The on ly ex cept ion s o ccur a t tho se 33 kV , 11 kV and 6~5 kV sub sta t iwh ere the jus t i f icat ion for prov iding busbar pro tect io n is m arginal and the s imppossible system is usually f i t ted.

Non-phase-segregated metalc lad switchgear should, for obvious reasons , a lsoprovided wi th busbar p ro tec t ion which wi l l opera te fo r phase fau l t s and eafaults .

The method of achieving ear th-faul t only or phase- and ear th-faul t protect iondescr ibed la ter on in th is C hap ter.

13.3 The c learance of busba r faul ts by no n-u ni t c i rcui tp r o t e c t i o n

13 .3 .1 Back-up ove rcu r ren t and ea r th -f au l t r e l ays

F or the reasons given in Ch apter 8 , faults c leared by b ack-up o vercurren t and eafaul t protect ion wi l l usual ly be nondiscr iminat ive , causing unnecessary spl i t t ingthe system w hich m ay leave som e sect ions w i th insuff ic ient generat ion to me et load, leading to widespread load shedding by vol tage redu ct ion and q ui te poss ithe d i sconne c t ion of som e consumers .

Rel iance on back-up protect ion to c lear busbar faul ts i s nowadays conf inedradia l sys tems a t the lower d is t r ibut ion vol tages of 11 kV and 6-6 kV where



84 8usbar protection

r a re ly b e j u s t i f ied . Where pe rm i t t ed b y t h e su bs t a t i on con f ig u r a t i on , so m e improment in the secur i ty o f supp l ies can be ach ieved by des ign ing the p ro tec t ion so ta s a f ir s t s t ep , r e lays op era te to t r ip a l im i ted num ber o f c i rcu i t b reakers so assp li t the busbars in to sec t ions , each wi th i ts own in feed and as a second s tep to

those in feeds s ti ll ca r ry ing fau l t cu r ren t a f t e r a t ime <le lay o f , say, 0 4 s . W hermore d i sc r imina t ive scheme i s necessa ry i t wi l l usua l ly be o f the f rame-ear th tdescr ibed in Sect ion 13.4 .2 .

13 .3 .2 Dis tanceprotection

F rom Ch a p t e r 9 i t was s een tha t f e ede r s f i tt ed w i t h d i s tanc e p ro t ec t i on h ave a tcharac te r i s t i c a s shown in F ig . 13 .3A. Th is means tha t the d i s t ance p ro tec t ion f ia t end A of the feeder A B wi l l , fo r a f au l t F on the bu sbars a t B , ope n the c irb reak er a t A in the Zo ne 2 t im e , w hich w i ll be se t be tw een 0 -4 and 1 .0s.

W here a ll f eeders c onn ec ted to a busb ar s t a t ion a re p ro tec ted by 3 -zone d i s t ap ro t ec t i o n , t h is c an , in some c i r cum s t an ce s , p ro v id e a l im i ted deg r ee o f busp ro t ec t i on . I f , howeve r, som e o f t he c i rcu i ts c onn ec t ed t o t he b us ba r s conce r nare genera to r s , on e m us t cons ide r how these w ould be t r ipped fo r a busb ar faThe o n ly gen era to r p ro tec t ive sy s tem s w hich o pera te fo r f au l ts on the h igh-vo l t

c i rcu i t b reaker o r the remote s ide the reof a re ea r th - fau l t and /o r overcur ren t anega tive phase -sequence p ro tec t ion . These re lays w ould a ll be un acce p tab ly slowc lea r i ng busba r f au l t s and , due t o t he d ec r e m en t o f t h e ge ne ra t i on con t r i bu t i onthe fau l t cu r ren t , overcur ren t and ea r th - fau l t r e lays may even fa i l to opera te a t F u r the r m o re , t he negat ive phase -s e q uenc e p ro t ec t i on on l y ope r a t e s fo r unb a l anfau l ts . There fore d i s t ance p ro tec t ion can no t be re li ed on to c lea r busb ar fauwhere genera t ion i s connec ted to the same busbars .

Time _

(secs)1.0

i

protectionequipment

Z o n e I

Zone 2. . . . . . .




- - 0 -

j - ~ . . . .

z r

, -

"-" I-"

z c

- - o , I

-o OE) -II

Bus sec t ion II

Z. , , , , , m . - . . - .

- - 0 ,

- - E l - -

- - E ] - -

( ~ = C i r cu i t- b re a k e r s t ri p p e d Z - Dis tance p r o t e c t i o n e q u i p m e n t

Fig . 13 .3B Lim ited busbar prote ction by distance pro tection

F ig. 13 .3B shows how the p ro t e c t i on o f busba rs can be a f fo rded by d is tapro tec t ion where the fo rego ing cons idera t ions do no t app ly. Here the in feedfau l t F on busbar B f rom the four feeders is in te r rup ted by the op era t ion o f td i s tance p ro tec t ion a t the remote ends a t subs ta t ions A and C.

i t is a lso to be n o ted tha t the w hole o f the busbars a t s t a t ion B a re c leared remote d i s tance p ro tec t ion , a l though i t i s on ly necessa ry fo r the bus sec t ion , tf eeder c i rcu i t b reakers and one t rans former c i rcu i t b reaker to opera te to c lfau l t F.

On a c lose ly in te rcon nec ted sys tem such as the 132 kV Br i ti sh Gr id , whthere a re a num ber o f shor t f eeders o f insuff ic ien t l eng th to pe rm i t p ro tec t ingd is tance p ro tec t ion , un i t p ro tec t io n has to be used . Such un i t p ro tec t ion remas tab le fo r busb ar fau l ts , and so the back -up overcu r ren t re lays , w i th the i r slopera t ing t ime , have to be re li ed upo n to c lea r the fau l t.

Be t te r d i sc r imina t ion m ay be possib le by a lso f it t ing d i s tance p ro tec t io n to coup le r an d sec t ion sw i tches and poss ib ly ge nera to rs a t l argo dou ble busb ar ss ta t ions , bu t pe r fec t d i sc r imina t ion cou ld s t il l no t be guaran teed und er

c i rcumstances .F rom the fo rego ing , i t is appa ren t t ha t non -un i t t ypes o f p ro t ec t i on such



86 Busbarprotection

on a l a rge in te rconnec ted sys tem. Consequent ly, un i t fo rms have been deve lopto ensure shor t c lea rance t im es and cor rec t d isc r imina t ion fo r such fau lt s. The opresen t d ay app l ica t ion fo r d i s tance pro tec t ion as a l im i ted fo rm o f busba r p ro tt ion is on d i s t r ibu t ion sys tem s a t 33 kV .

13 .4 U ni t sys tem so f b u s b a r p r o t e c t i o n f o rm eta lc lad d i s t r ibu t ion swi tchgear

13 .4 .1 Genera l cons ide ra t ions

The m ain requ i remen ts o f un i t p ro tec t ion whe n f i tt ed to p ro te c t busbars a re thamus t :

(a) Have a shor t op erat ing t im e, especia lly w here faul t levels are h igh, in ord eminimise damage to the swi tchgear and to ass is t sys tem s tabi l i ty.

(b) Be cer ta in to ope rate on in ternal faul ts . Busbar fa ul tsa re rare , especia llyfor indoor meta l c lad equ ipments , a s they a re l ess a ffec ted by a tmospher ic plu t ion e tc . O nly by very ca re fu l des ign and regu la r com prehens ive rou t ine t es tingthe busbar protect ion can the des i red re l iabi l i ty be achieved.

(c) Re m ain s table dur ing a ll externa l faul ts . Since m an y m ore faul ts occexternal ly to busbars than in ternal ly, busbar protect ion is ca l led upon to s tabi lm any m ore t imes t han t o ope ra t e . The m ax im um va lue o f ex t e rna l f au lt cu r roccurs wh en a fau l t i s ad jacen t to the busba r and ma y be equa l to the rup turcapac i ty o f the swi tchgear. On the o ther hand , the cur ren t a t which the busbpro te c t ion is requ ired to op era te on an in te rna l fau lt m ay on ly be one f i f t ie ththis value . The protect ion in th is ins tance is thus sa id to require a s tabi l i ty facof a t leas t 50.

(d ) Disc rimina te cor rec t ly, tha t i s dec ide on w hich sec tion of the busbars tfau l t has occur red , and then t r ip rap id ly on ly those c i rcu i t b reakers connec tedtha t sec t ion . I t is som et imes necessary to t r ip the rem ote ends o f some c i rcu i ts , th is d epen ds on the loca t ion of the cu r ren t t rans form ers in the sw i tchgear anddescribed in detai l later.

(e ) Be imm une f rom m alopera t ion . S ince busba r p ro te c t ion has to t r ip a lanum ber o f c i rcu it s , i t i s m os t im por ta n t tha t it does no t do so w hen there is no tactual faul t on the busb ar. Th us , bes ides requ ir ing a h igh s tabi l i ty facto r, as d iscusin i t em (c ) the equ ipment and c i rcu i t ry should be as fa r as poss ib le immune to effect of faul ts in wir ing, auxi l iary swi tches and human errors . These addi t ioprecaut ions agains t maloperat ion are descr ibed in deta i l la ter.

This Sec t ion dea l s wi th f rame-ear th busbar p ro tec t ion which i s commonly app l



8usbar protection 87

busba r p ro tec t ion uses the same pr inc ip le as tha t fo r t ransm ission subs ta t ions dew ith in Sect ion 13_5.

1 3 . 4 .2 F r a m e e a rt h s y s t e m s

W ith m eta lc lad swi tchgear, a very s imple fo rm of busba r ea rth - fau l t p ro tec t ion be ach ieved by l igh t ly insu la t ing f rom ear th a l l the meta l f ramework . The f ramw ork is then conn ec ted to e a r th a t on ly one po in t and a cur ren t t rans form er f itover th i s connec t ion . A typ ica l phys ica l a r rangement i s shown in F ig . 13 .4 .2A the m ain cable gland in F igs. 13.4 .2C a nd 13.4 .2D. I t is seen that there are tea r th bars , which run the l eng th o f the swi tchboard . One i s ca l l ed the swi tchg

bon ding bar and in te rconnec ts each cub icle o r f ram ew ork . W here the c ircbreakers a re w i thdraw able , e .g . t ruck ty pe sw i tchgear, the m oving po r t ion malso have a he avy cur ren t ea r th co nne c t ion to the bon ding bar to p rov ide a pa th any ear th fau lt o ccur r ing wi th in the c i rcu i t b reaker i tse lf . The second ear th bknown as the cable-sheath ear th bar, i s l ight ly insula ted f rom the swi tchgear cubor f ram ew ork and prov ides a d i rec t ea r th con nec t ion fo r those cab le shea ths w hrequire to be ear thed; th is wi l l not be used for every c i rcui t because , for exampsingle-end or midpoint ear th ing may be specif ied . Al l cables wi th an ear thed she

ir respect ive of the point of ear th ing, require an insula ted cable gland to ensure tcur ren t f rom an ear th fau l t wi th in the swi tchboard can on ly f low to ea r th v ia t

B u s.b ar ( o n l y o n e p h a s e s h o w n )

Switchgearh{}nding bar

ind ica tion I nsu lat ed~ '- ' ~ " ' - J II g ~ . d f o r

r " 7 ~ ' t ' ~ " 7 " , , -I II aux i l i a ry~ Z..~.Z_..~q [ [ ~ Frame-ear th cable

c u r r en t t r a n s f o r m e r

' " ~ • foun da t ion

Ci rcu i t s ide I ~ / ~p lug / socke t Wi thdrawableand m ain c i rcu i t breaker Cable sheath I 'a r th

cable g land (one on ly sho wn ) ear th bar e lec t rode



88 8usbar protection

S w i t c h g e a r f r a m e w o r k S w i t c h ge a r

Local infeed Itransformer

|

3 4 IF

All cable glands Cable she athinsulated from earth barswitchgear

RI R2

- - - - - I Qo l c ~ 1- . ? -O I C ~ 2

===r" To trip coils0 1 0 - - 3- r - '

. , ~ - - 4

F i g . 1 3 . 4 . 2 B Frame~arth busbar protection without c h e c k

~ R I

swi tchgear bonding bar and f rame ear th - fau l t cur ren t t rans former whi le , fo r cafau lt s , any cur ren t re tu rn ing dow n the shea th f lows d i rec t ly to ea r th and no t v ia

cur ren t t rans former.The bonding bar, ea r th bar, the in te rconnec t ion be tween them and a l l t

bonding connec t ions re fe r red to above mus t be adequa te to ca r ry the ra ted sht im e cu rrent of the swi tchgear for the specif ied du rat io n, w hich is usual ly thseconds .

Re lay R1 fed f rom th i s c . t . would de tec t any fau l t to the meta lwork of tswi tchgear as show n in F ig . 13.4 .2 B.

Re lay R1 could th en ope rate a mu l t iconta ct d .c . repeat re lay R 2 to t r ip a ll c i rcu i t b reakers conn ec ted to the fau l ted bu sbar as show n.

Care has to be taken to ensure tha t the swi tchgear f ramework has no oth




fou nd at ion s used for sw itchgear are adequa te for insulation f rom the general bof the grou nd provided the holding do w n bol ts are clear of any re inforcing roBreaks must be made in any venti lat ing pipes or earthed conduits carryisecondary wir ing, and both main and auxi l iary cables must have insulated glanA typical m ain cable gland with a n insulated barr ier is show n in F ig. 13.4 .2C.

I t i s possible to check this gland insulat ion only by disconnect ing the eacon nec t ions , since bo th s ides of i t are norm al ly conn ected to ear th . F or reasonsafe ty, th i s wo uld requi re an ou tage of the com ple te swi tchboard . A m ore e labogland which can read i ly be tes ted wi thout b reak ing the main ear th connec t ionshown in Fig. 13.4.2D. Here, the insulation is in two parts with a metal is land lbetween. Normal ly, th is layer is jo ined to the main switchgear metalwork by a l

connect ion. The cable gland is tes ted by opening this l ink and the insulatm easured betw een the is land layer and the cable sheath, and also betw een the islayer and the swi tchgear meta lw ork . I f bo th insu la ted sec tions are sound , thencable must be insulated f rom the switchgear metalwork.

Switchgearbondin~ ba

I r f J ( ) l I S w i t c h g e a rm e t a l w o r k

e a r t hc o n n e c t i o n

~ RI

Gland insulation

, l " 1| -l. t | ~ , ~ Wiped joint

- -" - ( . ' ab le sh ea the a r t h b a r

C a b l e l e a d s h e a t h

F i g . 1 3 . 4 . 2 C Simple nsulated cable gland

The e lem entary scheme of busbar pro tec t ion show n in F ig. 13 .4 .2B suffe rs fthe disadvantage tha t i t m ay ope rate for spur ious currents thro ug h the f ram e-eor c . t . , which , for example , could be produced by insu la t ion fau l t s on secondcircuit w ir ing on the switchgear. Also, inadve r tent ope rat ion of re lay R1 by, sa

mechanical b low would cause a l l the switches to t r ip . This l imita t ion can overcom e by f i t t ing an add i t ional re lay R3 w hich is called a check re lay, as sho



90 Busbarprotection

Switchgearbonding bar

l"rame~orkearthconnection

~ R I

Link

Double insulation

island layer

Fig. 13.4.2D

Cable sheathearth bar

Double insulated ca ble gland

I f t h e i n fe e d t o t h e s w i tc h g e a r is f r o m a l o ca l t r a n s f o r m e r t h e n t h is re l a y R 3 b e f e d f r o m a c . t . o n t h e t r a n s f o r m e r n e u t r a l c o n n e c t i o n . O n l y a n e a r t h f a u l t i n

s w i tc h g e a r w i ll c a us e o p e r a t i o n o f b o t h R 3 a n d R 1 .I f t h e r e a r e n o s u i t a b l e t r a n s f o r m e r n e u t r a l s a t t h e s t a t i o n , t h e n a c o r e - b a l ac . t. f i t te d a r o u n d t h e i n c o m i n g c a b l e c o u l d b e u s e d t o s u p p l y R 3 . I f m o r e t h a n c a b le ( o r l o c a l t r a n s f o r m e r ) w e r e f e e d i n g t h e s u b s t a t i o n , c .t .s m u s t b e f i t te d a r oe a c h c a b le ( o r n e u t r a l ) a n d p a r al le l ed t o e n s u r e o p e r a t i o n s h o u l d o n e o f t h e c a( o r t h e t r a n s f o r m e r s ) b e o u t o f s e rv ic e .

(a) Single busbar sw i tchbo ard w i th on e sect ion c i rcui t breaker :So fa r on ly as ing le sec t ion sw i tchbo ard has been con s ide red . I f a sec t ion c ircu i t b rea ker ise l u d e d , t h e n , w h e n a n e a r t h o c c u r s i n o n e s e c ti o n , t h e b u s b a r p r o t e c t i o n c a na r ra n g e d t o o p e n o n l y t h e b u s s e c t i o n a n d f e e d e r a n d t r a n s f o r m e r c i r c u it b r e a ko f t h a t s e c t i o n t o i s o la t e c o m p l e t e l y t h e f a u lt f r o m t h e p o w e r s y s t e m . To d o tt h e f r a m e w o r k o f t h e b u s s e c t i o n c i r c u i t b r e a k e r m u s t b e i n s u l a t e d f r o m a d j a c en t s w i tc h g e ar o n b o t h s id es a s s h o w n i n F i g . 1 3 . 4 . 2 F. T h e t h r e e f r a m e ws e c t i o n s a r e t h e n s e p a r a t e l y e a r t h e d , w i t h a r e l a y f e d f r o m a c . t . o n e a c h e a

c o n n e c t i o n . T h e c h e c k r e l a y R 4 i s s h o w n i n t h i s c a s e f e d f r o m a c . t . o n t h e l ot r a n s f o r m e r n e u t r a ls . T h e d . c . t ri p p i n g c i rc u i ts a r e s o a r ra n g e d t h a t t h e i n a d v e r



Busbar protec tion 91

R 3

l I " l ]

i i -

i |

i~ I I ll. .° ,

•

1 2 4 R I

L o c a l i n f e e dt r a n s f o r m e r

' 11_o

IIIIIII

J

R 1 R 2

IR4 I

/ o ..l_ c~ - - - - ? . ~ 1

" - " ? d . ~ 2, ' - ' ~ I O ' 3

~ 4

@

To t r i p c o i l s

Fig . 13 .4 .2E Frame-earth busbar pro tec tion with neutral check

F r o m t h e s e c i r c u it s i t w i ll b e s e en t h a t t h e b u s s e c t io n c i rc u i t b r e a k e r a lwt r ips on a busbar f au l t , and tha t fo r the fau l t F a in the bus sec t ion i tse l f, evc i r c u i t b r e a k e r c o n n e c t e d t o t h e b u s b a r s m u s t b e t r i p p e d . T h i s i s n e c e s s a r y b e c at h e r e la y R 2 w i ll o p e r a t e f o r f a u lt F a o n t h e p r i m a r y c o n n e c t i o n o r c o n t a c t sthe s ide c on ne c ted to the L .H. busb ars as we l l a s fo r a f au l t in a s imi la r po s i t iont h e r i g h t -h a n d s id e , t h a t is i t c a n n o t d i s c r im i n a te b e t w e e n t h e t w o , a n d s o w h o l e s w i t c h b o a r d i s m a d e d e a d .

(b) D oub le busbars: I n t h e c a s e o f s w i t c h b o a r d s f i tt e d w i t h d o u b l e b u s b a rb e c a u s e o f t h e g r e a t d i ff i c u l ty o f in s u l a ti n g b e t w e e n m a i n a n d r e s er v e b u s

c a sin g s , i t is n o t p r a c t ic a l t o d i s c r im i n a t e b e t w e e n f a u lt s o n o n e b a r a n d t h e o t hThere fo re i t i s necessa ry to employ the same p r inc ip le a s fo r s ing le busbars .



9 2 B u s b a r r o t ec t io n

Insula t ion barr ie rs

B u s s e c t i o n

R2 | R3

+ , - - ~ . . . . . . . . .

- ~ + E ++ +~_ ~+++.To t r i p c i r c u i t s To t r i p c i r c u i t so f 1 , 2 , 3 and o f 1 , 2 , 3 , 4 , S , 6b u s s e c t i o n a n d b u s s e c t i o n

I

To t r i p c i r cu i - t so f 4 , 5 , 6 a n db u s s e c t i o n



Bus section


. - # . , ,

|

Insulation barriers

))

Fig. 13.4.2G Frame-ear th protect io n for double busbar switchbo ard

selected to the lef t -hand s ide of the main bar, the bus sect ion swi tch, and feeders se lected to the reserve bar. Auxi l iary swi tches on the busbar se lectors wobe connected to achieve th is .

(c) Performanceo f frame-earth protection:In F ig . 13 .4 .2E the f ram e ear th . fau l tre lay R1 and the neu t ra l (o r core -ba lance) check re lay R3 a re usua lly bo th a t t racarmature types and the i r opera t ing t imes toge ther wi th R2 do no t usua l ly exce80 ms . I t i s to be no ted tha t the check re lay wi l l opera te fo r ea r th fau l t s bo th

the busb ars an d a lso fo r a ll exte rnal faul ts near to the s ta t ion. I f the ins ta lla t ioof suff ic ien t impor tance , a fu l ly d i sc r imina t ive check scheme can be ach ievedf i tt ing e xtra c . t. s to every feed er. T his ar rang em ent i s descr ibed in deta i l in Sec t13 .5 and w ould respond on ly to fau l ts a t the su bs ta t ion .

13 .5 U ni t sys tem s of busba r p ro tec t ion fo r t ransm ission subs ta t ions

1 3 . 5 .1 G e n e r a l c o n s i d e r a t i o n s



94 Busbarprotection

busbar p ro tec t ion employed a t the h igher swi tchgear vo l tages inc lud ing tha tthe supergr id sys tem , wh ere the un i t f ea ture is ach ieved by com par ing the cur rin each c ircu it conne c ted to the p ro tec ted busbar.

A num ber o f sy s tem s were developed by p ro t ec t i on r el ay man ufac tu re r s w

the ins ta l la t ion of u n i t bu sbar p ro te c t ion was found to be necessary. Mos t o f them ploye d a b ias fea ture to ach ieve th e requ ired s tab il ity and i t was no t unc om mto use a cur ren t sum m at ion technique to enab le ea r th fau lt s and phase fau l tsbe detected by a s ingle phase re lay. However, as conf idence in the high- impedacircula t ing current d i fferent ia l protect ion pr inciple increased, i t was used in the to the exclus ion of o ther sys tems. Al though some older substa t ions are sequ ipped w i th one or o the r o f the ea r li er des igns , they a re no longer f i tt ed on nswitchgear and are therefore not descr ibed here; readers requir ing informatthere on a re re fe rred to the f ir s t ed i t ion of th is boo k an d to the b ib l iography.

High- impedance c i rcula t ing current d i fferent ia l protect ion was in t roduced .Chapter 4 de al ing w i th protect ive t ransform ers . T he pr inciple is o f suff ic iim por tan ce , howe ver, to wa r ran t a fu r ther exp lan a t ion here , beg inn ing f rom basic pr inciples of the c i rcula t ing current sys tem.

13.5.2 C u r r e n t b a l a n c e u s i n g c i r c u l a ti n g c u r r e n tprinciple

A very s imple fo rm of un i t p ro tec t ion of a c i rcu i t can be ach ieved by co m parthe curren ts enter ing i t w i th those leaving, the c i rcui t being he al thy i f the y equa l , and fau l ty if they d i ffe r by m ore than a ce r ta in am ou nt .

In ord er to und ers tan d the pr inciple m ore eas i ly, the appl icat ion to a shs ingle phase feeder carrying load current wi l l f i rs t be considered, and Fig . 13.5shows the two equa l ra t io c . t . s so connec ted to each o ther tha t the i r secondcur ren ts f low in opp os i te d i rec t ions in the re lay R con nec ted ac ross them .

I f the c . t. s were per fec t , the i r seco ndary c ur ren t s would be iden t ica l in bom agni tude and phase ang le , bu t as no tw o c . t. s pe r form a bso lu te ly a like , then the tw o a lm os t equa l c .t . seconda ry cur ren t s caused by the load cur ren t beIa and It , .

E n d A E n d B | To l o a dF e e d e r c u r r e n t ~ m | o r_ _f au lt

l a _ b

R e l a y c u r r e n t is p h a s o r d i f f e r e n c e (1 a - l b ) A m p e r e s

F1




The ins tan ten eou s va lues o f c . t . cu r ren t s a re show n dur ing the pe r iod o f t iw hen the ha l f -cyc le o f p r im ary cur re n t is f low ing f rom A to B . As these flowopp os i t e d i rec t ions th ro ug h the re lay, the resu l t ing re lay cur ren t w ould be thd i ffe rence (Ia - Ib ) .

I f a f a u lt e x t e rna l t o f e ede r AB w ere t o occu r a t F~ bey o n d en d B o f the f e ethe c . t . secondary cur ren t sla and Ib would f low exac t ly as fo r the load cur ren tcons id er e d ab o ve , bu t t hey wo u ld b e i nc r ea s ed in v a lue m a ny t im es . F o r th i s fa t F l , r e l ay R m us t no t ope ra t e an d so t he c . t.d i f f e r e n c e cu r r en t ( Ia - I b) f lowingth rough t h e r e l ay mu s t no t exceed t he v a lue o f t he cu r r e n t ne ed ed t o o pe ra t e re lay. Th i s con d i t ion m us t r em ain sa ti s fi ed up to the la rges t va lues o f th ro ucu r r en t t ha t c a n f low in to an y f au l t j u s t beyo n d e nd A o r en d B .

End A End B

t F2

t1'a ~ la / ~ R lb

Fig. 13.5.2B

Relay cu rrent is phasor sum (1 a + Ib) A m peres

Circulat ing current protec t ion internal faul t )

F ig . 13 .5 .2 B shows a d i s t r i bu t i o n o f i n s t an t aneou s va lue s o f c u r r en t whe nfau l t occurs ins ide the p ro tec ted c i rcu i t a t F2 . Here i t i s seen tha t the re lay nreceives the s u m o f t he c . t . s e conda r y cu r r e n t s an d t h e r e l ay mu s t be d e s igne d ope ra te rap id ly fo r th i s va lue o f cur ren t . I f no in feed to th e fau l t ex i s t ed a t enthen t he r e l a y cu r r en t wo u ld be on l yI a and th i s mus t a l so pos i t ive ly opera te there lay do w n to the lowes t va lue o f fau l t cu r ren t l ike ly to ex i s t on the p r imsys t em , t ha t i s du r ing m in im um p l an t co nd i t i on s .

1 3 . 5 . 3 C o n n e c t i o n s f o r c i rc u l a ti n g c u r r e n t b u s b a r p r o t e c t i o n

F or busba r p r o t ec t i on t he app l i c a ti o n o f t he c i rcu l at ing cu r r e n t p r inc i p le is sh oin F ig . 13 .5 .3A fo r a busbar wi th four c i rcu i t s connec ted to i t . The c . t . s fo r tp ro tec t io n a re sh ow n f i t t ed on the feeder s ide o f the c i rcu i t b reakers so tha t p ro tec ted zone inc ludes a ll c i rcu i t b reakers as we l l as the busbars .

The s ingle ph ase sys tem is s t il l con s idered for s im pl ic i ty, and the c . t . s a re conec ted in pa ra ll e l w i th the re lay R . F or the fau l t a t F1 on feeder Da n assum ed pr imf au lt cu r r en t d i s t r i bu t i on is show n , f r om w h i ch i t w il l b e s e en t h a t t he sum m a tof the c t secondary cur ren t s resu l t s in ze ro cur ren t in the re lay R and there fo





S e c t i o n 1

Bus s e c t i o n

8usbar protection 97

S e c t i o n 2

Z o n e 1 I / \ / ~ I Z o n e 2

s y s t e m

F i g . 1 3 . 5 . 3 C

C h e c k s y s t e m I 1

Circulating curre nt duplicate l ineo f d e f en c e busbar protect ion

check sys tem re lay R3 to t r ip a ll ci rcu it b reakers in zone 1 , toge ther w i th the

sec t ion c i rcu it b reak er.To app rec ia t e t he pu rpose o f emp loy ing a check sys tem cons ide r now

cond i t i on o f a h ea l t hy sy s t em ca r ry ing no rma l l oad cu r r en t s. I f one o f t he l eto , say, c . t . ( a ) were to become open-c i rcu i ted , then re lay RI would have a cur rf lowing in i t due to the l ack o f the c . t. cu r ren t (a ) ba lanc ing o u t those o f c . t. sand (c ) . Th is cur ren t in R1 m ay no t be la rge enou gh to ope ra te i t, bu t whenex te rna l fau l t occurs the cur ren t cou ld w ell be la rge enou gh to cause R1 to pup . Tr ipp ing of zone 1 w ou ld , how ever, s ti ll be p reven ted b y the fac t tha t check sys tem re lay R3 wi l l no t have opera ted .

The method of us ing separa te check and d i sc r imina t ing sys tems a l so covaga ins t the ina dver ten t ope ra t ion o f any one re lay due , fo r exam ple , to v ib ra to r o p e r a ti o n b y h a n d .

(b) D ouble busbar arrangement:F ig . 13_5.3D shows the a r rangem ent requ i red tcover d up l ica te busb ars , a s ingle phase sys tem s ti ll be ing considered a t th is s tag

he lp in more eas ily dem ons t ra t ing the p r incip les invo lved .Here , four d i sc r imina t ing re lays a re requ i red , one fo r each main and rese

sec t ion o f busbars . I f f eeder A on the l e f t -hand s ide o f the d raw ing were se lec tethe rese rve busbar, th en the aux i l i a ry swi tch opera ted by the se lec to r w ould c lso se lec ting i ts d iscr imina t ing c . t . to the reserve zone 1 d iscr im inat ing sys temfeeder B were se lec ted to the main bu sbar, then the rese rve zone 1 d i sc r imina tsys t em wou ld on ly have two c . t . s connec t ed t o i t , n ame ly t hose o f f eede r A athose o f the bus couple r (b ) . Main zone 2 w ould o n ly have two c . t .s se lec tedi t, nam ely feeder B and the c . t. ( c ) o f the bus sec t ion .

In o rde r to mak e one o f the two f ines o f de fence imm une f rom defec t s wh



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D i s c r i m i n a t i n g relays

F i g . 1 3 . 5 . 3 D C i r c u l a t i n g c u r r e n t p r o t e c t i o n f o r d o u b l e b u s b a r s a .c . c i r c u i t s )

f or e c o v e r s th e w h o l e s u b s t a t i o n a n d d o e s n o t d i sc r i m i n a t e b e t w e e n b u s b as e c t io n s . T h u s t h e c h e c k s y s t e m h a s o n l y t h e fe e d e r c .t .s p e r m a n e n t l y c o n n e c t eto i t ; ad d i t i ona l f eede r s wo u ld have t he i r c . t .s add ed in pa ra ll e l w i th t he se tw o .

T h e c . t. s i n t h e b u s s e c t i o n a n d b u s c o u p l e r a re p e r m a n e n t l y a ss o c i a te d w i t h t hd i s c r i m i n a t i n g z o n e s o n t h e o p p o s i t e s i d e o f t h e i r b r e a k e r s t o i n c l u d e t h e b r e a k ein b o t h z o n e s o n e i th e r s id e o f it . T h i s m e a n s t h a t f a u lt s o n b u s c o u p l e r s a ns e c t i o n s c a u s e t h e l o s s o f t w o s e c t i o n s o f b u s b a r.

I n t h e d o u b l e b u s b a r a r r a n g e m e n t s h o w n t h e r e ar e d i s c o n n e c t o r s 5 a n d 6 f i tt e din t he r e se rve busba r, bu t no c i r cu i t b r eake r. The r e fo re , i f t he fo rm er ar e c lo se dt h e ir a s s o c i a t e d a u x i li a r y s w i t c h e s ( 5 ) a n d ( 6 ) j o i n t h e b u s w i r e s o f z o n e s 1 a n d t o g e t h e r , a n d u n d e r t h e s e c i r c u m s t a n c e s a fa u l t o n t h e r e se r ve b u s b a r w o u l d r e q u ira ll c ir c u i ts s e l e c te d o n i t t o t r i p . T h i s w o u l d r e s u lt f r o m t h e o p e r a t i o n o f R 1 , R

and C .T h e d . c . t r i p p i n g c i r c u it s r e q u i re d f o r t h e a . c . s c h e m e i n F ig . 1 3 . 5 . 3 D a re s h o w



B u s b a r p r o t e c t io n 9 9

c i rcu i ts to ensure tha t on ly the c i rcu i ts se lec ted to a par t icu lar zon e a re t r ippedS i n c e b o t h t h e b u s c o u p l e r a n d b u s s e c t i o n m u s t b e t r i p p e d f r o m t w o z o n e s t h e

need two t r ipp ing r e l ays .T h e c h e c k r e l a y c o n t a c t i s c o n n e c t e d i n t h e c o m m o n n e g a t i v e l e a d o f t h

indiv idua l t r ipp ing re lays , whi l s t the d iscr imina t ing re lay contac ts a re in the pos i t ivl ead . Th i s ensu res tha t n o t r ipp ing wou ld occur i f a pos i t i ve supp ly werinadv e r t en t ly app l i ed to the wi r ing o f any t r ip r e l ay, a s , fo r exa m ple , a t po in t P ofeede r A in F ig . 13 .5 3 E . Th i s cou ld hap pen b ecause the l eads to the d i scon nec taux i li a ry swi t ches m ay b e o f con s ide rab le l eng th and p ass th rou gh seve ra l j un c t iob o x e s .

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s w i t c h e s

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T o t r ip To t r i p

f e e d e r A b u s c o u p l e r

I

C h e c k Ire la~/ L

C

T o t r i pf e e d e r B

I A 3

A I , A 2 , A 3 , A 4 - Z o n ed e f e c t i v e a l ar m r e l a y s

T - I n d i v i d u a l c i r c u i t t r i p p i n g r e l a y s

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100 Bu sba r rotection

R]

~, (z

2

To trip To trip To trip To tripfeeder bus bus feeder

A couple r section B

Fig. 13.5 .3F Circulat ing current protect ion for double busbars d .c . c i rcu i ts) / mu lt icon tacttripping relays

The individual c i rcui t - t r ipping re lays arrangement shown in Fig . 13.5 .3E has advantage that regular proving tes ts of c i rcui t t r ipping can readi ly include provthe t r ipp ing f rom the busbar p ro tec t ion re lay T. Wi th the a l te rna t ive a r rangemeof mul t icon tac t t r ipp ing re lays shown in F ig . 13 .5 .3 .F, ca re fu l cons idera t ion hto be given before operat ing such re lays , in order to ensure that dur ing ' t r ip tes t ion ly the desired c i rcui t i s t r ipped .

(c) Protection fo r ea rth faults:I f only ear th-faul t protect ion of the busbars isrequired , then the c . t. s can be con nected as show n in Fig . 13.5 .3G where i t wi l l

seen that only one relay element is required since the c. t .s of al l phases aparalleled together.

If the outgoing circuits are cables (ei ther three core or three single cores) , al ternative arrangement of c. t .s is to f i t a single core-balance c. t . on each circuas show n in F ig . 13 .5 .3G.

With th i s fo rm of ea r th fau l t p ro tec t ion , as wi th f rame ear th - fau l t p ro tec t idescr ibed ear lier, the check feature m ay be by a re lay fed f ro m the sum of tcurrents in the neutra ls of the local t ransform ers con nected to the busbars . Tdisadvantage of th is arrangem ent how ever, is tha t th is relay wi l l op erate for exterear th faul ts. Also, the re lay wil l no t op erate i f there is no n eutra l cu rrent due to tlocal t ransformers being out-of-service . The most re l iable check arrangement current balance as shown in Fig . 13.5 .3C.

(d) Protection fo r phase and earth faults:Fig. 13.5 .3H shows the pr inciple ofsuch a curren t balance system w hich is, in effect , three separate system s, ea

cover ing one phase an d joined toge ther a t th e s tar po int of each set of c .t . s to sin mul t icores . For s impl ic i ty, par t (a ) shows on ly two pr imary c i rcu i t s connec t



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Singlepole relay

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Busbar pro tection 103

and yel low re lays wi l l opera te . In the event o f a b lue pha se- to-ear th faul t , as sha t F in par t (b ) o f F ig . 13 .5 .3H then on ly the b lue e lemen t wi ll opera te .

13.5.4 T h e i n f l u e n c e o f c . t . p e r f o r m a n c e o n t h r o u g h . f a u l ts tabi l i ty

The c i rcula ting curren t pr inciple ca n be exten ded for appl icat ion to var ious ss ta t ion con f igurat ions w i th m ul t ip le sect ions o f s ingle or dou ble busba r swi tchgbu t befo re d oing th is a pract ica l l imi ta t ion to the sys tem descr ibed so far m uste l im ina ted . T here wo uld be no prob lem w i th such a sys tem i f the ou tpu t o f cur ren t t rans formers was a fa i th fu l reproduc t ion of the p r imary cur ren t a t t im es . H ow ever, one m ust a l low for the fact that faul t curren t m ay , as expla i

in Ch apte r 4 , con ta in a t rans ien t d .c . com po nen t cur ren t w hich can cause sa tu ra tof the cur ren t t rans form er cores and d i s to r t ion of the seconda ry cur ren t , e ffewhich can only be avoided by increas ing the core s ize substant ia l ly. Since the cwhich are connected together to form a c i rcula t ing current zone wi l l each generbe ca r ry ing d i ffe ren t m agni tudes o f cur ren t and hence d i ffe ren t m agni tudes o fco m po ne nt , the degree o f d i s to r t ion wi ll va ry f rom one c . t. to ano ther, w i th resu l t tha t ( fo r an ex te rna l fau l t ) the secondary cur ren t wi l l no t sum to ze ro a

there wi ll be an unbalanc e (spil l) curre nt in the re lay. I f the m agn i tude of the scurrent exceeds the re lay se t t ing for suff ic ient t ime, operat ion of the re lay wocc ur an d the pro tect io n wi ll be unstab le . Ma ny ear ly appl icat ions of c i rcula tcu rrent pro tec t ion used re lays w i th a re la t ively shor t ope rat ing t ime and thoccas iona lly opera ted incor rec t ly fo r an ex te rna l fau l t. The re lays were f reque nof the a t t rac ted a rmature type of fa i r ly low impedance wi th a nomina l opera tt im e of ab ou t 100 m s . At th is t ime it was no t c lea rly und ers too d w hy the remisbehaved in th i s way. I t was found , however, tha t i f a re lay wi th a much lonopera t ing t ime was used , fo r exam ple an indu c t ion d isc re lay i t d id no t -opera teexternal faul ts . I t i s now known that th is was because i t provided t ime for thecomponent to decay and for s teady s ta te condi t ions to become es tab l i sheU nfo r tuna te ly, the long fau l t c lea rance t im es resu lt ing f rom the use o f such re lwere fo und to be unaccep tab le and , fo r a t ime , b iased sys tem s were cons ideredbe the o n ly so lu t ion .

13.5.5 Bas ic p r inc ip l e s o f h igh - im ped anc e c ir cu l a t ing cu r r en t busba r p r o t ec t i on :s t ab i l i t y

The requ i rem ents o f h igh- im pedance c i rcu la ting cur ren t p ro tec t ion a re , in p r inc ipthe same w hethe r i t is app lied to a two-ended zone , such as a shor t in te rconn ec tor to the mul t i -ended zones o f busbar p ro tec t ion . The no tab le d i ffe rences occurt h e s tab i li ty leve l and the increased com plex i ty o f busbar p ro tec t ion requ i red preve nt the un w an ted t r ipping o f the several c ircui t breakers in one zon e, e .g . d

to the inadver tent operat ion of a s ingle re lay.The d i sadvantages o f c i rcu la t ing cur ren t p ro tec t ion us ing low impedance re la



104 Bu sb ar rotection

s a tu r a t i o n . I t was found t ha t t h is cou l d b e ov e rco m e by t he c o r r ec t cho i ce o f a nd r e la y c i rcu it com pon en t s , in wh a t ha s be com e k now n a s h i gh . i mpedac i rcu la t ing cur ren t p ro tec t ion .

Cons ider a sys tem to p ro tec t a zone have on ly two c i rcu i t s connec ted to

The mos t onerous cond i t ions fo r s t ab i l i ty a r i se when :

( i ) M a x i m u mfau l t cur ren t en te r s the zone th ro ug h one c i rcu it and l eaves th rou gthe o th e r to a f au l t jus t ou t s ide the zo ne , as show n in F ig . 13 .5 .5A.

H i g h i m p e d a n c e r e l a y Stab i l is ing res i s to r

A L l ~ / L I B

C.T. secondary winding ItS" ~ _ .. .L . I L~ C .T. turnsresistance = Rct ~ . ~ _ . _ i ~ ~ ratio T

I F i1 ,

Fig. 13.5 .5A H ig h mpedance scheme

( ii ) One c .t . , s ay tha t on c i rcu i t B , sa tu ra tes com ple te ly due to ass ym m eof the fau l t cu r re n t , whi le the o the r c . t . does n o t en te r sa tu ra t ion a t a l l, am a in t a in s it s o u tpu t a s a f a it h fu l r ep r od uc t i on o f t he p r im a r y cu r r en t .

In Chap te r 4 the equ iva len t c i rcu i t o f a c . t . was desc r ibed and th i s can be used

i l lus t ra te the cond i t ions ex i s t ing in the sys tem be ing cons ide red , a t which po inshou ld be no t ed t ha t t he c a l cu l at ed pe r fo rma nc e o f the p ro t ec t i on c an o n ly ob ta in ed w i th the des i red re l i ab i li ty i f the c . t. s a re o f the low reac tance type de f ined in BS 393 8 . Th i s be ing the case , the p r im ary and sec ond ary l eakage ret ance s c an be omi t t ed f rom the equ iva l en t c i r cu i t , wh i ch i s s hown in F i g .13 .5 .5Becaus e th e c . t. on c i rcu i t B is s a t u r a te d by t he d . c . c om p o nen t o f t he p r imacur ren t i ts m agne t i s ing b ran ch m ay be assum ed , pess im is t ica l ly, to have zeim ped ance . H ence i t i s va lid to inse r t the con nec t ion PQ. In the absence o f

socond ary e an . f . , thec . t . on feeder B wi ll behave as a res is tor having a value equ ato the res i s t ance o f the secondary wind ing .



|11 ~ I

i

' A RCT

5gZ

R L A

2 ~

RLB

3 ~

wit ]

8 u s b a r p r o t e c t i o n 1 0 5

RCT

i iII

I I

/~ I Zero seconda rye.m.f , due to

I saturat ion

N o n - s a t u r a t e d c . t . 1Q iagnetising and Satu rated c.t .core Joss branch

F i g . 1 3 . 5 . 5 B Equivalent circuit of high-impedance scheme

making the fo l lowing assumpt ions :

(a ) F or the c . t . on c i rcu i t A , the imped ance o f the magn e t is ing b ran ch and res i s tance o f the core loss b ranch a re h igh re la t ive to the o ther pa ths th rowh ich the second ary cur ren t can f low ( i. e. the c . t. is pe r fo rm ing a lmper fec t ly ) .

(b) Since , in genera l , al l c . t. s ins ta l led for the busbar p rot ect io n o f a g iven ss ta t ion wi l l be iden t ica l , the i r s eco nd ary w inding w i ll have the same res is tashown in the equivalent c i rcui t asRct.

The second ary cur ren t f rom the c . t . on c i rcu it A wi ll d iv ide be tw een the rc i rcui t and the seco nd ary w inding of the c . t . on c ircui t B. I f , as we shall sub sequ ef ind to be the case , the re lay c i rcui t res is tance is h igh compared wi th the lcom pr i s ing the sa tu ra ted c . t. and i ts conn ec t ions , the la t t e r can be cons ideredcar ry the whole o f the cur ren t . The resu l tan t vo l tage d rop appears ac ross XY therefore across the re lay c i rcui t and th is must be insuff ic ient to opera te the re l

Perfect c.t.

RCT ~,_ _

4.5FZ

24A

p

S R

R

5 . 5 ~

Sa tu r a t ed c . t .




the p ro tec t ion is to rem ain s tab le . The l ead burd ens wi l l near ly a lways d iffe r duvar ia tions in the d i s tance f rom the re lay to each th ree -phase o f c . t. s and a re shoas R L A and R L B .

A similar ca lcula t ion m ust be m ade assuming th at the c . t . on c i rcuit A sa tura

and tha t on B behaves as a perfect c . t . Whichever vol tage is the h igher is referreas the s tabi l i ty vol tage of th e sys tem . ( In th is ins tance , fo r the values o f res is tagiven in Fig . 13.5 .5C, the former wi l l be h igher) .

Th e re lay c i rcui t is show n as the re lay ope ra t ing coi l, R, and a ser ies res is tor, I f the re lay is of the current -opera ted type wi th adjus table se t t ings , i t s res is tam us t be ascer ta ined fo r the se t ting to be em ploy ed . The se lec tion o f the cur rse t t ing is d iscussed in Sect ion 13.5 .6 . Alm ost invar iably, the pr od uc t o f se t tcurr ent and the corresp ond ing res is tance is less tha n the s tab i l i ty vol tage referto above and the va lue o f the res i s to r SR i s ca lcu la ted so tha t the p roduc t o f re lay se t t ing current and the sum of re lay res is tance and s tabi l i s ing res is tor (SRequal to , or s l ight ly greater than, the s tabi l i ty vol tageV . I t i s apparen t f rom th i show the s tabi l is ing res is tor becam e so-called .

Summar i s ing the s tab i l i ty ca lcu la t ion , the s tab i l i ty vo l tage mus t be de te rmif rom:

V = 1F T ( RL + Ro t) 13 .5 .1

and , fo r relays w i th cu r ren t ca l ib ra t ion ,

I S (R R + R S R ) " ) V s 13.5.2

w hi le , for re lays ca l ibra ted in voltage"

set t ing ) V 13.5.3

where :

fau l t cur ren t in p r imary amps

se t ting o f re lay in amp s

Rct = secon dary w ind ing resi s tance o f c . t. a t 75°C

R L = R L A , R L B= res istance of leads be tw een re lay and the c . t . oc i rcu i ts A or B ( to ta l fo r the loo p)

RR ~-" res istance of re lay coi l



Busbar prote ction 107

T = tu rns r a t io o f c . t.

Vs = s tabi l i ty vol tage

Le t us t ake , a s an example , the c i r cu i t o f F ig . 13 .5 .5B wi th r e s i s t ances a s showa c . t . ra t io of 1•2 00 0 t u r n s a n d a m a x i m u m f a u l t c u r r e n t o f 5 0 k A .

Su bs t i tu t ing in eqn . 13 .5 .1 to ob ta in the s t ab i l i ty vo l t age , f i r st ly a s sumisa tu ra t ion o f the c . t . o f c i r cu i t A :

5 0 0 0 0Vs = ( 2 + 2 - 5 )

2 0 0 0

= 112-5 V

and th en fo r sa tu ra t ion o f the c . t . o f c i rcu i t B"

5 0 0 0 0Vs = (3 + 2-5)

2 0 0 0

= 137.5 V

I f the r e l ay to be used is o f a typ e ca l ib ra ted in cu r re n t and hav ing a bu rd ens e t ti n g o f 0 . 5 V A w e c o u l d s e l e ct o n e h a v i n g a s e t ti n g r a n g e o f 2 0 t o 8 0 % o f and use the 0 .5 A t ap , a t wh ich the impedance i s :

VA 0 . 5

I s ~ 0.52 - 2 1 2

Unless the r e l ay has a low pow er f ac to r, t h i s m ay be t ak en o n the co i l r e s i st anR R ; o t h e r w i s e t h e r e sis tiv e c o m p o n e n t , R n s h o u l d b e d e t e r m i n e d .Rear rang ing eqn . 13 .5 .3 to g iveR S R ,

zsR S R ~ - - - _ R g

I s

137.5. . . . . . 2

0-5

273 ~2

A l te rna t ive ly, a se t t ing range o f 10-40% o f 0 -5 A cou ld have been chose n and ca lcu la t ion based on the use o f the 2 0% (0-1 A) t ap , in w hich case the s tab i li s




I f a re lay ca l ibra ted in vol tage w ere used in th is app l ica t ion , i t w ould m erelnecessary to se lec t a se t t ing equal to , or greater than, 137.5 V. One des ign of sure lay has se t t ings in 25 V s teps f ro m 25-17 5 V, and in th is case the 150 V se twould be used .

1 3 . 5 . 6 Bas ic p r inc ip les o f h igh- impedance c i rcu la t ing cur ren t busbar p ro tec t iope ra t i on

One o f the m ain requ i rem ents to ensure rel iab le and fast opera t ion o f th is typpro tec t ion on in te rna l fau lt s is tha t a l l cur ren t t r ans form ers m us t have a knee-pvol tage w hich is a t leas t twice the se t t ing vol tage o f the re lay c i rcui t . F or

exam ple jus t cons idered , the re fo re , the c . t. s shou ld have a knee-p o in t vo l tage oleast 275 V (or 3 00 V in the case w here the re lay is se t to 150 V) .

The maximum in te rna l fau l t cur ren t wi l l usua l ly be the same as tha t fo r s t ab i l i ty condi t ion and i f we look a t the cur ren t f lowing in the secondary c i rc(as show n in F ig . 13 .5 .6A ) i t is seen tha t the c . t. a t t em pts to d r ive 25 A th roug hrelay c i rcui t . S ince , in one solut ion to the previous example , the res is tance of re lay c i rcui t was 1375 I2 (and could be higher for a vol tage ca l ibra ted re laywould appear tha t the vo l tage ac ross the re lay c i rcu i t cou ld be 34-4 kV or mDue to the l imi ta t ion impo sed by i t s core en te r ing sa tu ra t ion o n each ha l f .cyc lepract ica l c . t . could produce a vol tage of th is value but i t could produce spikesvery h igh vo l tage a roun d the ins tan t s o f ze ro f lux as exp la ined in the cha p te rc.t .s and v.t .s.

A fo rmula due to Mathews ( see Bib l iography) g ives a reasonab le approx imat ionthe peak vo l tage p roduced under these condi t ions :

Vpk = 2 ~ 13 .5 .4

where :

V.k is the peak va lue o f the d i s to r ted vo l tage waveform

v: is the r.m.s , va lue o f the vol tage th at w ould app ear i f the c .d id no t sa tu ra te

vk i s the knee-point vol tage of the c . t .

F or e xam ple , Vf is the vo ltage o f 3 4 4 kV a l ready re fe r red to and Vk is say 275jus t mee t ing the requ i rement o f twice the se t t ing vo l tage .

H e n c e ,



8usbar prote ction 109

To pro te c t the c . t. s , the secon dary w i ring and the re lay f rom dam age due to suchigh vol tage, a nonl inea r res is tor i s con ne cted across points X Y as sho w n in 13 .5 .6A i f the peak vo ltage w ould exceed 3 kV . This res i sto r is o f the typ e w hreduces in value as the voltage across i t increases and is selected with characteris

w hich l im i t the vo l tage to be tw een 1 kV and 2 kV. The nonl inear res is to rs used th is purpose conta in a largo propor t ion of s i l icon carbide or s imi lar mater ia l mafac tured as d iscs. The proper t i es o f the m ate r ia l , coupled w i th the d imen s ions , the required character is t ics .

[ ]50 kA j k0

A f

ii

I t , s ,N o n - l i n e a rre s is tor 3 2 $ £~

~ 2 5 AR 50f~

, , , , I ,

l . 18

F i g . 1 3 . 5 . 6 A Connect ion of vol tagedimitingd e v i c e

The vol tage is appl ied be tw een the tw o c ircular faces of the disc by co nne ct i

wh ich ensure c on tac t w i th the m ate r ia l over the w hole o f each a rea .Th e res is tor curren t and appl ied vol tage are re la ted by the fol low ing express io

V = klO 13. 5.5

where/3 var ies f rom 0.2 to 0 .25 and k is control led by the dimensions .For nonl inear res is tors used for h igh- impedance protect ion, k usual ly has a va

be tw een 200 and 1000 . Because the m anufac tur ing process is such tha t the re large to lerance on bo th g and k , it is an advantage to specify the l im i ts of voltagthe maximum cur ren t and a l so the maximum va lue of /3 .

The re la t ionship in eqn. 13.5 .5 only appl ies to ins tantaneous values of curreand vol tage and therefo re i f the curre nt is s inusoidal the vol tage w aveform w ill be . F or th is and a nu m ber o f o the r reasons ( thermal ra t ing , fo r exam ple) ca lcu la tifor the se lect ion of sui table nonl inear res is tors for busbar protect ion should sup po r ted by the resul ts o f t es ts on a typ ica l ins ta l la t ion .

A necessary par t o f the des ign of h igh- impedance c i rcu la t ing cur ren t p ro tec t ii s the calcula t ion of the faul t se t t ing, somet imes referred to as pr imary operat



110 8usbar pro tec tion

An in te rna l f au l t , who se m agni tude is jus t su ff i c ien t to cause op era t ion o f re lay, wi l l resul t in a vol tage be tw een X and Y (Fig . 13 .5 .6A ) equal to the se ttvo l t age o f the re lay c i rcu i t . The cur ren t s which , a t th i s vo l t age , pass th rough eco m po ne n t and a l l the c . t. magn e t is ing and core loss b ranc hes w i ll , w hen add

toge ther and re fe r red th rough the idea l c . t . , equa l the fau l t se t t ing in p r imary amThe re levan t c om po nen ts and par t s o f the c . t. equ iva len t ci rcu i ts are show n in F13.5 .6B and i t wi l l be seen tha t an ad di t ion al res is tor,R a i l , has been added inpara l le l wi th the re lay c i rcui t . This shunt se t t ing res is tor i s f i t ted when i t i s requito ra i se the fau l t se t t ing above tha t which would o the rwise ob ta in . Reasons doing th is are g iven in Sect ion 13.6 .10.

F i g . 1 3 . 5 .6 B

Tu r n s trati~ T

[ ] . . .~ ], , ¢ -

RLA RLB

2 _ R s

, , I L I

Equivalent circuit for in ternal faul t condition

The pr imary faul t se t t ing i s ca lcula ted f rom"

where"& = ( h i e + rlR + [SR "t" ]NLR+ IsH ) + T 1 3 . 5 . 6

n = nu m be r of c . t. s in para lle l

le = exci t ing cur ren t o f each c . t . a t re lay c i rcui t se t t ing vol tage

IR = re lay cur rent a t se t t ing

INLR = curr ent in non l inear res is tor a t re lay c i rcuit se t ting vol tage

1St = shu nt se t t ing res is tor curr ent a t re lay c i rcui t se t t ing vol tage




r = nu m be r o f r e l ays in pa ra l le l

T = c u r r e n t t r a n s f o r m e r t u r n s r a t io .

I f the re lays have a r ange o f cu r ren t se t t ings , i t i s f r equen t ly poss ib le to avo id need fo r a shun t se t t ing res i s to r by se lec t ing the re lay se t t ing to g ive the requ ifau l t se t t ing . W here , how ever, the re is a va r iab le nu m be r o f r e l ays in se rv ice depein g o n t h e r u n n i n g a r ra n g e m e n t s o f t h e s u b s t a t i o n , f o r e x a m p l e fo r b u s b a r p r o t e c td iscr iminat ing zones , i t i s usual to se lec t as low a re lay current se t t ing as poss ib leorder tha t the fau l t se t t ing does no t va ry too wide ly and to f i t a shun t se t t iresis tor.

I t is usua l to a s sume tha t :

( i )(ii)

l e is in phase wi th the res is t ive currentsINLR is s inuso ida l wi th a peak va lue ca lcu la ted f rom eqn . 13 .5 .5 .

These assum pt ions s impl i fy the ca lcu la t ion and t end to g ive a s l igh t ly pess imiresu l t , t ha t is a h igher se t ting than wi l l be fo und by in jec ting cur ren t th ro ug h oc . t . and inc reas ing the cur ren t un t i l t he re lay op era t ion . The ca lcu la ted se t t ing

a d e q u a t e f o r m o s t p u r p o s e s . T h e c . t. e x c i ti n g c u r r e n t is u s u a ll y o b t a i n e d f r o m m a n u f a c t u r e r ' s e s t i m a t e d c u rv e b u t c a n b e o b t a i n e d f r o m t es t c e r ti fi c a te s m e a s u r e m e n t s m a d e o n s ite .

1 3 . 5 .7 E x t e n s i o n o f t h e b a s ic p r i n c ip l e s t o b u s b a r p r o t e c t i o n

The bas ic p r inc ip les jus t desc r ibed a re app li cab le to busbar p ro tec t io n w i th

changes .In the s t ab i l i ty ca lcu la t ion , the re i s a l a rge r number o f c . t . s to cons ide r and

l ead resi s tance be tw een the re lay and each c . t . in tu rn m us t be cons ide red in oto f ind the c i rcui t w hic h g ives the h igh es t s tabi l i ty vol tage . I f the c . t .s do n othave the sam e secon dary res i s tance , th i s shou ld a l so be t a ke n in to ac co un t ; occurs ch ie f ly w he n a subs ta t ion is ex tend ed w i th a d i ffe ren t des ign o f swi tchgo r s e p a r a t e l y m o u n t e d p o s t - t y p e c . t. s . M o d e r n p r a c ti c e is t o c o n n e c t t h e c .t .sbusw i re r ings as wi l l be desc r ibed l a t e r and the res is t ance be twe en re lay and m u s t t a k e a c c o u n t o f t h e p a r a ll el p a t h s f o r m e d b y t h e t w o r o u t e s p r o v i d e d b y r ing.

When ca lcu la t ing the fau l t se t t ing the re wi l l a l so be a l a rge r number o f c . t . spa ra ll e l co r respo nd ing in genera l to the nu m be r o f c i rcu i t s con nec ted to a g izone . Also , a s can be seen f rom F ig . 13 .5 .9A , the nu m be r o f c . t. s in pa ra ll ed o u b l e b u s b a r s u b s t a t io n s c a n d e p e n d o n t h e n u m b e r o f c i rc u i ts se le c te d t o a gb u s b a r. W i th b u s b a r p r o t e c t i o n i n p a r t i c u la r, i t is n o w c o m m o n p r a c ti c e to f i t m

t h a n o n e r e la y p e r z o n e a n d t h e c u r r e n t p a ss e d b y t h e a d d i t io n a l r e la y s m u s tinc luded w hen ca lcu la ting the fau l t se t t ing . F or the reasons g iven in Sec t




been decided and the c . t .s des igned, th is i s achieved m ainly by the se lect ion of value for the shunt resistor.

13.5 .8 Typ es of h igh-impedan ce re lays

A co m m on form of re lay used is the a t trac ted a rm ature type who se cur ren t se ttm ay be arou nd 30 m A, and i t has a re la t ively high s tabi lising res is tor con nectedser ies wi th i t to give i t the v ol tage se t t ing determ ined by the s tabi l i ty requirem eIts ope rat ing t ime w ill be less than 60 ms a t twice the se t ting cu rrent .

Another form of a t t racted armature re lay c i rcui t incorporates a capaci tor series wit h the op eratin g coil , w hich ren ders the relay insensitive to an y d.c. volwh ich m ay be present dur ing the f irst few cycles of faul t curren t . The re lay is g ithe required vol tage se t t ing b y the add i t ion o f l inear and non l inear res is tors , ai ts c i rcuit is shown in F ig . 13.5 .8A . T he use of the la t ter ensures high ope ratspeeds at small increm ents abov e th e set t ing voltage, s ince the cu rren t r ises mrapidly than the vol tage. With th is re lay i t i s not necessary to take in to account poss ibi li ty o f a lmo st twice the calcula ted s inusoidal a .c . curren t f lowing dur ing first few cycles.

Developed from this is a similar relay in which the nonlinear resistors

replaced b y ohm ic res is tors .Because of the d i s to r ted waveform of the cur ren t th roug h the re lay dur

s tabi l i ty and operat ion, only cer ta in re lays are sui table for h igh4mpedancircula t ing current protect ion. For th is reason, re lays should be se lected f rom tywhich have been tes ted in conjunc t ion wi th typ ica l c . t . s to demons t ra te tha t thgive the perform ance predic ted b y calcula t ion. In those tes ts the cu rrent or the lburden wil l have been increased unt i l the system becomes unstable , showing tthere is a fa ctor of safe ty in the calcula t ions varying, for typical re lays , f rom 1-42. From the earl ier calculat ion of stabil is ing resistor value, or relay voltage set t ii t appeared that the re lay wo uld be jus t on the point of ope rat ing for a faul tm axim um sever ity and w i th the m os t adverse of o ther cond i t ions . H owever, cause of the pess imis t ic assumptions made in the calcula t ion, th is factor of safensures that the required s tabi l ity is achieved w itho ut in t rodu cing o ther factorsincrease the set t ing voltage above the value given by eqn. 13.5.1.

Volts I*/5C-Plugsettingbridge

150 125 IO0 75 50 25- RelayA ~'~ A ~ ~'~ Linear °Pcer°~ing

, , , ' , ' I I " i - . _ oI

Non - l ine ar res i s tor Ca pac i tor Ik j r - 7

tabi l is ing resis tor



8usbar protec tion 113

13.5.9 Pract ical hish-im ped ance instal lat ions

For the reasons a l ready g iven , mos t o f the h igh- impedance schemes which hbeen ins ta l l ed a re o f the phase and ea r th - fau l t type , employ ing th ree -po le re l

ins tead o f the s ingle-pole re lays w hich wou ld be requ ired for ear th-faul t oschemes . The th ree -po le re lay schem e has the add i t iona l advan tage o f enab llower ear th-faul t se t t ings to be achieved for the same s ize of swi tchboard.

The ea r ly ins ta l la t ions em ploy ed on ly one re lay fo r each check and d isc r imina tzone , as show n in F ig . 13 .5 .9A. The d i sadvan tage o f busbar p ro tec t ion schememploy ing re lays con t ro l l ing the t r ipp ing of more than one c i rcu i t b reaker was tthe p ro tec t ion t ended to be t a ken ou t o f serv ice mo re o f ten than was necessabecause of the fear of causing mal t r ipping whi le tes t ing. I t i s now recognised tthe consequence o f a busbar fau l t , w hen the busbar p ro tec t ion is no t in se rv ice , be c ons iderab ly mo re se rious than tha t o f a possib le m alopera t ion whi le t e s tingwas f rom the des ire to reduce the r i sks o f malop era t ion to a m in im um and e l imina te the d i ffi cu lt ie s assoc ia ted wi th t ak ing the busbar p ro tec t ion ou t o f servfor norm al com miss ion ing and main tenan ce purposes tha t the schem e o f F ig . 13 .5was deve loped fo r 400 kV subs ta t ions . Main tenance o f th i s type o f ins ta l l a t iemploying separa te a .c . re lays for each c i rcui t breaker, i s very much eas ier than

is wi th ins ta l la t ions em ploy ing on ly one re lay per zone , and can be ca r r ied ou t wthe pro tect io n in service . The absence of auxi l iary swi tches in the d .c . c i rcuresul ts in a s impler schem e an d there fore one w hich is less l ike ly to fa il to op erw hen requ i red to do so . The schem e i s des igned in accordance w i th the p r inc ipdescr ibed in S ect ion 13.5 .5 an d 13.5 .6 , and a l tho ug h there are now severa l re laypara l le l per zone a sa t i s fac tory faul t se t t ing can s t i l l be obta ined by us ing re lw i th a low cu r ren t a t se t t ing so tha t the to ta l cur ren t requ i red fo r the i r ope ra tis stil l low .

Since individual re lay room s are provided a t Br i t ish 40 0 kV dou ble busbsubs ta t ions us ing open - te rmina l swi tchgear, tha t is to say one ro om ad jacen t to ec i rcu i t b reaker, thep e r c i r c u i t high- impedance re lays and the associa ted t r ip re layare acco m m od ated the re in . I t wi ll be seen tha t bus sec t ion and bus couple r swi tcrequ i re th ree h igh- impedance re lays, nam ely one fo r the check zone , and two d i sc rimina t ing zones . Each re lay ro om is equ ip ped wi th a 1 l0 V ba t te ry and tsupplies the local relays.

S ince the in t ro du c t ion o f th i s sys tem wi th i tsp e r c i r c u i t re lays , the need toachieve re liable , fas t c learance of busb ar faul ts has assumed even greater im po rtanAlthough re lay fa i lures have been smal l in number, i t was considered necessaryca te r fo r such a s i tua t ion by connec t ing to the a .c . buswi res pe r zone re lays , con tac t s o f which a re connec ted to d .c . buswi res (now known as back- t r ip bwires) in order to t r ip each c i rcui t breaker se lec ted to the faul ty zone so provida second ind epen den t t r ipp ing rou te fo r each ci rcu it b reaker.

A s impl i f i ed d iagram, F ig . 13 .5 .9C shows how thep e r z o n e re lays are incorp or-a ted w i th separa te t r ip re lays and wi th the use o f th ree t r ipp ing ba t t e r i es , i.e .and '2 ' in the re lay rooms of c i rcu i ts 1 and 2 , respec tive ly, and 'ST N ' , the sta t





B u s b a r p r o t e c t io n 1 1 5

M a i n I. , , . . .

b u s b a r

i?

Rl /

m l /

R e s e r v e b u s b a r

/R 3 /R 4

B u s s e c t i o n

IR 2 /

M2 /

N ~ t e :

B u s c o u p l e r

I I L I

? ,

l * "

R e l a yr o o m

R e l a y Ir o o m _ . . j

( r l )R e s e r v e z o n eb u s w i r e

I ) i s c r i m in a t i n gm a in z (~ n e b u sx ~ i r e

( r 4 ) ( r 2 )

°1m 2 ) n

I ) i s cr i m i n a t i n gm a i n 2 z o n e h u s ~ v i re

( I ) A l l c . t . s a r c I / 2 0 0 0t u r n s r a t i o( 2) R e l a y s a re h ig h i m p e d a n c e t y p e s h u n t e d b y

n o n - l i n e a r r e s i s t o rs (n () t s h ( ~ n )

M a i n 2b u s b a r

C h e c kb u s w i r e s

F i 1 3 5 9 B S i l i f i d d d i i t f b b t t i f 4 0 0 k V b t t i



1 1 6 B u s b ar p r o t e c t i o n

1

I I I

STN

2, ~ o : ~ o _ . . . o - G >

I

Back t r i pp ingbuswi re s

C.T. hus~vires

BTC H - Back t r ip check rece ive re layBTD - Back t r ip d is cr im ina t ing rece ive re layCH - Chec k zone h igh impe dance r e l ayI ) - Disc r imi na t i ng zone h igh imp eda nce re layT - Tripping relay



Busbar protec tion 117

du pl ica t io n of the faul t -detect ing an d t r ip-process ing arrang em ents . Even the fa io f on e of the d .c . suppl ies could no t , by i tse l f, be the cause of a c i rcui t bre ake r t r ipping w hen required to d o so for a busb ar faul t . This re l iabi l ity is a ided cs iderab ly by the ex i s tence o f du p l ica te t rip co ils - a s t andard fea tu re o f 400

c i rcu i t b reakers . T he back- t r ip d i sc rimina t ing re lays wou ld be conn ec ted to buswires v ia auxi l iary swi tches on the bus se lec tor d isconnectors , but these hbeen omit ted in th is d iagram to i l lus t ra te the bas ic pr inciple more c lear ly.

The a r rangem ent in F ig . 13 .5 .9C has , in England and W ales, becom e k now nMk I I 400 kV busbar p ro tec t ion and tha t in F ig . 13 .5 .9B as Mk I .

i A

1 2

I

t, 'l 'J t 'l"J2

( ' H - C h e c k z o n e h i g h i m p e d a n c e r e l a yI ) - D i s c r i m i n a t i n g z o n e h i g h i m p e d a n c e r e l a y

T - T r i p p i n g r e l a y

F i g . 1 3 . 5 . 9 D Simp l i f ied a r r angemen t o f M k l 1 27 5 k V b usb a r p ro t e c t i on

A la rge par t o f the 275 kV sys tem was comple te be fore the p r inc ip le o fpercircuit high- impedance re lays was adopted fo r 400 kV busbar p ro tec t ion so tha l though s imi la r a rguments app l ied , the p rac t i ce o f depending onper circuit re lays

had b eco m e w el l es tabl ished. This fac tor, co upled w i th the exclusive use com m on re lay room s led to the requ i red impro vem ents be ing ach ieved by dup l i




v

ii

f :

_ ( "

bSTN ~"C H - C h e c k z o n e h ig h i m p e d a n c ere layD - D i s c r i m i n a t i n g z o n e h ig h i m p e d a n c e r e la yT - T r i p p i n g r e la y

F i g . 1 3 . 5 . 9 E Sim pl i fied a r rangement o f M k. I 132 k V busbar p ro tec t ion

s ta t ion ba t t e r i e s have been ve ry re li ab le , th i s sing le source o f d . c . was cons idea d e q u a t e b u t d u p l i c a te s u p p li es h a v e b e e n d e r iv e d , w h e r e p r a c t i c a b l e , f r o m

ba t t e ry d i s t r ibu t ion board v ia separa te ly cab led and fused r ing mains o r r adfeeders . The s impl i f i ed d iagram of Mk I I 275 kV busbar p ro tec t ion i s shownFig . 13 .5 .9D.

At 132 kV the m easures r equ i r ing dup l i ca t ion o f r e lays and assoc ia ted c i rcuare no t jus t i fi ed and the s impl if ied d iagram of F ig . 13 .5 .9E shows a typ ia r r a n g e m e n t .

13 .6 Prac t ica l cons ide ra t ions

1 3 . 6 .1 F a c t o r s a f f e c t in g th e p o s i t i o n o f c . t. s i n b u s b a r s

Although i t i s common fo r one o r more bus sec t ion c i rcu i t b reakers to be ins ta lin the m ain b usba r, p rov i s ion fo r sec t iona l isa t ion o f the rese rve busb ar i s, w i tf ew e x c e p t i o n s , b y m e a n s o f d i sc o n n e c t o r s .

To o b t a i n c o r r e c t p e r f o r m a n c e o f b u s b a r p r o t e c t i o n , i t is n e c e s sa r y t o c o n s i

a t the ou t se t where the c . t . s a re to be loca ted . P rev ious re fe rences to the zonesb u s b a r p r o t e c t i o n h a v e i m p l ie d t h a t t h e r e is o n l y o n e c h e c k z o n e . F o r t h e p re s




or in com ing c i rcu i ts s ince the su m m at ion o f the cur ren t s in a ll such c i rcu i ts s h o w w h e t h e r o r n o t t h e r e i s a f a u lt w i t h i n t h e o v e ra ll b u s b a r z o n e s h o w nF ig . 1 3 . 2 . 1 A . B u s se c t io n a n d b u s c o u p l e r c i r c u it b r e a k e r s d o n o t h av e c h e c k zc . t. s s ince the c i rcu i t b reak ers are on ly ca r ry ing cur re n t be tw ee n d i sc r imina t

zones wi th in a g iven swi tchboard . Th i s i s shown in F ig . 13 .5 .9A, f rom which a lso seen tha t the d i sc r imina t ing c . t. s a re s i tua ted on the s ide o f the c i rcu i t b rear e m o t e f r o m t h e z o n e t o w h i c h t h e i r s e c o n d a r y w i n d i n g is c o n n e c t e d . T h is e n st h a t t h e f a u lt o f F I s h o w n i n F i g . 1 3 . 6 . 1 A , a n y w h e r e b e t w e e n t h e c i rc u i t b r e aco n tac t s an d the c . t. s on the Main 1 s ide o f the c i rcu i t b rea ker, w ou ld be de te cas w i th in the Main 1 Zone and resu l t in the t r ipp ing o f a ll c i r cu it b reak ers se lect o t h e M a in 1 B u s b a r. A fa u l t in t h e p o s i t io n j u s t m e n t i o n e d w o u l d a ls o b e d e t e cby the M ain 2 Zon e , pe rha ps u nnecessa r i ly s ince open ing the bu s sec t ion c i rb r e a k e r m a y c le a r t h e i n fe e d f r o m t h e M a in 2 Z o n e . I f, h o w e v e r, t h e f a u l t is w ithe c i rcu it b reak er i t se lf , i t m igh t n o t be in te r ru p ted w he n the c i rcu i t b reaop ene d and the t r ipp ing o f a ll c ir cu i t b reakers se lec ted to Main 2 Busbar w ou lde s se n ti al t o t h e i n t e r r u p t i o n o f t h e f a u l t. S im i la r a rg u m e n t s a p p l y f o r t r ip p i n g bo f these sec t ions fo r a f au l t a t F 2 . Th i s over lapp ing o f the d i sc r im ina t ing zon e a t a bus sec t ion (o r bus coup le r ) c i r cu i t b reaker i s the re fo re necessa ry fo r the rac lea rance o f such fau l ts , no t w i ths tand ing the fac t tha t the p rov i s ion o f c . t. s

b o t h s id e s o f b u s s e c t io n a n d b u s c o u p l e r c ir c u it b r e a k e r s is , w i t h s o m e t y p e sswi tchgear, an expens ive i t em.

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F ig . 1 3 . 6 .1 AD i s c r i m i n a t i n g m a i n z o n e c o n n e c t i o n s a t a b ~ s se c t io n

I t wi l l be no ted f rom F ig . 13 .6 .1B tha t c . t . s a re no t r equ i red in the rese rb u s b a r b e t w e e n t h e R e s e rv e 1 a n d R e s er v e 2 Z o n e s w h e n t h e s e a r e o n l y s e p a r ab y d i s c o n n e c t o r s . I n th e a b s e n c e o f a c ir c u it b r e a k e r b e t w e e n d i s c o n n e c t o r s R 3 aR 4 i t is n e c e s s a r y, w h e n t h e se a re b o t h c l o s ed , to c o n n e c t t h e R e s er v e I Z o n e a

R e s e rv e 2 Z o n e b u s w i r e s t h r o u g h a u x i l ia r y s w i tc h e s o n R 1 a n d R 2 . In t h e a b se no f s u c h a c o n n e c t i o n , a fa u l t a t F 1 s h o w n i n F i g. 1 3 . 6 . 1 B w o u l d r e su l t in i n s ta b i




appear as a secondary cur ren t in the buswires and there fore each zone would unba lanced by th e co r re spond ing am oun t , wh ich wou ld f low th rough the h igimpedance re lays and any para llel pa ths . I f on ly one o f the d i sconn ec tors is c los

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say R3 , the sec t ion o f busbar be tw een R3 and R4 beco m es par t o f the associadiscr iminat ing zone.

1 3 . 6 . 2 E f f e c t o f c .t . l o c a t i o n i n o u t g o i n g c i r cu i t s

The p os i t ion w here the cur ren t t rans form ers fo r the c i rcu it and the busbar p ro tt ion are f i t ted on o utgoin g c i rcui ts varies according to the ty pe of the swi tchgear.

(a) C.t.s overlapping the circuit breaker:Fig . 13 .6 .2A(a) shows the a r rangementof c . t .s f it ted on b o th s ides o f the c i rcu i t b reaker wh ich is co m m on in ou td oo r buoff type s an d m etalc lad SF6 insula ted swi tchgear. The perform anc e o f the prott ion wil l now b e cons idered fo r the fau lt s Fz to F 4 .

A fau lt a t F I is a busbar fau lt which shou ld be c lea red by the bu sbar p ro tec t ioFaul t F2 is a c i rcui t faul t and should be c leared by the c i rcui t protect ion. Fa i




pro tect ion to o pera te and the faul t wil l be c leared, a l thoug h the c i rcuit breakethe rem ote end of the c i rcuit m ay also be t r ippe d. Al tho ugh F 4 is a c i rcuit faumay be de tec ted by both c i rcu i t and busbar pro tec t ion depending on the i r re laoperat ing t imes. Thus, c i rcui t breakers se lected to the busbar may be ope

unnec essari ly for a circuit fau lt . This disadvantage is acce ptable in view o f the incidence of suc h faul ts .(b) C. t . s on the c i rcui t s ide o f the c i rcui t breaker:Fig. 13.6 .2A(b) shows themost common ar rangement of cur ren t t ransformer loca t ion for a i rb las t swi tchgand o the r designs em ploying post typ e c . t .s . F aul ts F~ and F2 should be correc learedas before , bu t F3 wi l l on ly cause opera t ion of the busbar pro tec t ibecause the faul t i s outs ide the c i rcui t protect ion zone. Thus, the faul t may remfed f rom the rem ote end of the c i rcu i t. A rrangem ents mus t , therefore , be madcause the c i rcuit break er a t the rem ote end to t r ip in these c i rcum stances .

( i) Direct tr ipping of the rem ote c ircui t breaker(s) w i tho ut in t roducing any tdelay is the fastest o f the available al ternatives bu t, fo r feeder circuits , dep endthe ex istence of in te rt r ipp ing equipm ent w hich , on some c ircu it s, w ould otherwise be required.

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( ii ) Each feed er c i rcui t can be f i t ted w i th a t ime- lag re lay se t a t ab ou t 100 t hi s b e in g s t a r t e d b y o p e r a t i o n o f t h e b u s b a r p r o t e c t i o n h a v in g i ts c o n t a c t s cnec ted to uns tab i l i se un i t p ro tec t ion o r to acce le ra te o r unb lock d i s t ance p ro tec t iTh i s g ives a measure o f d i sc r imina t ion in tha t on ly the c i rcu i t on which the fa

ex i st s wi l l have i ts r em ote end c i rcu it b reak er t r ipped .

( ii i) I f c i rcui t bre ak er fa i l pr o te c t io n is ins ta l led , i t w ould be co nn ecte d so thais in i t ia t ed b y the busba r p ro tec t ion t r ip r e lay as we l l a s by the c i rcu i t -p ro tec tt r ip re lays . Ci rcui t bre ak er fa il pro tec t ion is descr ibed la ter in Sec t ion 13.7 , heris su ff i c ien t to s t a t e tha t any fau l t cu r ren t in fed to F au l t F 3 f rom the rem ote w ou ld , p rov ided the in i t i a tion s igna l has been rece ived , m a in ta in a cu r ren t chre lay in the ope ra ted pos i t ion . I f th i s con d i t ion s til l pe r s is ts a f t e r a p red e te rm int i m e t h e c i r c u i t b r e a k e r s a t t h e r e m o t e e n d o f t h e c i r c u i t w o u l d b e t r i p p e d uns tab i l i s ing un i t p ro tec t ion o r by acce le ra t ing o r unb lock ing d i s t ance p ro tec t iI t shou ld be no te d tha t the re is no fa i lu re o f the c i rcu i t b reak er in the s i tua tjus t desc r ibed b u t the fea tu res o f the c i rcu it b reaker f ai l p ro tec t ion a re beused to advan tage .

( iv ) An ad d i t iona l r e l ay ca ll ed an in te r locked ov ercur ren t r e l ay can be f i t t ed

de tec t an y pow er in fed a t F 3 a f te r the c i rcu it b reak er has ope ned . When c ircbreake r f ai l p ro tec t ion is no t f i t t ed , i .e . a t 132 kV and low er vo l tages , th is m e thcan be app l i ed to f eeders hav ing un i t p ro tec t ion and to genera to r s and t r ans fo rmeThis r e l ay can be e i the r a th ree -po le , o r a s ing le po le wi th summat ion wind iinduc t ion-d i sc overcur ren t type hav ing an ex t remely inverse t ime charac te r i s t i c wan ope ra t ing t ime o f abou t 0 -3s . I t s lower mag ne t c i r cu it is on ly com ple ted whthe bu sbar p ro tec t ion keeps op era ted , and so i f the fau l t pe r si s ts a t F3 a f t e r c i r cu i t b reaker opens , the busbar p ro tec t ion wi l l r emain opera ted , so pe rmi t tthe in te r loc ked ove rcur ren t r e l ay to fun c t ion and send an in te r t r ipp ing o r s tabi l i s ing s ignal to the remote end of the c i rcui t .

(v ) W here g rid t r ans fo rm ers f eed busbars a s soc ia ted w i th meta lc lad swi tchgear, busb ar p ro tec t ion cover ing these busba rs is a r ranged to t r ip d i rec t ly bo th the and the l .v. c ir cu it b reake rs o f the t r ans fo rm er so e l imina t ing any p ro longed in fto a faul t a t F3 .

Tw o gr id t r ans fo rm ers f eed ing d i ffe ren t sec t ions o r vo lt ages o f busbars m ay ban ked on a co m m on h .v. c i rcu i t b reaker. In th i s case the low vo lt age busbpro tec t io n wo uld f ir st tr ip on ly the g r id t r ans fo rm er l.v. c i r cu it b reake r a s soc iaw i th the fau l ty busbar. I f the fau l t were a tF3, then the s t age 2 o f the s t andbyear th - fau l t p ro tec t ion o r s tage 2 o f the ov ercur ren t p ro tec t io n wo uld l a t e r ope ra tet r ip the o the r t r ans fo rm er 1 .v. and the co m m on h .v. c i r cu i t b reakers.

(v i) I f the rem ote ends o f the feeders con nec ted to the busbars a re f i t ted wdis tance p ro tec t ion , the in feed to the fau l t a tF 3 wil l be c lea red by the opera t ionof the d i s t ance p ro tec t ion on the fau l ty f eeder in second zone t ime (0 .4 -1 .0



8usbar protec tion 123

(c) C. t . s on the busbar s ide o f the c i rcui t breaker:In F ig . 13 .6 .2A(c) the fau l t sa t F ~ and F2 wi l l be cor re c t ly c l ea red . A fau l t a t F 3 w i ll cause the c ircu i t p rot i o n t o t r ip t h e c i rc u i t b r e a k e r, b u t t h e f a u l t w i ll r e m a i n f ed f r o m t h e b u s b a r s . Tbu sbar p ro tec t io n wi ll no t op era te a s F 3 is ou t s ide i ts zone . Aga in , an in te r lock

overcur ren t r e l ay i s used , bu t in th i s case , s ince i t i s the c i rcu i t p ro tec t ion whremains opera ted fo r the fau l t a t F3 , i t i s th i s p ro tec t ion which i s used to in i t io p e r a t i o n o f t h e i n t e r lo c k e d o v e r c u r r e n t r e l a y. I f F 3 p e rs is ts f o r a b o u t 0 . 3 s , i n t e r lo c k e d o v e r c u r r e n t r e la y t h e n o p e r a t e s t h e t r i p p i n g r e la y s o f t h e p r o t e c to f t h e s e c t io n o f b u s b a r t o w h i c h t h e c i rc u i t is s e l ec t e d .

1 3 . 6 . 3 M u l ti p l e c h e c k z o n e s

U n t i l n o w w e h a v e c o n s id e r e d b u s b a r p r o t e c t i o n s y s t e m s w i t h a s in g le c h e c k z oa n a r r a n g e m e n t h a v i n g t h e a d v a n t a g e o f r e la ti ve s i m p l ic i ty a n d e c o n o m y. A t l a rg e r s t a t i o n s , h o w e v e r, t h e r e m a y b e s t r o n g r e a s o n s f o r d e p a r t i n g f r o m tc o n c e p t o f a sin g le c h e c k z o n e a n d u s in g i n s te a d t w o o r m o r e c h e c k z o n e s . Oc o n s i d e r a t i o n i s t h a t t h e t o t a l n u m b e r o f c i r c u i t s m a y b e s u c h t h a t i t i s nprac t i cab le to ob ta in a su i tab le p r im ary fau l t se t t ing f rom a s ing le check zoc o v e ri n g t h e w h o l e s u b s t a t i o n b e c a u s e o f t h e n u m b e r o f c .t .s a n d , if a p p l ic a b l e , c i rc u i t r e la y s in p a ra ll el . F u r t h e r m o r e , t h e p r o v i s i o n o f m o r e t h a n o n e c h e c k zl ea d s t o g r e a te r s e c u r it y o f t h e b u s b a r p r o t e c t i o n d u r i n g c o n s t r u c t i o n , c o m ms io n i n g a n d m a i n t e n a n c e a n d m a y b e s p e cif ie d f o r th i s r ea s o n a l o n e .

A t s u b s t a t i o n s w h e r e o n l y b u s s e c t io n c i rc u i t b r e a k e r s o r p e r m a n e n t b r e a k s p rov ide d to sec t iona li se the m ain and the rese rve busb ars , the sp l i tt ing o f chezones is a s imple m at te r, o n ly requ i r ing the ad d i t ion o f check zo ne c .t .s e i the r s

o f the b us sec t ion c i rcu it b reakers . T he over lapp ing zones tha t th i s c rea tes sho w n in F ig . 13 .6 .3A. H ow ever, w here the re is ne i the r a c i r cu it b reaker n op e r m a n e n t b r e a k ( as is t h e c as e w i t h t h e r es e rv e b u s b a r a t t h e m a j o r i t y o f d o u bbusb ar swi tch ing s t a t ions ) i t is necessa ry, in o rd e r to sec t iona li se the ch eck z oto adhere to the fo l lowing ru les :

( i) The check -zone de m arca t ion p o in t s sha ll in a ll cases co inc ide w i th d i sc r im it i n g z o n e d e m a r c a t i o n p o i n t s .

( ii ) Disc r imina t ing and ch eck-zon e cur ren t s sha ll each be m easured by separc . t. s a t the d em arca t ion po in t s so as to sa t i s fy the reasons m en t ion ed ea r li e r p r o v i d i n g t w o o r m o r e c h e c k z o n e s .

( ii i) There sha ll be no aux i l ia ry swi tches in the c . t . c i r cu it s o f the ch eck zon e .

( iv ) W i th bo th o f the sec t ion d i scon nec to r s ope n in the absence o f a c i r cb r e a k e r ( o r w i t h t h e s in g le d i s c o n n e c t o r o p e n i f o n l y o n e is f i t t e d ) a fa u l t o n a




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s e le c te d t o t h a t b u s b a r b u t m u s t n o t r e su l t in t h e o p e r a t i o n o f t h e p r o t e c t i o n ot h e t r i p p i n g o f c i rc u i t b r e a k e r s a s s o c ia t e d w i t h t h e a d j a c e n t b u s b a r.

(v ) I f b o th (o r the s ing le ) sec t ion d i sco nn ec to r ( s ) r e fe r red to in ( iv ) a re c losef a u lt o n a n y p a r t o f o n e b u s b a r m u s t r e su l t i n t h e i m m e d i a t e t ri p p i n g o f a ll c i rc

b r e a k e r s s e le c te d t o t h a t s e c t io n o f b u s b a r a n d t o t h e s e c t i o n c o n n e c t e d b y d i s c o n n e c t o r ( s ) in q u e s t i o n .

( v i) O n i n s ta l la t io n s h a v in g d u p l i c a te s e c t io n d i s c o n n e c t o r s , w h e n o n e s e c td i s c o n n e c t o r is c lo s e d a n d t h e o t h e r o p e n , a f a u l t o n a n y p a r t o f o n e b u s bi n c l u d i n g t h a t p a r t c o n n e c t e d t o i t b y t h e c l o s e d d i s c o n n e c t o r, m u s t r e s u l t i n i m m e d i a t e t ri p p i n g o f a ll c i rc u i t b r e a k e r s c o n n e c t e d t o t h e f a u l te d b u s b a r, b u t mn o t r e su l t i n t h e o p e r a t i o n o f p r o t e c t i o n o r in th e t r ip p i n g o f c i rc u i t b r e a ka s s o ci a te d w i t h t h e h e a l t h y s e c t i o n o f t h e b u s b a r.A n a r r a n g e m e n t w h i c h m e e t s a ll t h e s e r e q u i r e m e n t s f o r t h e c a se o f a s in g le s e c td i s c o n n e c t o r i s s h o w n i n F i g . 1 3 . 6 . 3 B . T h e m a i n b u s b a r s a n d b u s s e l e c t o r d i s cn e c t o r s h a v e b e e n o m i t t e d f o r c la r i ty. A f au l t o c c u r r in g a t F I w i t h t h e d i s c o n n e co p e n is w i t h i n c h e c k z o n e 2 a n d d i s c ri m i n a t in g z o n e R 2 ; c . t. s o n c i rc u i ts t h r o uw hich cu r ren t flOWS to the fau l t wi ll ene rg ise the check zon e 2 and d i sc r im ina trese rve 2 zone a . c . buswi res , opera t ing theper zone a n d per circuit r e la y s c o n n e c t e d

the re to , so t r ipp ing a l l c i r cu i t s connec ted to the rese rve zone . Ne i the r check zonnor d i sc r imina t ing rese rve 1 zone a . c . r e l ays wi l l opera te fo r th i s f au l t whe the r



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rese rve 1 bu sbar w i ll be t r ipped f rom the zon e 2p e r z o n e re lays v ia the back - t r ipp in gs y s t e m .

I f w e n o w c o n s i d e r t h e f a u lt t o b e a t F 2 , a g a in w i t h th e s e c t io n d i s c o n n e co p e n , t h e c h e c k 1 z o n e r e la y s w i l l o p e r a t e b u t c h e c k 2 z o n e w i ll b e s t a b l e . S in c e

d i s c r i m i n a t i n g z o n e c . t . s i n t h e b u s b a r a r e d i s c o n n e c t e d b y t h e d i s c o n n e c t o r s wthe i r o u tp u t wi l l no t be ava il ab le to s tab i li se the rese rve 2 zone and so i tsp e r z o n e

a n d p e r c i r c u i t re lays wi ll op era te . The bac k- t r ip r ece ive re lays o f Sec t ion 2 wi l l o p e r a t e d b y t h ep e r z o n e re lays o f chec k 1 zon e and d i sc r im ina t ing rese rve 2 zonA f a u l t i n t h is p o s i t i o n w i t h t h e d i s c o n n e c t o r c lo s e d w o u l d b e d e t e c t e d b y c h e cz o n e a n d d i s c r im i n a t in g r es e rv e 1 z o n e w h i c h w o u l d r e s u lt in t h e d i r e c t t r ip p i n gc i rc u i t b r e a k e rs i n S e c t i o n 1 w h i l e t h o s e o f S e c t i o n 2 w o u l d b e o p e n e d v ia b a c k - t r ip p i n g s y s t e m .

I f t w o d i s c o n n e c t o r s a re i n s ta ll e d b e t w e e n s e c ti o n s o f b u s b a r, t h e c .t . s e c o n d am u s t b e c o n n e c t e d a s s h o w n i n F i g . 1 3 . 6 . 3 C . Wi t h th e l e ft -h a n d s e c t io n d i s cn e c t o r o p e n a n d t h e r ig h t - h a n d o n e c l o s e d , a f a u l t a t F 2 w o u l d b e c le a r ed i n same w ay as has jus t b een desc r ibed fo r F 2 in F ig . 13 .6 .3B. Co nverse ly, a fau lF 1 , w i t h t h e l e f t- h a n d s e c t io n d i s c o n n e c t o r c lo s e d a n d t h e r i g h t -h a n d o n e o pw ou ld b e c lea red in a s im i la r ma nn er, bu t w i th the bac k- t r ip rece ive re laysS e c t i o n 1 o p e r a t e d b y t h ep e r z o n e r e la y s o f t h e c h e c k 1 z o n e a n d t h e d i s c ri m i n a t in g

reserve 2 zone .W i th b o t h d i s c o n n e c t o r s c l o s e d , a f a u lt i n e it h e r p o s i t io n w o u l d b e d e t e c t e d

t h e c h e c k a n d d i s c r i m i n a t i n g z o n e s w i t h i n w h i c h i t h a s o c c u r r e d , r e s u l t i n g i n d i r e c t t r ip p i n g o f c i r c u it s s e le c te d t o t h e c o r r e s p o n d i n g s e c t io n o f b u s b a r, w ht h o s e o f t h e a d j a c e n t s e c t i o n w o u l d h a v e th e i r c ir c u it b r e a k e r s o p e n e d v ia t h e b at r ip p i n g s y s t e m .

1 3 . 6 . 4 B u s b a r s e l e c t o r a u x i li a r y s w i t c h e s

In a l im i ted nu m be r o f bu sbar ins ta l l a t ions i t is no t poss ib le to ca r ry ou t on - lt r ans fe r s o f a c i r cu i t f rom , say, the m ain b usba r to the rese rve . In th i s case inecessa ry to have au x i l i a ry swi tches f i tt ed in on ly one o f the bu sbar se lec t( sa y t h e m a i n b u s b a r ) , a n d t h e y a re c o n n e c t e d s o t h a t w h e n a c i rc u i t is n o t s e le cto the m ain busb ar, i t is a s sum ed tha t i t is se lec ted to the rese rve , F ig . 13 .6 .s h o w s t h is . W i th th e m o r e u s u a l o n - lo a d s e l e c ti o n p o s s ib l e w i t h o u t d o o r s w i tc h gt h e c o n d i t i o n o f b o t h s e le c to r s s im u l t a n e o u s l y c l o se d m u s t b e a l l o w e d f o r, a n daux i l i a ry swi tch con tac t s a re f i t t ed in each busbar se lec to r, a s shown in F1 3 . 5 . 9 A .

T h e r e l a t i v e t i m e s o f o p e n i n g a n d c l o s i n g o f b u s b a r s e l e c t o r c o n t a c t s t o t haux i l ia ry sw i tches is imp or tan t . W hen c los ing a busb ar se lec to r, i ts aux i l ia ry sw im u s t c lo s e b e f o r e t h e m a i n c o n t a c t s c lo s e o r p re - a rc . W h e n o p e n i n g t h e s e le c tt h e p r i m a r y c o n t a c t s m u s t a l l o p e n b e f o r e t h e a u x i l i a r y c o n t a c t s o p e n . T h e n e e d

th i s can be seen in F ig . 13 .6 .4B where an on- load changeover o f a f eeder i s bem a d e f r o m t h e m a i n t o th e r e s er v e b u s b a r s . L e t t h e c u r r e n t d i s t r i b u t i o n i n t



1 2 8 B u s b a r r o t e c t i o n

In F ig . 13 .6A B, the cur ren t o f th ree un i t si s en te r ing the main busbar f rom feederA, pass ing thr ou gh the bus cou pler and leaving the reserve bus bar v ia feeder B. c . t. secondary cur ren t s c i rcu la te as show n in F ig . 13 .6 .4B(a) and bo th m ain rese rve busbar d i sc rimina t ing re lays remain uno pera ted . I f now the con tac t s

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cu r ren t d i s t r ibu t ion , a s sho w n in the F ig. 13 .6 .4B (b) caus ing ins tab i l i ty may a

before the aux i l ia ry swi tches pa ra l le l bo th m ain and rese rve d i sc r imina t ing zoness h o w n b y t h e d o t t e d l in e s.B y c o n s i d e r a t i o n o f s im i l ar c u r r e n t d i s t r i b u t i o n d u r i n g o p e n i n g a b u s b a r s e l e c

i t can be seen tha t ins tab i l i ty w i ll aga in occ ur i f the aux i l i a ry swi tches do no t oa f t e r th e m a i n c o n t a c t s , t h a t is , a u x i l ia r y s w i tc h e s m u s t m a k e e a r l y a n d b r e a k l a t

M e a s ur es m u s t b e t a k e n t o e n s u r e t h a t , w i t h b o t h b u s b a r s e l e ct o rs f o r a g ic i rc u i t o p e n , t h e c . t. s e c o n d a r y c o n n e c t i o n s a re n o t l ef t o p e n - c i r c u i te d . W hper circuit high- im pedan ce re lays a re f i tt ed these w ou ld st il l be con ne c ted to t

c . t. s a s can be seen f ro m F ig . 13 .5 .9B . I n o th e r cases aux i l ia ry swi tches m us tco nn ec ted to sh or t -c i rcu i t the d i sc r imina t ing c . t .s a s sho w n in F ig . 13 .6 .4C. I f tw ere l e f t o pen -c i rcu i t ed the re is the r isk o f dam age to the insu la t ion o f s e c o n d a r y w i n d i n g d u e t o , s a y, m a i n t e n a n c e e a r t h s o n e i th e r s i de a ll o w i n g c u r rin t h e s u b s t a t i o n e a r t h m a t t o b e d i v e r te d t h r o u g h t h e c . t. p r i m a r y w i n d i n g d u r i nfau l t e l sewhere in the subs ta t ion , o r even ex te rna l ly to i t , a s shown in F ig . 13 .6 .

1 3 . 6 . 5 C . T. t e s t l i n k s

To e n a b l e c . t . s e c o n d a r y c u r r e n t s t o b e r e a d il y m e a s u r e d w i t h o u t d i s tu r b i n g aseco nd ary w i r ing , c . t . t e s t links a re f i t ted in eve ry se t o f d i sc r im ina t ing and chc . t . s . These can be seen in F ig . 13 .6 .4C. They a l so have a changeover pos i tw h e r e b y th e c . t. s a re d i s c o n n e c t e d f r o m t h e s c h e m e w i r in g a n d s h o r t - c i rc u i t e d ,

th is po s i t ion m ay be used wh en a f eeder is ou t o f serv ice and hav ing cur rein jec ted th rough i t s c . t . s fo r t e s t purposes , a s i t pe rmi t s the busbar p ro tec t ion



Busbar pro tec tion 131

13.6.6 Precaut ions agains t malopera t ion of busbar protec t ion

As descr ibed ear lier, the use o f du pl icate l ines of defenc e, requir ing the s imtaneou s ope ra t ion of separate d i sc r im ina ting and check sys tems before t ripp

takes p lace , i s one of the mos t impor tan t methods of avo id ing malopera t ionbusbar p ro tec t ion . W hereas ma lopera t ion of the p ro tec t ion fo r an ind iv idual c i rnorm al ly resu lt s in the wrong t r ipp ing of one or two c i rcu it b reakers , malope ra to f busba r p ro t ec t i on m ay com ple t e ly shu t dow n a who le s ec t ion o f busba r. Abusbar p ro tec t ion d .c . c i rcu it s a re a rranged so tha t the inadver ten t op era t ion o f one re lay wi l l, a t the m ost , o nly t r ip a single c i rcuit break er.

Rel iabi l i ty of busbar protect ion is improved by avoiding the use of auxi l iswi tches in the check sys tem. Where they occur in the discr iminat ing sys tem Bripract ice fol low s one o f the fol low ing a l ternat ives :

( a )

(b )

When disconnector auxi l iary swi tches have been specif ica l ly approved swi tching current t ransformer c i rcui ts , s ingle contacts are used throughout .When the auxi l iary swi tches have not received such approval , two s i lvpla ted sw i tches con nec ted in para l le l are used w here con tacts are op en w hthe disconnector i s open and one s i lver-pla ted swi tch is used where conta

are c losed wh en the d i scon nec tor is ope n .

F ur ther p recau t ions can be t aken by f it ting re lays to con s tan t ly superv ise integ ri ty o f the c. t . circuits . Th e pres ent m eth od is to f i t a sensi tive supervisre lay S in para l le l wi th the main pro tec t ion re lay E . The sys tem pr imary locurrents w i ll cause an out-of-balance c . t. seco nda ry cu rrent i f any conn ec t ionc . t . becomes fau l ty. Th is method depends on there be ing suff ic ien t ly h igh lo

curre nt in the pr im ary c i rcuit wh ich has the defect ive c . t. (or wir ing) to operrelay S.

With th is in mind, i t i s usual to specify that the pr imary operat ing currentthe supervis ion re lay lies betw een 1 and 5% of the ra ted curren t of the swi tchgfor d iscr iminat ing zones th is curre nt wi ll vary w i th the nu m be r o f c ircui ts se lectethe zone (because of the num ber o f c . t. s conn ec ted to the re lay a .c . c i rcu i t) bu t m in im um and m axim um va lues should be wi th in the fo regoing l im i ts . C i rcu i ts w

a low ful l load current re la t ive to o ther c i rcui ts in the same zone present a probsince the norm al load curre nt m ay , in the even t o f an op en c i rcui t in one o f i ts seco nda ry c i rcuits , be insuff ic ient to ope rate the supervis ion re lay. Th e only acttha t can be tak en in such c i rcum stances is to se t the supervis ion re lay to give as a pr imary se t t ing as poss ible wi thin the l imi ts ment ioned previously.

Ine vi tably there wi l l be s i tua t ions w here , fo r par t o f the t im e, the supervisre lay would no t opera te on the occur rence of an open c i rcu i t in the secondcircui t o f one o f the c . t. s because the curre nt f low ing a t the t ime is below t

pr imary se t t ing. The r isk of ins tabi l i ty due to such an open c i rcui t remainiun de tecte d is acce pted in view of the low incidence o f op en c i rcui ts in the




The opera t ion of the supervis ion re lay s tar ts a t ime lag re lay which af ter, s3 - 5s b r ings up an a la rm tha t the p ar t i cu la r d i sc rimina t ing o r che ck zon e i s de fec tOr ig ina lly an aux i l ia ry re lay, w hich opera ted a f te r the t ime de lay, was a r rangedswitch out th is defect ive zone by shor t -c i rcui t ing the c . t . bus wir ing and in t

rupt in g the d .c . suppl ies . The supervis ion re lay w ould a lway s have op era te d fobusbar fau l t , and the t ime lag was p rov ided to ensure tha t ope ra t ion o f p ro t ec t i on w ou ld n o t be i n t e r fe r ed w i th .

A reassessment o f the p r io r i ti e s l ed , w i th the adv en t o f the 4 00 kV sys tem ,the abandonment o f buswi re shor t ing . Re ta in ing the busbar p ro tec t ion in se rv icethat i t wi l l opera te for a genuine busbar faul t i s now considered essent ia l . Thecor rec t ope ra t ion o f the re lays in the fau l ty zone on the occur rence o f a faex te rna l to tha t zone , shou ld one happen before the open-c i rcu i t connec t ionlocated and repai red , presents a to lerable r i sk par t icular ly s ince i t would require complementa ry zone (check or d i sc r imina t ing , a s appropr ia te ) to become uns tabefore c i rcu i t b reakers were t r ipped . The absence o f buswi re shor t -c ircu i ting redoes requ i re spec ia l cons idera t ion o f the ra t ing o f re lays and assoc ia ted co m pon eas expla ined in Sect ion 13.6 .10.

1 3 . 6 . 7 . Tr i p p i n g a n d a la r m c ir c u i t a r r a n g e m e n t s

The t r ipp ing sequences o f the d .c . r e lays in busbar p ro tec t ion ins ta l l a t ions a r ranged such tha t the ope ra t ion o f a ny re lay in e r ro r, o r by v ib ra t ion , does cause more tha n on e c i rcu it to t r ip . F igs . 13 .5 .3E and F show the tw o mco m m on a r rangem ents . The a r rangem ent comp r i sing one t r ipp ing re lay per c i rcas shown in Fig . 13.5 .3E, permits the t r ip c i rcui t se lec t ion to be achieved in

re lay op era t ing coi l c i rcui t ins tea d o f in the a c tual c i rcui t break er t r ip coi l c i ras is don e in the m ul t i . con tac t t r ipp ing re lay m etho d shown in F ig . 13 .5 .3Fla rge subs ta t ions wi th c i rcu i t b reaker t r ip co i l cur ren t s o f 30A or more , the vo l td rops in the l eads m ay well p roh ib i t the use o f mu l t i con tac t t r ipp ing re lays . Ttes t ing f rom busbar p ro tec t ion can a l so be more sa fe ly ca r r i ed ou t f rom schemusing individual t r ipping re lays .

Whenever p rac t i cab le , the d .c . wi r ing fo r busbar p ro tec t ion shou ld be segregaf rom o ther w i r ing to reduce the p oss ib i li ty o f mul t ip le ina dver ten t t r ipp ing .

F ig . 13.6 .7A show s one poss ible ar ra ng em ent o f d .c . i so la ting f inks f i t ted in con tac t c i rcu i t s o f the ind iv idua l t r ipp ing re lays on the busbar p ro tec t ion panThe advan tage o f the dup l ica t ion o f l inks , a s show n in the f igure , is tha t w hen des ired to m ake every wi re dead in the busbar p ro tec t ion pane l , a s , fo r ins tanw hen m aking a l t e ra t ions o r ex tens ions to the pane l , the links a re remov ed a t evc i rcu it r e lay pane l. S imi la r ly, the l inks a re remov ed a t the bu sbar p ro tec t ion panei t is des i red to m ak e the re lay pan el wir ing dead . I f seco nd ary in jec t ion is be

car ried ou t to p rove the b usbar p ro tec t ion re lays , i t is necessa ry to rem ove the l ion ly a t the busbar p ro tec t ion pane l . To prove the f ina l t r ip t e s t ing o f the c ircu




With the increased complexi ty of protect ion on c i rcui ts a t the higher systvol tages , coup led w ith the use of dup l icate t r ip coi ls , the n um ber o f l inks forpurposes has tended to b ecom e unacceptab ly h igh , bo th f rom cons idera tion ofspace required and the apprecia t ion by the f ie ld engineer of the purpose of th

As a result o ther arrangem ents wi l l f requen t ly be foun d such as the f i t t ing of l ior d isconnect ing points in only one wire of a pai r of t r ip re lay contact connect io

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Circuit relay panel

Fig . 13 .6 .7A Arrangement o f t r ipp ing l inks for busbar p ro tec t ion

A disconnec t ing p oin t is usually an in tegral par t o f a re lay panel termina l b losuch as a removable plug con nect ing the two wires to a given w ay.

As described earl ier, a 'busb ar p ro tec tion d efective ' alarm is given after a t idelay of 3-5 s when a sensit ive supervision relay operates for a fault in the cci rcui ts , leads , buswir ing e tc . The operat ion of in termediate manual ly reset drelays, i f an y, can also be arranged to give an a larm.

The d .c . suppl ies fo r busbar p ro tec t ion can be moni tored by a normalenergised protect ion supply supervis ion re lay which gives a 'protect ion supply fa larm when i t drops off .

On the occurrence of a busbar faul t an indicat ion is g iven showing exact ly wh

zone of busbar p ro tec t ion has ope ra ted , th is be ing achieved by contac t s o f the drepeat re lay o f the check re lay in ser ies w i th co ntacts o f the d .c . repeat re lay of



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138 8usbar pro tection

13.6 .8 Back- t r ipping

Where per circuit high- impedance re lays a re used , a s a t 400 kV subs ta t ions , theis , a t a f i r s t g lance , no need for d .c . buswires s ince each c i rcui t breaker in a d

c r imina t ing zone con ta in ing a busbar f au l t , wou ld rece ive a t r ip s igna l in i t i a t edits per circuit re lay.

However, a f ac i l i ty known as back- t r ipp ing i s r equ i red to fu l f i l t he fo l lowfunc t ions"

( i) To t r ip a ll c ir cu i t b reake rs w hich m us t be ope ned to c lea r a busb ar f ad e t e c t e d b y t h e i m g h - i m p e d a n c eper zone c h e c k a n dper zone disc r imina t ing re laysas a safeguard ag ains t the fa i lure o f aper circuit re lay.

( ii ) On the occu r rence o f a f au l t in a sec t ion o f the rese rve busb ar a t a subs ta tw i t h m o r e t h a n o n e c h e c k z o n e , t o t r ip t h e c i r cu i t b r e a k e r s o f a d j a c e n t s e c tiwhen the sec t ion d i sconnec to r s a re c losed .

( ii i) To t r ip a l l c i r cu it b reakers con nec ted to the sec t ion o f busbars to w hichcon nec ted a gene ra to r c i r cu i t b reak er o f the p ressuri sed h ead a i r-b last typ e in

even t o f a sudd en loss o f a ir p ressure . Th i s is to guard aga ins t the conse quen cean unw ante d rec losure o r in te rna l f la shover o f the c i rcu i t b reak er wh ich cou ld car e c o n n e c t i o n o f t h e g e n e r a t o r o u t o f s y n c h r o n i s m .

( iv) W here c i rcui t brea ke r fa il pro tec t ion is f i t ted , to t r ip a ll the loca l c i rcb reakers n ecessa ry to d i sconn ec t a c ir cu i t b reak er w hich has f a il ed to c l ea r a fw h i c h h a s b e e n d e t e c t e d b y t h e a p p r o p r i a t e p r o t e c t i o n .

A s t a n d a r d b a c k - t r ip p i n g s y s t e m h a s b e e n d e v e l o p e d f o r t h e B r i ti sh 4 0 0 k V d o ubus bar sub s ta t ion s . I t is des igned to effec t the h igh-speed t r ipp ing o f a ll c i rcb reakers se lec ted to a pa r t i cu la r busbar and to ad jacen t busbars th rough busse lec to r d i sconnec to r s o r busbar sec t ion d i sconnec to r s .

T h e b a c k - t r ip p i n g f a c il it y c o m p r i s e s a d i s c ri m i n a t in g s y s t e m a n d a c h e c k s y s ta r ra n g e d s o t h a t b o t h s y s t e m s m u s t m a l o p e r a t e b e f o re i n c o r r e c t c i rc u it b r e at r i p p i n g d u e t o s e c o n d a r y e q u i p m e n t d e f e c t s c a n o c c u r. B o t h b a c k - t r i p p i n g s y s t ehave dou b le -po le swi tched in i t i a t ion in o rde r tha t the back - t r ip rece ive re lays no t have to mee t the more d i ff i cu l t r equ i rements app ly ing to t r ip r e l ays .

O n e b a c k - t r ip p i n g s y s t e m d i s c ri m i n a t e s b e t w e e n s e c ti o n s o f b u s b a r, e m p l o yaux i l i a ry swi tches on busb ar sec t ion d i scon nec to r s fo r th i s purp ose , and i s kn ow nt h e d i s c ri m i n a t in g s y s t e m . T h e o t h e r b a c k - t ri p p i n g s y s t e m is k n o w n a s t h e c hs y s t e m .

Rece ip t o f back- t r ipp ing by the d i sc r imina t ing re lays assoc ia ted wi th ind iv idc i rcu i t b reakers is qua l i f ied by aux i l i a ry swi tches on the ap prop r ia te busb ar se lec

d i sconnec to r s in o rde r tha t the requ i red c i rcu i t b reakers , and no o the r s , a re t r ippThe bas ic conn ec t ion s o f the b ack- t r ipp ing fac i li ty a re show n in F ig . 13 .6 .



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13 .6 .9 Tes t fac i l i t ies

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140 Busbar pro tection

cut-off" (p .c .o . ) swi tches and then swi tching on the tes t suppl ies , the la t ter beinter locked thro ug h the p .c .o , swi tch 'off ' con tacts . Th ere is e i ther a separcu t-off swi tch for each discr im inat ing zone or a s ingle sw i tch for cut t ing -off whole ins ta l l a t ion . Usua l ly, ind ica t ing lamps on the busbar p ro tec t ion pa

opera ted by the p .c .o , swi tches show w hethe r the p ro tec t ion i s in o r ou t o f servand in large substa t ions remote indicat ions are a lso given in the control room.

With the main tes t swi tch in posi t ion 2 and a l l the protect ion cut-off swi tcope n , an ou tp u t f rom the t es t t r ans form er can be in jec ted to check each c r imina t ing and check zone re lay in tu rn by opera t ing the appropr ia te t es t swi tthe se t t ing cur ren t and vo ltage be ing no ted on the ins t rum ents A and V . By openl ink D, the se t t ings of individual re lays can be checked. When on load the re

spill curren t is read o n the am m eter A b y changing over the m ain tes t sw i tchpos i t ion 1 and l ink D to the do t ted pos i t ion . The cu r ren t f rom the buswire untes t, M1 in th is case , then f lows to the t es t buswire , th roug h the am m eter to re tt o t he com m on wi re a s shown by the do t ted a r rows . The ou tpu t f rom the c .t .s be s imi lar ly read by changing over l ink B. Before the l ink se lect ions , descr ibabov e, can be carr ied ou t , i t is necessary for the sh or t ing links A and C to be c loand opened again as appropr ia te , to avoid open-ci rcui t ing.

To obta in the magnet isa t ion character is t ics of any c . t . the main tes t swi tch

turned to posi t ion 2 and l ink B changed to i ts do t ted posi t ion. In th is case tp r imary c i rcu it m us t be o ff load .

O ther faci li ties to help tr ip test ing are desc ribed in the sect ion covering tr ippcircui t ar rangements .

13 .6 .10 Fau l t s e t t i ngs

(a) E arth faults: With a res is tance ear thed sys tem, the faul t current wi l l bcontrol led by the value and number of neutra l res is tors in service . I f poss ible neutra l ear th ing res is tor c i rcuit sh ould be conn ected to each sect ion of the busbThe ear th -fau l t se t ting of the busbar p ro tec t ion should be f rom 10-30 per cen tthe smal les t expected faul t current , to ensure high speed re lay operat ion.

I f i t i s not poss ible to obta in such a low re lay se t t ing, then l imi ta t ions may hato be p laced on the sys tem opera t ing condi t ions , whereby the requ is i t e numberneutral resistors are maintained in service.

With sys tems having sol id mul t ip le ear th ing, such as the 132 and 275 kV grsystems, the ear th-faul t current i s comparable to the phase faul t value , and so earfault set t ings are usually not diff icult .

(b) Phase faults: The value of the current which f lows for two- and three-phasbusba r f au lt s depends on t he num ber o f i n feeds t o t he busba r and the am oun tgenera t ing p lan t running . The lowes t va lue may be when the major i ty o f t

genera ting p lan t is shu t dow n dur ing the n igh t , bu t lower va lues o f cur ren t mar ise i f the o nly infeed to the bu sba r i s a s ingle long feeder. I t m ay no t a lways



Busbar pro tectio n 141

accep tab le r isk i f i t is kn ow n tha t i t is a s itua t ion th a t w i ll occu r ve ry ra re ly. Othwise , back-up p ro tec t ion wi l l have to be p rov ided to c lea r such a fau l t i f i t i s pa r t o f t h e p r o t ec t i on a l r eady r equ i r e d .

In genera l , theoverall fau l t se t t ing shou ld be b e tw een 10 and 50% of the min

m um fau l t cu r ren t ava il ab le , a range wh ich ca te r s fo r long t e rm changes as power sys tem deve lops and fo r shor t t e rm changes due to swi tch ing and the genet ion capac i ty requ i red a t any par t i cu la r t im e . Theoverall fau l t se t t ing is de te rm inedby the h ighes t o f the ind iv idua l check and d i sc r imina t ing zone fau l t se t t ings .

F o r un i f o rm i ty o f app l i c at ion and t o avoid ce r ta i n p rob l ems wh ich w i ll be fe r red to sho r t ly, the fau l t se tt ings o f ind iv idua l p ro tec t ion zones shou ld norm am ee t t h e f o ll o wing r equ i r em en t s , a l ways b ea ri ng in m in d t h e f o reg o ing co m m eso t ha t o n ly e xcep t iona l l y w i ll th e s e t ti ng ex ce ed 50 % o f t he m in im u m f au lt cu r ravai lable f rom the c i rcui t providing the lowest infeed:

(a ) The fau lt se t t ing o f the check zone and the min im um fau l t se t t ing o f tind iv idua l d i sc r im ina t ing zones shal l norm al ly be equa l to ap pro x im ate ly 5of the fu ll load cu r ren t r a t ing o f the assoc ia ted bus bar.

(b ) The m ax im um fau l t se t t ing o f the ind iv idua l d i sc r imina t ing zones sha ll n

norm al ly exceed the fu ll load ra ting o f the assoc ia ted bu sbar.

(c ) W hen two or more sec t ions o f busbar a re conn ec ted toge ther v ia sec t id i sconnec to rs , the cor responding d i sc r imina t ing zone buswi res a re pa ra l l e l cnec ted th roug h the bus sec t ion d i sconn ec to r aux i l ia ry swi tches . W here the com bindiscr im inat ing zo ne thu s form ed w ould o therw ise g ive r ise to a faul t se t tingexcess o f the fu l l load cur ren t r a t ing o f the assoc ia ted busbar, the min imum fa

se t t ing o f the ind iv idua l d i sc r imina t ing zones sha l l be reduced be low the va lspec i f i ed in (a )above .

Refe rence has been m ade to the reduc t ion in p r imary fau l t se t ting tha t can resf rom ado p t i ng m ore t han one ch eck zone. A po in t t ha t m us t be bo rne i n m indtha t a t those sub s ta t ions w here the reserve busbar con ta ins n o p e rm ane n t b reaks ,rese rve d i sc r imina t ing zon e ( i f the re i s on ly o ne fo r the s t a t ion ) o r the co m bine d

serve d i sc r imina t ing zones (w here the re a re sec t ion d i sconn ec to rs and these a re c loscan a lso be h igh. In fac t , i f a l l c i rcui ts are se lec ted to the reserve busbar, there wusua l ly be m ore c . t. s in pa ral le l than w ould be con nec ted to a sing le overa ll chzone because the re wi l l be bus coup le r c . t . s connec ted to the rese rve d i sc r imina tzones b u t no t to the check zone , r esu l t ing in the fo rm er hav ing a s l igh t ly h igp r im ary s e tt in g t han t he l a t t e r. A l t hou gh t h i s w o u ld app ea r t o d e t r a c t fr om advan ta g e s o f em p loy ing m ore t ha n on e ch eck z one , it w i ll n e a r ly a lwa ys b e case th a t , w i th a ll c i rcui ts se lec ted to the reserve bu sba r, the fau l t level wi l l exce

t he m ax im um p r imary f au lt s e t ti ng o f t h e ( co m bi ned ) r e se r ve d i s c rim i na t ing zoby an adeq ua te m arg in . Wi th fewer c i rcu i t s se lected to the rese rve bus bar, the fa




where the se t ting is cons tan t and there fore m us t be less than 50% of the min imfau lt in feed to an y on e busbar sec t ion .

The aba ndo nem ent o f buswire short -c i rcu i ting m ent ioned earl ie r impol im i ta t ions on the fau lt se t ting and cur ren t ra ting of secon dary equ ipm ent . T

secondary current f lowing in the re lay c i rcui t wi th an open-ci rcui t current t raformer secondary connec t ion i s re la ted to the p r imary cur ren t and there fore ccorresp ond to full load. The re lay(s) , ser ies and shu nt res is tors including nonl inres is tors mu st the refore be ra ted to carry th is curren t cont inu ou sly since i t m ayan apprec iab le t ime before the fau l ty connec t ions can be loca ted and repa i redthe faul t se t t ing is hal f the ra ted current of the largest c i rcuit these com po nemust wi ths tand twice the se t t ing vol tage cont inuously. This can usual ly be achievbut , s ince the power diss ipated by the res is tors for the condi t ion considered wi l lvery large , adequa te ven t i la t ion m ust be provide d. W here the faul t se t t ing is lthan ha l f the ra ted curre nt of the largest c i rcui t or of the busba rs , w hichever is greater, the solut ion is m ore di fficul t and the ins ta l la t ion mu st be des igned w i ththese cons t ra in t s t ake n in to ac cou nt .

Idea l ly, the faul t se t t ing o f bu sbar pro tect io n should be greater than the fload current of the larges t c i rcui t , so that i f a .c . t , becomes open-ci rcui ted no reoperat ion would occur. This i s not a lways poss ible wi th res i3tance ear thed sys te

The sensi t ive supervision relays f i t ted to detect open-circuited c. t .s usually havse t ting of 1 -5% of the swi tchgear ra ted cur ren t .

In the h igh-impedance c i rcu lat ing cur ren t scheme of busbar p ro tec t ion , becauthe re lays require only 10-30 mA, the id le c . t . magnet isa t ion current decides tfau lt se t ting of the scheme. T hus , fo r schem es cover ing on ly ea r th fau l ts , wheach c i rcui t 's c . t. s are para l le led, the fault se t ting wi l l be ap pro xim ately three t imhigher than that of a schem e cover ing bo th phase and ear th faul ts , s ince in t

case on ly o ne th i rd of the c . t. s are para lle led on to a re lay e lem ent , as i t now ocovers one phase .

13.6.11 Stab il i ty l imits

Busbar protect ion schemes are des igned to remain s table on external c lose-two - and three-phase faul ts up to the ra ted current break ing cap aci ty of the swi tgear be ing pro tec ted . For ea r th fau l t s on mul t ip le so l id ly ea r thed sys tems , a t v

large substa t ions , the ear th-faul t current may be even larger than for phase fauland d ue a llowance m ust be m ade for th is . On res istance ear thed sys tem s, par t iculawh ere l iquid ear th ing res istors are f i t ted , a l lowan ce m ust be m ade for the po ss ibiof thei r f lashing over, so perm it ting m uc h larger ear th-faul t curren ts to f low.

13.7 Circuit breaker fail pr ote ct io n

13.7 .1 Pr inc ip le of opera t ion




1970s. I t s purpose is to deal wi th the s i tuat ion in which a c i rcui t breaker fa i lsin te r rup t the cur ren t which i t i s ca r ry ing in sp i te o f the opera t ion of a t r ip re lA m on g the poss ible reasons are :

(i)

0i)

(iii)

F a i lu re o f the t r ip com m and to reach the c i rcu i t b reaker t r ip co il . Th is isunl ikely cause where dupl icate t r ip coi ls are f i t ted and Mark I I s tandardsd.c . c i rcui t ry are em plo yed to ensure t ripping in the event of the fa i lure an y s ingle device or su pply .Fa i lu re o f the c i rcu i t b reaker mechanism due to an e lec t r i ca l o r mechanifault .Fa i lu re o f the c i rcu i t b reaker cur ren t in te r rup t ion dev ice due to a defec t

the inadv er tent op era t ion o f the c i rcui t break er o uts ide it s l im i ts o f pfo rmance .

The po l icy of p rov id ing two fau l t-de tec ting sys tems is app l ied to feeder p ro tec tand, where poss ible , p lant protect ion. The arrangements are such that , as far aspracticable no single secondary electr ical fai lure should result in an uncleasystem faul t . In general , dupl icat ion ceases a t the c i rcui t breaker t r ip coi ls . Tposs ibi l ity tha t a c i rcui t brea ker m ay fa il to pe rform i ts func t ion , w hen ins t ructo d o so , is h igher than for the pro tect io n and t r ip c i rcui ts . This is due to the ftha t m echanica l, pneum at ic o r hydrau l ic t r ipp ing m echanisms and in te r rup tors no t be dupl ica ted fo r p rac t ica l and economic reasons . The ava i lab le methodssys tem back-up p ro tec t ion a re general ly inadeq ua te to dea l w i th the s i tua t ion w hw ould ex i s t due to the fa ilure o f such com pon ents and the re fore c i rcu it b reaker p ro tec t ion has been deve loped for th is purpose .

The bas ic p r inc ip le o f th is p ro tec t ion is the m easurem ent o f the du ra t ion of fa

cur ren t f rom the ins tan t a t which any one t r ip re lay opera tes to t r ip the c i rcbreaker. I f , a t the end of a preselected t ime delay, current i s s t i l l f lowing, i t considered that the c i rcui t breaker has fa i led to t r ip and the t r ipping of a l l o thc i rcu i t b reakers connec ted to the connec t ions on bo th s ides o f the c i rcu i t b reawil l be in i t ia ted . A t dou ble busb ar substa t ions th is is effected b y m eans of back- t r ipp ing sys tem, whi le a t mesh and o ther types o f subs ta t ion , c i rcu i t -b reafail d .c . t r ipping c i rcui ts m ust be provide d.

Circui t breaker fa i l protect ion is f i t ted to each c i rcui t breaker and comprises tcur ren t check re lays and fou r t im er e lements ( tw o a t m esh subs ta tions ) .

A s impl i f ied c i rcui t d iagram for c i rcui t breaker fa i l protect ion is shown in F13 .7 .1A f rom which cer ta in dupl ica ted re lay e lemen ts have been o m i t ted s impl ic i ty as have the b ack - t r ip chec k busw ires and associa ted receive re lay.

Detec t ion of the c i rcu i t b reaker fa i l condi t ion i s governed by the cur ren t cherelays , which are s ta t ic ins tantaneous overcurrent re lays; these are only permitto opera te fo llowing opera t ion of one or m ore o f the c i rcu it t r ip re lays and he

coincidenta l ly wi th the energisa t ion of the associa ted c i rcui t breaker t r ip coiA l though the c . t. seconda ry cur ren t passes th rough the re lay when ever p r im a




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F i g . 1 3 . 7 . 1 A Sim pl i f ied d iagram of b reaker fa i l p ro tec t ion fo r a 40 0 kV double busbarsubstation

Assum ing that the c i rcuit brea ker has fa iled to c lear a faul t fo l low ing ope rat iof a t r ip re lay, the fol low ing sequence of events wi ll occur :

The d .c . c i rcui t of the cu rrent check re lay wil l be energised, and i f the sec ondcurrent is in excess of the set t ing, the relay contacts wil l close, in turn energisthe t ime delay re lay f rom the d .c . supply to the t r ip coi l .

At the exp ira t ion o f the t ime de lay, the c urren t check re lay is s ti ll energised b y tun interr up ted faul t curre nt . Posi tive and negative supplies are con ne cted to the ba

t r ip d iscr iminat ing buswires associa ted wi th the busbar to which the fa i led c i rcbrea ker is se lected. Energisa t ion of these bus-wires t r ips a ll o th er c i rcui ts conne cto the sam e busb ar. Rec eipt of back- t r ipping s ignals by individual c ircuit breakis effected via the sam e busbar se lector d isco nne ctor aux i liary swi tches as w ouldused by th at c i rcuit for back- t r ip in i t ia t ion.

13.7 .2 Precau t ions against m alop erat ion

At a double busbar substa t ion where c i rcui t breaker fa i l protect ion operates inthe b ck t r ipp ing s s tem the conseq ences of n incor rec t oper t ion re co



Busbar prote ction 145

therefore adopted to achieve a h igh secur i ty agains t such incidents :

i )

(a)

Tw o cur ren t check re lays a re em ployed and bo th m us t opera te before tt ime de lay re lays a re energ i sed and bo th mus t remain opera ted fo r the bac

t r ip busw ires to b e energised.Two t ime de lay re lays mus t opera te before the back- t r ip buswires aenergised.

These points are shown in the diagram of the d .c . connect ions for a feeder c i rcin F ig. 13.6.8 A.

13 .7 .3 Cur ren t che ck r e lay s e t t i ngs

The se t t ing for the curre nt chec k re lay m ust be se lected so tha t posit ive op eratis ensured for faul ts wh ich occu r a t the e lectr ica l ex t rem ity of the pro tecte d c i rca t min imum p lan t condi t ions .

Ideal ly, in order to obta in minimum operat ing t imes for c i rcui t breaker fp ro tec t ion , the cur ren t check re lay se t t ing should be above the res i s to r cur ren tthe c i rcuit brea ker (w here res is tors are f i t ted) , so that the re lay resets as soo n as main arc i s ext inguished. This shor tens the t ime a l lowed to e lapse before tdecis ion is mad e tha t the c i rcuit b reak er has fa iled . The disadvantage of th is polis that one poss ible fa ilure m od e of som e designs of c i rcui t brea ker i s that in w hthe main con tac t s in te r rup t the fau l t cur ren t bu t the res i s to r con tac t s do no t b rethe res is tor current , which cont inues to f low causing the res is tors to burn out . Sudam age to the c i rcui t breake r i s a hazard to the sys tem s ince i t m ay lead to in ternal f lashover to ear th (e i ther as a busbar fa ul t or c ircui t faul t or bo th) a nd ev

if th is does not occur the c i rcui t breaker wi l l be unl ikely to in terrupt subsequefaul ts sa t is factor i ly. Therefore , i t i s preferable that the current check re lay se t t ii s not more than two-thi rds of the res is tor current ( to provide a margin for re l iaop era t ion) even tho ug h th is en ta i ls the ad op t ion o f t ime r se tt ings giving longoverall operat ing t imes .

A nothe r fac tor m us t be cons idered in the case o f c i rcu it b reakers which swigenerators . In such ins ta l la t ions , the re lay se t t ing must be low enough to cater the s i tuat ion in which the c i rcui t break er fa ils to t r ip fol lowing the dete ct ion oboi le r, tu rb ine o r o ther none lec t r i ca l fau l t condi t ion . The resu l tan t cur ren t d rafrom the t ransmiss ion sys tem is smal l in the per iod immediate ly fol lowing c i rcbreake r fa ilu re w hen there is no s team input and the exc i ta t ion has been suppressThe se t t ing should be suff ic ient ly below this curren t (wh ich var ies w i th m achis ize and des ign) to ensure re lay op era t ion.

13 .7 .4 C i r cu it b r eake r f a il t im er s e t t i ngs

The se t t ing appl ied to the t im e de lay re lays is governed by the fol low ing factors :



146 Bu sb ar rotect ion

2 8 0 ~

2 6 0 ,

2 4 0 -

2 2 0 -

2 O0

1 8 0

1 6 0

1 4 0

1 2 0

I 0 0

8 0

6 0

4 0

2 0 "

. . .

p.

26.';

,

v- £

d

0

V H S I ' 0 l R e m o t e c .b . s .

;v i a i n t e r t r i p

( . ' a r r i e r

L o c a l c . b . s , v i a b a c k t r i p

T ~ t a l c l e a r a n c e t i m e s> fi~ r 4 0 n t s c . b . s ,

B a c k t r ip r e c e i v e t r ip r e l a y , J p e r a t e d

B ,a ck t r i p r e c e i v e r e l a y s & i n te r l , ~ c k e t l c u r r e n t t r i p r e l a y ~ p e r a t e d

( ' .B . f ai l t i m e r ~ p e r a t i ~ n c = ~ m p le te d - b a c k t r i p i n i t i a t e d

C . B . f a i l c u r r e n t c h e c k re la ~ d r ~ p - , K f " c . b . f a il t i ,l lc r s t,~ p p e d

C . B . t r i p ( ~ p e r a t i o n c , ) m p l e t e d

- C . B . f a il ti m e r s ta r t e d - c u r r e n t c h e c k r e la y c o n t a c t s o p e r a t e dC i r c u i t p r , ~ e c t i o n t r i p r e l a y ,~ p e r a te d - c .b . t r i p in i t i a t e d

- ' - - " C i r c u i t p rc J te c ti ,J n o p e r a t i o n

S y s t e m f a u l t

F i g . 1 3 . 7 . 4 A Ti m e s eq u en ce f o r c i r c u i t b r e a k e r f a i l p r o t e c t i o n - 4 0 0 k V w i t h t w o - c y c l e



Busbar pro tection 14 7

t ion o r res i s to r con tac t separa t ion , depending on the c r i t e r ion adopted fo r se t tthe cur ren t check re lay. Th is t ime mus t be tha t app l icab le under maximum shocircui t co ndi t io ns a t n orm al t r ip coil voltage an d, w here appl icable , no rm al pressure.

(b ) The min im um a llowable marg in o f t ime necessa ry to ensure d i sc r im ina tbe tw een the cor rec t ci rcu it b reak er t r ipp ing ope ra t ion and in i t ia t ion o f back- t r ipp(or equ iva len t ac t ion a t mesh subs ta t ions e tc . ) due a l lowance be ing made fo r sca t t e r o f opera t ing t imes o f re lays and c i rcu i t b reakers .

(c ) The m ax im um fau l t c lea rance t ime accep tab le f rom loca l back-up pro tec t( in th is case the c i rcu i t b reak er fai l p ro te c t ion) . A co m m on f igure a t the p rest ime is 300 ms . F ig . 13 .7 .4A show s a typ ica l opera t ing t im e sequence fo r the c i rb reaker fa i l p ro tec t ion and the resu l tan t loca l back- t r ip and remote in te r tfunc t ions . A d i sc r imina t ing m arg in o f 60 ms is typ ica l and has been a l lowed in case, r esu lt ing in a t ime se t ting o f 105 m s w hen , as he re , the n om ina l opera t ing to f the c i rcu i t b reak er is 40 ms .

The t ime delay re lay se t t ing is der ived f rom the fo l lowing const i tuent t imes:

( i ) The m in im um c i rcu i t b reaker t r ip opera t ing t ime (e i the r to main a rc ex t inc tor to res is tor contact separa t ion as d iscussed ear l ier ) for the par t icular typeci rcui t breaker, p lus

( ii ) the cur ren t ch eck re lay m ax im um d rop .off t ime fo r the par t i cu la r re lay ty(assumed as 10 ms fo r the p urpose o f F ig . 13 .7 .4A) , p lus

( i ii ) a d iscr iminat ing m argin o f 60 ms to a llow for scat ter in c i rcui t bre ak er a

re lay ope ra t ing t imes , less( iv ) The cur ren t che ck re lay m in im um p ick-up t ime fo r the par t icu la r re lay ty(assumed as 5 ms fo r the purpose o f F ig . 13 .7 .4A) .

Should the es t imated fau lt c lea rance t ime be in excess o f the a l lowable m axim umreduc t ion in the d i sc r imina t ing t ime marg in may be accep ted p rov ided due cs idera t ion is g iven to the poss ible var ia t ion in the opera t ing t imes of the c i rcbreaker and associa ted protect ive re lays .

13 .8 Te rmino logy

The separa te func t ions o f busba r p ro tec t io n , c i rcu i t b reake r fa il p ro tec t ion aback- t r ipping have been descr ibed. Whi le , in many ins ta l la t ions a l l three wi l l exand wi l l be c lose ly re la ted bo th in t e rms of accommodat ion and e lec t r i ca l cnec t ions care is required to avoid the use o f loose descr ip t ion s w hich ignore



148 8usbar prote ction

13.9 Bibliography

BooksThe protect ive gear handbo okby F E Wellman (Pitman, 1968)

Pro tective rela ys: their theory and practice(V ol.1) by A R Van C W arrington(Chapman & Hall , 1962)Protective current transformers and circuitsby P Mathews (Chapm an & Hall,1955)Pro tective relays application guide(GEC Measuremen ts, 1975)

Articles'Busbar protect ion' by I A R eid,(Electr. Rev.June 1957)'Instantaneous bala nc ed current p rotection' by J R ushton and F E WeU(Metrovick Gazette,May/June 1951)'Busbar protect ion' by F L Ham ilton(Reyrolle Rev.Spring/Sum m er 195 8)'Rec ent develop m ents in bu sba r protect ion' by H D Nunney (I EE ColloquDigest No. 1 968/19, p. 69)



Protection of motors, reactors,boosters and capacitors

by P.M.Dolby

C h a p t e r

1 4 . 1 I n t r o d u c t i o n

This chap ter deals w i th the general character ist ics o f m oto rs , reactors , boo sters acapaci tors, w i th the appl icat ion o f such plant to a c losely in terco nn ected po wsystem and with the se lect ion and appl icat ion o f sui table auto m at ic protect iequ ipm ent for the p lant concerned . To specify adequate pro tect io n i t is necessato have an apprecia t ion of the way in wh ich the plant is con structed , it s charactis t ics and how i t i s incorporated in the system, and so a s ignif icant proport ion the chapter is devoted to these aspects.

The main func t ion of au tom at ic p ro tec t ion i s the de tec t ion o f a fau lt condi t ioand, through the opening of the appropr ia te c i rcui t breakers , the disconnect ionthe faul ty i tem of plant f rom the remainder o f the system. Co upled with th is is need to l imi t to an abso lu te min im um the damage caused to the a ffec ted equipm e

W hils t d i scr imina t ion i s, wi tho ut dou bt , the m os t im po r tan t requi rement o f aprotect ive system, the need for rapid operat ion is f requent ly a considerat ion a lmost equal importance, bear ing in mind the need to minimise damage, to saguard system stabil i ty and to reduce as much as possible the r isk to l ife and l imb.

1 4 . 2 M o t o r s

1 4 . 2 . 1 C h a r a c t e r is t ic s o f d . c . a n d a . c. m o t o r s

D.C. Mo tors

D.C. motors are c lass i f ied as ser ies , shunt or compound motors depending upthe way in which the f ield winding is con nec ted wi th respect to the arm ature wiing, tha t is the m eth o d of exc itat ion used (see Fig. 14.2.1A ). The characterist icsthe mo tor a re de te rm ined by the m etho d o f exc i ta t ion .

In a d .c . mo tor the a rmature cur ren tla adjusts i t se l f to produ ce a torqu e balance,so tha t the torqu e developed is equal to the to ta l opposing torqu e, th is opp osi



150 Pro tect ion o f m otors, reactors , boosters and capacitors

Vl 4 - - -

E

M

V

( a ) S e r i e s e x c i t a t i o n ( b ) S h u n t e x c i t a t i o n

V

Fig . 14 .2 .1A

(c) Compound exc i ta t ion

M e t h o d s o f e x c i t in g d . c . m o t o r s

torqu e of the load. Dev eloped torqu e, as the nam e impl ies, is the torq ue del iveto the shaf t and avai lable to do useful work .

I f the f lux q~ is sensibly constant , as wi th the shunt motor,la will vary as thetorq ue T. If • varies w ithI a, as wi th the series and c om po un d m otors , there la t ionship betweenla and the torqu e is no longe r linear.

The equat ion for the back e .m.f , induced in a d .c . armature winding is

E - C N ~ b

w here N is the spee d, • is the f ield f lux and C is a co ns tan t for the pa rt icum achine c onc erned . The bac k e .m.f . , how ever, is a lso given by

g = V- Ia R a +R sf ) - V b

where V is the suppl~, vol tage, usual ly assum ed con stan t ,R a is the armature resist-ance, R s f is the resistance o f any series f ield w inding a nd l /'b is the brush volta

drop, usual ly of the order of two or three vol ts .Since the voltage-drop in the armature circuit including any series f ield circui



P rotec tion of m otors, reactors, boo sters an d capacitors 151

s tan t and hence the p rodu c t o f N and • is a lso reasonably cons tan t .The deve loped to rque is p rop or t iona l to the p rodu c t o f the flux • and t

a rmature cur ren tIa.

[ -

q u eE

z"

0 Ill ' 11 '

A r m a t u r e c u r r e n t I a

F i g .14.2.1 B S p e e d a n d t o r q u e c h a r a c t e r i s t ic s o f a d . c. s e rie s m o t o r

(a) Th e series m otor. In the ser ies m oto r, the f lux • is pro du ced b y the arm ature cur ren t la , the speed and to rque charac te r is ti cs o f the m oto r, p lo t ted as funt ions of the arm ature curre nt , being as sho w n in Fig . 14.2 .1B. I t wi ll be seen thw h e n la and there fore ~ , i s smal l, the speed of the m oto r is ve ry h ig h . At lovalues ofla, ~ i s d i rect ly propor t ional toIa and the torque increases as the squareof the arm ature curren t . A t h igher values ofIa the e ffec t s o f sa tu ra t ion a ndarmature react ion cause cI , to become re la t ively constant , the torque then increapro po r t iona l ly wi th the arm ature current . At s ti ll h igher values ofIa, thepro no unc ed e ffec t o f a rm ature reac t ion resu lt s in an ac tua l reduc t ion in • fo r increase inla , w ith the consequence tha t the to rq ue then increases a t a lower rathan the a rmature cur ren t .

Because of i t s very high speed a t low loads , i t i s important that a ser ies motshould never be s ta r ted wi tho ut load . Fo r the same reason , a m oto r o f th is tyshou ld no t be used i f there is a po ss ibil ity o f the ent i re load being los t whrunning.

( b ) T h e s h u n t m o t o r : Inthe shunt m oto r, the f lux • is p rod uce d by the cur renIf in the shunt- f ie ld winding, the current I f being sensibly constant for a constasupply vo l tage V. The speed an d to rq ue charac te r is t ics o f the m oto r a re show nFig. 14.2 .1C, the m oto r having a s l ight ly droo ping spe ed/c urren t character is tAssuming constant terminal vol tage, an increase inIa will result in increases inarmature react ion and to ta l vol tage-drop, and s ince these la t ter two quant i t ies haoppo s i te e ffec t s o n the m oto r speed , they will t end to cance l each o ther to g ivealm ost con stan t speed character is tic . In the norm al case , the vol tage-drop effepredo m ina tes so tha t the speed c ur ren t charac te r is t ic has the gen t ly d roop ing foshown .

I t w i l l be no ted tha t the to rque varies a lm os t l inear ly wi th a rm ature cur renexce p t a t the h igher values o f a rm ature cur ren t where the e ffec t s o f a rm atu



152 Protec tion o f m otors, reactors, boosters and capacitors

Fig . 14 .2 .1C

ed

i • i i l |

A r m a t u r e c u r r e n t I a

Speed and torque characteristics o f a d.c. shun t mo to r

A method of con t ro l l ing a shunt motor i s shown d iagrammat ica l ly in F i14.2 .1D from w hich i t w i l l be seen tha t one end of the f ie ld winding conn ec ted d i rec t to the supply and the o ther end con nec ted v ia a f ie ld rheos tW hen i t is required to increase the speed o f the m ot or th is is achieved by weakeing the f ie ld by increas ing the amount of res is tance connected to i t .

An a l t e rna t ive method of con t ro l l ing a shunt motor i s to employ a var iabres is tance in ser ies w i th the arm ature . The res is tance m ust , of course , be sui tabfor con t inuous energ i sa t ion , bu t the method i s no t a par t i cu la r ly sa t i s fac tory obecause of the relat ively heavy losses in the resistance.

1 I_ O

Field Fieldwind ing rheos ta t

F ig . 14 .2 .1D Simple method of speed cont rol of a d.c. shunt moto r

(c ) The compound m oto r : Fig. 14 .2 . tA shows that the compound motor i sprovided w ith both a series and a shun t w indin g and tha t the characteris tics of themotor depend chiefly upon the relat ive s trengths of the m.m.f .s produced by thetw o f ield w indings. If the series w indin g is connected so that i t produces an .m .m.f .

in the same di rec t ion as the shunt wind ing the m otor is know n as a cum ulat ive lycompounded m otor ; i f the connect ion i s such tha t the series wind ing m .m.f .



Pro tectio n o f m otors, reactors, boosters an d capacitors 153

( a ) ' S h or t- s h u n t ' c o n n e c t i o n ( b ) ' L o n g - s h u n t ' c o n n e c t i o n

Fig . 14 .2 .1 E A l te r n a t iv e c o n n e c ti o ns o f a c um u l a ti ve c o m p o u n d m o t o r

By ad jus ting the ser ies f ie ld o f the d i ffe ren t ia l ly co m po un de d m oto r so tha t tspeed a t fu l l - load is equal to the speed a t no- load then the speed a t any in tm edia te load will be app rox im ate ly con s tan t . Th is is no t as advantageous as i t mseem s ince there is a t endenc y for th is type o f m oto r to s ta r t up the wrong wThis i s possible because of the in it ia l rush of current thro ug h the ser ies f ie ld aarmature , the shunt f ie ld taking ra ther longer to bui ld up than the ser ies f ie ld . Whboth f ie lds are ful ly es tabl ished there may be only a re la t ively smal l net t f ie ld , ahence to rque , to d r ive the motor.

A no ther d isadvantage is tha t i f over loade d, the resul ting decrease in f lux wtend to fo rce up the speed so tha t the m oto r is over loaded still m ore . I f the cur rreaches a cri t ical value the motor wil l begin to race in much the same way as a serm oto r depr ived of load . Thus the d i ffe ren tia lly com po un ded m oto r should be uon ly in c i rcum stances w here there is no l ikel ihood o f over loading.

In the cum ula tive ly com po un ded m oto r an increase in the load app lied produan increase in f lux and a corresponding decrease in speed, a l though the constaf lux pro du ce d b y the s hu nt winding prevents th is decrease being as rapid as in p la in ser ies m oto r. Th us i ts speed / torqu e character is t ic wi ll be ap pro xim atelyshow n in Fig . 14 .2 .1F, i t s pos i t ion re la tive to the character is t ics of the ser ies a

I

¢)

= I

11111

0 , 1 . . . . .Load cu r ren t

F i g 1 4 2 1 F

D i f f e r e n t i a l c o m p o u n d

S h u n tC u m u l a t i v e c o m p o u n dSeries

C i f d / t h t i t i f d i h t d d



154 P rotec tion o f m otors, reactors, boosters an d capacitors

shun t m otors depending up on the num ber o f tu rns on the ser ies wind ing . advan tage o f the cumula t ive ly compounded motor i s tha t the speed fa l l s whensudden , heavy load i s app l ied , thus he lp ing to o ff se t the burden of the inc reaseload.

( d ) C o m p ar is on o f s e ri es , s h u n t a n d c o m p o u n d m o t o rs :A compar i son o f speed /cur ren t and to rq ue /cu r ren t charac te r i st ic s o f the se ries , shun t and co m po um oto rs i s show n in Figs . 14 .2 .1F an d 14.2 .1G. I t wi ll be seen f rom Fig . 14.2 .

r J

G

Load cu r ren tF i g . 14 .2 .1G Com par i son o f to rqu e /cu r ren t cha rac te ri s ti c s o f d .c . ser ie s, sh un t and co m po un d

m o t o r stha t the shun t m oto r charac te ri s ti c li es be tw een those o f the d i ffe ren t ia lly cum ula t ive ly com po un ded machines . The sign if ican t d i ffe rence in charac ter i sob ta ined by connec t ing the se r ies wind ing of the compound motor e i the r to as

or to o ppose the shun t w ind ing should a l so be no ted .A summ ary o f the charac ter i st ic s o f the th ree types o f mo tor is g iven be low.

Seri e s m oto r : (j)

0 i )i i i )

High s tar ting torq ue varying as the square of thcurrent a t low current values .High speed a t low values of load.Decrease in speed as torqu e increases .

S h u n t m o t or : ( j)

( i i )

Prac t ica l ly cons tan t speed f rom noqoad to fu lload .Prac t i ca l ly cons tan t f lux and the re fore to rquepropor t iona l to the a rmature cur ren t .

C o m p o u n d m o t or : D i f fe r e n t ia l( i) Ap prox im a te ly cons t an t speed f rom no - load t

fu l l4oad.( i i) Ov er loading tend s to cause rapid increase in speed



Pro tect ion of m otors , reactors , boosters and capaci tors 155

C u m u l a t i v e( i ) S teeper decrease in speed w i th increas ing torq ue

than i n t he shu n t mo t o r.( ii ) Spe ed /cur ren t and to rq ue /c ur re n t characte ri s ti c s

l i e be tween those o f the se r i es and shun t motors .

A . C . m o t o r s

P o l y p h a s e i n d u c t i o n m o t o r s :Induc t ion motors fa l l in to two d i s t inc t ca tegor iesnam ely the squ i rre l cage m oto r and the w ou nd ro to r, o r s lip- ring , m oto r. As name impl ies , bo th opera te on the induc t ion p r inc ip le , on ly the p r imary s t awind ing be ing connec ted to the supp ly.

The squ i rre l cage m oto r is o f t en iden t i f ied wi th it s inheren t ly po or s t a rtqual i t ies , but i t wi l l be seen la ter tha t measures are taken to improve i t s perfoance in th is r espec t . In a squ i rre l cage m oto r the ro to r condu c tors a re copp er rla id in s lo ts in the ro tor core and r ive ted or brazed to a shor t -c i rcui t ing r ing a t eend, g iving the ap pearan ce of a cage. The absence of sl ip- rings ensures tha t spa rkwi ll no t occ ur and the cage cons t ru c t ion g ives a cheap and very robus t m oto r. d i sadvan tages a re low s ta r ting to rqu e , h igh s t a r t ing c ur ren t a nd d i ff i cu lty in p ro

ing an easy means o f con t ro l .The s lip- ring m ot or has a 'w ou nd ' ro to r, tha t is a g roup o f co il s fo rm ing windings i s car r ied on the ro tor i tse l f. This m akes i t m ore expensive tha n squir re l cage motor, but i t has the advantage tha t i t develops the same s tar tto rque as a comparab le squ ir rel cage mo tor wi th a cons ide rab ly lower s t a r t ing ren t . Unl ike the squ i r rel cage ro to r, the to ta l r e s is t ance o f the wo un d m ot or is f'L xed bu t can be var ied by ad jus tm en t of the externa l resis tors co nn ecte d in ro to r c i rcu i t dur ing s t a rt ing . An inc iden ta l advan tage o f the wou nd ro to r is t

m uch o f the hea t genera ted dur ing s t a r ting is d i s s ipa ted in the res is to rs . Sm oaccelera t ion is achieved i f the res is tance can be red uced in smal l s teps f ro m 'a l l in ' to the ' a l l out ' condi t ion .( a ) T h e s q u ir re l c ag e m o t o r :Fig. 14 .2 .1H shows the to rq ue / sp eed curves o f squ i rre l cage m oto r. Dur ing s ta r t ing the deve loped to rq ue fo l lows the upp er cuun t i l the m oto r a t t a ins a speed a t which the deve loped to rq ue is equa l to the lto rque . The m ot or wi ll r each it s ra ted speed i f the load to rqu e is no t in exces

the deve loped to rque .The to rque i s p ropor t iona l to the t e rmina l vo l t age squared , and a l though som e dr ives the squi r re l cage m o to r can be co nn ecte d d i rec t - to- line , i t i s ofnecessary to reduce the term inal vol tage a t s tar t ing as a m eans o f reduc ing the sing cur rent and the vol tage drop in the conne ct ions . This i s par t icular ly appl icawhen only a smal l s tar t ing torque i s necessary s ince the effec t i s to reduce s t a r t ing to rque avai lab le . The m etho ds used inc lude the inse r t ion o f

( i) ser ies resis tors or reac tors ;( i i ) au to t rans formers ;



156 Pro tect ion o f m otors , reactors , boosters an d capaci tors

b-

ob-

Running speed

0Speed

Synchronousspeed

Fig. 14.2.1H To r q u e / s p e e d c h a ra c t e ri st ic s o f a s q u i r r e l c ag e i n d u c t i o n m o t o r

( i ) S er ies res is tor or reac tor : By using this method the voltage applied to them otor at starting is reduced b y the sim ple exp edien t of intro du cin g a series resistoror reactor in the sup ply to the m otor, the resistor or reactor being short-circu ited assoon as the motor has reached its normal running speed. The starting current isthus reduced in p rop ort ion to the term inal voltage. The start ing torque , however, isreduced as the squa re of the term inal voltage, so tha t the use of a series resistor orreactor to drop the vol tage by 20% will have the effect o f reducing the s tartcurrent by 20%, the s tar t ing torq ue being thereb y redu ced to (0"8) 2 x 100%, tha64% o f the full voltage value.( i i ) Autotransformer:In this case (see F ig. 14 .2.1 I) the line cu rre nt is equ a

to the t ransformer pr imary cur ren t and the motor cur ren t to the t ransformsecondary current . Similar ly, the line vol tage is equal to the t ransfo rm er pr imvol tage and the m oto r voltage to the t ransform er secondary vol tage. Thus

l ine curren t = m ot or vol tage

m ot o r curren t l ine vol tage

If, for exam ple , the t ransform er second ary vol tage is 80% of the l ine vol tage,that the m o to r curren t i s 80% o f the full voltage value, then the l ine curren t0 .8 x 0 .8 x 100%, tha t i s 64% of the m oto r current a t fu ll vol tage. As wi th ser ies res is tor o r reactor m eth od , an 80% voltage value gives a m ot o r torq ue of 6of the full voltage value.(iii) Star-delta sw itching: By mean s o f a three-pole changeover switch the s ta towindings are s tar-connected dur ing the in i t ia l s tar t ing per iod and del ta-connecwh en the m oto r has run up to speed. In th is way the vol tage per phase a t s tar t

is reduced to 1/~/3 , th at is 58% of the su pply vol tage, and is res tored to ful l supvol tage for n orm al run ning. Switching f rom s tar to del ta shou ld be carr ied ou t a



Pro tection o f m otors, reactors, boosters an d capacitors 157

A

Au,o-,rans ormer

~ Starting

k R unning

Supply

M otorstator

Fig. 14.2,11 A u t o t r a n s f o r m e r s t a r ti n g o f a s q u ir r e l c ag e m o t o r

de l ta con nec t ion is made . Th is d i ff icu l ty can be avo ided by ma in ta in ing the c i rcth rou gh a res is to r dur ing the t rans i t ion f rom s ta r to de l t a conne c t ion .

( b ) T h e w o u n d - r o t o r m o t o r:The to ta l va lue o f the ro to r res i stance o f a wo undro to r m oto r cons is ts o f the f ixed resi stance o f the ro to r wind ing p lus the addexte rnal res is tance w hich is var iable .

As the load to rque changes so does the ra t io o f the to ta l ro to r res i s tancer t tothe s lip s . To overcom e the load to rqu e , the appro pr ia te va lues o f secondacu r r en t and f lux , wh ich t oge the r de t e rmine t he m o to r t o rqu e , mu s t be p roduc

and the motor ' s abi l i ty to achieve th is i s governed by the ra t ior / s . T h eslip, for agiven value of torq ue , is pro po r t ion al to the to ta l ro to r res is tance . I t wi ll be stha t fo r each va lue o f ex te rna l ro to r res is tance a spee d / to rq ue curve can be d rawand a se t of typ ical curves is show n in Fig . 14 .2 .1J . C urve (a) represe nts tcon d i t ion wh en the ex te rna l resi stance is ze ro and curves (b ) and (c ) the cond i t iw i th d i ffe ren t va lues o f ex te rna l res is tance conn ec ted .

By main ta in ing the ra t ior /s con s tan t , o r near ly so , by a s tep by step reduc t iono f t he ex t e rna l r e si st ance t he t o rque o f t he wound - ro to r m o to r can be kep t r ea soab ly cons tan t dur ing the runn ing-up per iod .

This contras ts wi th a squir re l cage motor in which the ro tor res is tance is f ixand the re fore on ly one sp eed / to rqu e charac te r is t ic is ob ta inab le .

S i n g l e - p h a s e i n d u c t i o n m o t o r s :There a re many types o f s ing le -phase induc t ionm oto r and a represen ta t ive se lec t ion wi ll be cons idered here . The major i ty o f sum otors have ro to rs o f the squ i r re l cage type .

Unl ike the po lyph ase indu c t ion m otor , the s ing le -phase version has no s ta r t ito rque and s ta r ting i s ach ieved by the in t rodu c t ion o f an aux i l ia ry s ta r ting wind



1 5 8 P r o t e c t i o n o f m o t o r s , r ea c to r s , b o o s t e r s a n d c a p a c it o r s

(c) (h) (a)

S p e e d

Fig. 14.2.1J

S y n c h r o n o u ss p e e d

T o r q u e /s p e e d c h a ra c te r i s t ic s o f a w o u n d - r o t o r m o t o r

r s = To t a l s e c o n d a r y r e s i s ta n c e= r 2 p l u s e x t e r n a l r e s i s ta n c e

a commutator and brushes on the rotor (as , for example, with the repulsion-

induct ion motor) .

a ) R o t a t i n g f i el d m et h o d : To obtain a rotating field, the axis of the startingwinding must be displaced in space with respect to the main s tator winding, and thecurrents in the two w indings m ust be out of phase. The f irs t requirement issat isf ied by placing the s tart ing w inding in em pty or part ly f il led s lots of the mainwinding and the second by the choice of sui table resis tance and reactance values for

the two windings, or by the addition of resistance, reactance or capacitance in seriesw ith the s tar t ing w inding. Som e typical appl ications are given b elow .

t A . C . supplyR

Runningw i n d i n g ~ S t a r ti n g

l i - - ° ° R

I |

F i 14 2 1



Protect ion o f m otors, reactors , boosters and capacitors 159

In the spl i t -phase motor, the res is tance and reactance values of the two windinare usua l ly such tha t an angle o f app rox im ate ly 30 ° be twee n the tw o cu r ren tobta ine d. To increase th is angle and so prod uce a s t ronger rota t ing fie ld , aconsequent ly a larger s tar t ing torque, a res is tor i s inser ted in para l le l wi th ts tar t ing w inding (usual ly categor ised as a res is tance-s tar t spli t-phase m o to r as shoin Fig. 14.2.1K). Again an increase in angle is achieved by using a react( reactance-s tar t spl it -phase m oto r) . A fur th er increase in the angle , m aking tcurre nt in the s tar t ing winding lead the term inal vol tage, is obta ine d by us incap aci tor in p lace o f the res is tor (capa ci tor-s tar t spl it -phase m oto r, as show n Fig . 14.2 .1L) . To prevent overheat ing of the s tar t ing winding i t i s necessary in these cases to d i sconnec t i t a s the m oto r approaches syn chro nou s speed and th i

done au tom at ica l ly b y a cen tr ifuga l swi tch on the ro tor.A.C.s u p p l y

TR(,' "tapacl or T ~Running

~i:3tcr2 ugalls~

Starting~vinding

I R

0 approaches 90°F i g .14.2 .1 L Connect ions and phaso r diagram o f capacitance-start spl it -phase ind uct ion m ot or

( b ) C o m m u t a to r m e t h o d:This method of s ta r t ing i s used for the repu ls ioninduc t ion m o to r wh ich is a fo rm o f i nduc t ion mo to r w i th com m uta to r and b rush

and wi th an a dd i t iona l squ ir re l cage wind ing in the ro tor. Th is type of m oto r mhave constant-spe ed or variable-speed character is t ics d epen ding up on the des ignthe windings . I t d i ffers f r om the pla in a .c . series m oto r in tha t no curre nt is lin to the armature , the brushes being shor t -c i rcui ted .

I f the brush axis is perpen dicular to the ma in f lux, as sho w n in Fig . 14.2 .1M (the e .m.f .s ind uce d in the tw o halves of the w inding wi ll cancel each othe r ou t , athe a rmature wi l l no t ca r ry any induced cur ren t nor p roduce any to rque .

I f the brush axis i s in the same direct ion as the main f lux then a current wi l l binduced in the a rm ature . The four par t s o f th is cur ren t will in tu rn p rodu ce fotorques which , as shown by the a r rows in F ig . 14 .2 .1M (b) , will cance l one ano thout .

I f now the brush axis i s incl ined a t an angle to the di rect ion of the main f luthen torqu es T1 and 7'2 are pro du ce d, as show n in Fig . 14.2 .1M (c) . Th us resul tato rque T = 2(7'2 - TI ) and i t can be show n tha t T is a m axim um wh en the ang lebetw een the brush axis and the h or izon ta l axis of the arm ature is 45 .0

T h re e -p ha se c o m m u t a t o r m o t o r s



(a)

(h)

T \c )

160 Pro tection o f m otors, reactors, boosters an d capacitors

Fig. 14.2.1M E f f e c t o f d i f f e r e n t b r us h p o s i ti o n s o n t he a c t io n o f a r e p u l si o n m o t o r

exc i t ed shun t commuta to r mo to r ( t he Sch rage mo to r ) i s by f a r t he mos t commoused, and th is sec t ion w ill be l imi ted to a d iscuss ion o f the sa l ient fea tures o f sucm o t o r.

The ou t s t and ing f ea tu r e o f t he Sch rage mo to r i s t ha t t he commuta to r a l l o

good speed con t ro l and power fac to r cor rec t ion to be ob ta ined in the one machThe speed o f an induc t ion m oto r can be co n t ro l l ed by impress ing on i ts secon d(s ta to r ) wind ing a vo l tage in the same d i rec t ion and of the same f requency, thathe s l ip f requency, as the e .m.f , induced in th is winding by the ro ta t ing f lSe lec tion o f the cor rec t phase and m agni tude o f the impressed vo ltage g ives conof the speed above and be low synchron ous speed . The th ree -phase shun t com mta tor m oto r, o r Schrage mo tor, is in e ffec t a com bined th ree-phase ro to r exc iinduc t ion motor and a th ree -phase commuta tor machine , a s shown in F ig . 14 .2 .1

Of the two ro to r w ind ings , one is con nec ted to the s lip- rings and the o th er to co m m uta tor segments . The s ta to r wind ing consi st s o f th ree separa te phase wind i



Pro tection of motors, reactors, boosters and ca pa citors 161

Compensating ~,.:.,a;.,~,

R o t o r w i n d i n g( p r i m a r y )

T h r e e - p h a s e s u p p l y

S t a t o r w i n d i n l( s e c o n d a r y )

CommutatorF i g . 1 4 . 2 . 1 N Con nect ions o f the three-phase Schrage m o to r

direc tion round the com m utator. The com m utator v oltage varies as the angulardistance between the brushe s of ea ch pair, being greatest whe n the d istance is180 electrical degrees, and decreasing to zero wh en the y are bo th on the samecom m utator segm ent, thus sh ort-circuit ing the stator w inding . In Fig. 14.2.1P(a)the brus he s are shown in this latter pos ition and the m achine w ill run as anordinary induc t ion m otor. In F ig . 1 4 2. lP( b) the connections are su ch that thepoten t ia l d i ffe rence be tween brushes x and ywill have a componen t i n d i r ec topp os i t ion to the e .m. f , induced in the s ta to r, r educ ing the m oto r speed to a vabe low synch rono us speed . By reversing the b rushes , a s shown in F ig . 14 2 .1 P(the commuta tor e . rn . f , wi l l have a component in phase wi th the ro to r induco .m. f. , thus inc reas ing the m oto r speed to a va lue above syn chron ous speed .

y i~ i ,xI l i l 11 ' 1 I I ~ l k l I 1 11 i

I I I

F i g . 1 4 . 2 . 1 P

(aj h )

Alterna t ive brush pos i t ions o f Schrage m ot o r

(c)

The phase o f t he c om m uta to r e .m . f, be tw een b rushes is de t e rmined by pos i t ions o f the cen t re po in t o f the b rushes re la t ive to the cen t re o f the rowind ing , so t ha t by mov ing t he b rush sy s tem a s a who le round t he com m uta to r phase of the e .m.f , in jec ted in to the seco nda ry c i rcui t wi ll be var ied re la tive to ro to r induced e .m. f .

Thus the angular separa t ion of individual brush se ts determines the speed vat ion above and be low sync hron ous speed , the angula r pos i t ion o f the b rush sysas a whole con t ro l l ing the power fac to r o f the motor.

From the typ ica l speed / to rque curves shown in F ig . 14 .2 .1Q i t wi l l be apprea ted tha t the speed is r easonab ly cons tan t up to app rox im ate ly 100% torq




F i g . 14 .2 .1Q

= 2 0 0 - - - I - - - - L - - - - I - - - - - 11 - - - - - I ". . . . . . . . ~ ~ I

/I I I

. . . . . J_ . . . . . 1-~ 160 - ~ l

= I I I - - - - -12o . . . . I I,, - - - - I " r - - T

=~:~ 8 0_ . . . _ _ _ . . i . _ _ _ 4 _ _ _ _ . 1 _ _ _ _ , . 1 . . _ - IIc~

5 i

I ; I t I~- 4o - - i ' l . . . 1 - - - i " - t :

l t tI

I ,0 5 0 0 0 1 5 0 2 0 0 2 5 0

% of fu l l load torque

Typica l speed~torque charac te r is t ics o f a th ree -phase co m m uta tor m ot o r

1 4 . 2 . 2 . A p p l i c a t i o n o f d . c . a n d a . c . m o t o r s

In choos ing a m oto r fo r a g iven app l ica t ion , cons idera t ion m us t be g iven to the tyof load i t w ill d r ive , and f rom th is the speed and to rque requ i rem ents can be dem ined . For loads such as c ranes, ho i s t s and e lec t r ic t r ac t ion , mo tors w i th spevarying w i th the load are requ ired and the d .c . series m ot or or the a .c . repulsm o to r a r e mo s t o f ten u sed . Fans , b lower s and un loaded compres so r s dem and ls tar t ing torque whi le pulver ised fuel mi l ls and loaded compressors s tar t under land d em and h igh s ta r t ing to rq ue to b rea k away f rom s tands t il l.

Cons tan t speed app l ica t ions inc lude motor-genera to r se t s , concre te mixers a

con s tan t speed conveyors and in these an inc rease in load to rqu e , fo r exam ple add i t ion o f coa l to a conveyo r, requ i res a p ro po r t iona te increase in pow er ou tpthe power o u tp u t be ing p ropo r t i ona l t o t he l oad t o rque . Of t he m o to r s dea l t win th i s chap te r, the d .c . shun t motor and the induc t ion motor would be su i t ab le such loads .

Var iable speed app l ica t ions m ay be c lassi fied as (i ) con stant torq ue , (i i) var iato rque , ( i i i ) cons t an t power-ou tpu t .

( i) A good exam ple o f cons tan t - to rque app l ica t ion is the au tom at ic m achtoo l where the ou tpu t i s d i rec t ly p ropo r t iona l to the speed . The w oun d- roinduc t ion m oto r i s the obv ious cho ice fo r th is app l ica tion , b u t a d .c . shun t m oprov ided w i th mean s o f speed ad jus tm ent by f i e ld con t ro l cou ld be used .

( ii ) Fans and blowers are typical of the var iable- torque appl ica t ion . W ith sloads the to rque var ies as the square o f the speed , and a d .c . shun t motor p rov i

w i th m eans o f speed ad jus tm ent by f i e ld con t ro l w ould be su i tab le .



Protec tion o f m otors, reactors, boosters and capacitors 163

they do , to rque vary ing inverse ly wi th speed . Aga in , the d .c . shun t mo tor as ( i ) and ( i i ) o r the wound- ro to r induc t ion motor cou ld be app l ied .

When f luc tua t ions in speed wi th vary ing load have to be ca te red fo r a ' shutype o f t h ree -phase co m m uta to r m o to r is u sed . The be s t know n o f t hese , Schrage motor descr ibed ear l ier, i s used extens ively in the speed-control sys tefor l a rge th ree -phase induc t ion motors p rov id ing power fac to r improvementthe same t im e. I t is appl ied w idely in the pr in t ing and text i le indust r ies and d riv ing some con veyo r equ ipm en t .

1 4 . 2 . 3 . M o t o r c o n t r o l

I t wi ll be seen l a te r tha t the ty pe o f p ro tec t ion used , in pa r t i cu la r the p ro tec taga ins t short -c ircu i ts in a mo tor o r i ts con nec t ions , is to some ex te n t de pen dup on the typ e o f swi tchgear used to control the mo tor. This fal ls in to tw o class

(a)

(b)

a contactor, wi th h igh-breaking-capaci ty fuses providing the shor t -c i rcp ro t ec t i on ; anda c i rcu i t b reaker, wi th shor t -c i rcu i t p ro tec t ion p rov ided by ins tan teneoa t t rac ted-a rmature type re lays .

In general , the cho ice o f swi tchgear w ill depe nd upo n the size o f the mo tor, exam ple con tac to rs and fuses fo r up to app rox im ate ly 150 h .p . and c ircu i t b reakfor larger motors .

Con t ro l by con tac to r and fuses :A con tac to r consi st s o f a mov ing con t ac t ope ra t edby an e lec t rom agne t a t t r ac t ing an i ron a rmature aga ins t the ac t ion o f a sp r ing . T

press ing o f the ' s t a r t ' push bu t ton energ i ses the e lec t romagne t , thus comple t ing ope ra t ing coi l c i rcui t . W hen the ' s top ' bu t to n is pressed the coi l c i rcui t is ope nde-energ i s ing the e lec t romagne t and a l lowing the moving con tac t to i so la te m oto r f rom the supp ly. Co ntac to rs m ay be o f the a ir insu la ted o r o il insu latype , the l a t t e r hav ing the advan tage tha t the m oto r cur re n t is b ro ken in o i l whgeneral ly perm its a smal ler overal l s ize of the c on trol uni t .

Co ntac to rs ca n be used where the cur ren t to be in te r ru p ted i s l imi ted

app rox im ate ly s ix t imes the i r r a ted cur ren t , the ra ted c ur ren t be ing o f the saorde r, or a li t tle h igher, th an the norm al fu l l load of the mo tor . Direct -ac t ing ovload t r ip dev ices m ay be incorp ora ted in the con tac to r, p ro tec t ion aga ins t shoci rcui ts being prov ided b y fas t op era t ing high-break ing-capaci ty fuses . The fushould b low a t cur ren t s in excess o f those which can be hand led by the con tacbu t shou ld no t b low a t cur ren t s wi th in i t s capac i ty.

S ince the con tac to r co i l wi l l ho ld the a rmature aga ins t the spr ing on ly whthere is suff ic ient m agnet ic f lux, a ny severe dro p in or fai lure o f the sup ply w

resu l t in the open ing of the con tac to r. Thu s an inhe ren t ' no-vo l t ' r e lease fea tis p rov ided ensur ing th a t the m oto r cann ot re - s ta r t a f t e r a pa r t ia l o r com pl



164 Pro tect ion o f m otors, reactors , boosters an d capacitors

When the contactor uni t i s smal l i t i s general ly located adjacent to the machibeing driven, so tha t co ntro l of the m achine is faci l i tated, in such cases the 's taand ' s top ' push bu t tons a re an in tegra l part o f the contac tor. When the contacis too large to be located c lose to the machine, local controls are provided a t t

machine.

Control by circui t breaker:Large motors are control led by c i rcui t breakers oei ther the a i r-break or off -break type, which should be capable of in terrupt ing highest current w hich can f low under the m ost severe faul t con di t ion. Oi l -brgear is seldom instal led in gene rating stat ion s chiefly because o f the add it iof ire hazard wh en oi l is present under faul t cond i t ions .

A ci rcui t breaker may be arranged for hand c los ing, e lect romagnet ic solenclosing o r spring closing. H and closing is usually provide d for small, infre qu enoperated c i rcui t breakers , solenoid c los ing for those operated f requent ly or f romremote point , and spr ing c los ing for c i rcui t breakers operated only occasionaand w here hand ope rat ion is undesi rable . Over load tr ip devices ope rat ing di ron to the t r ipping mechanism usual ly form an integral par t of the c i rcui t break

1 4 . 2 . 4 Ty p e s o f f a u lt

In general i t is necessary to pro tect a m oto r against abnorm al running and facon di t ions ar ising from :

( a )load:

prolonged over loading as a resul t of the appl icat ion of excess ive mechani

(b) s ingle-phasing caused, for exam ple , by the ruptur ing of a fuse or by open c i rcuit ing o f a con nec t ion in one phase of a three-phase m oto r. I f one phis open-ci rcui ted w hen the m oto r is running i t wi ll con t inue to ru n and provpow er even thoug h i t i s con nec ted to w hat i s, in effect , a s ingle-phase supply. I fload on the motor i s o f the order o f i t s ra ted ou tpu t , the cur ren t d rawn f rom supply wi ll be app reciably higher tha n the current for wh ich the windings designed and if the condit ion is al lowed to persist , severe damage may be caused

(c) shor t -c ircui ts between phases or between phase and ear th in the m owinding o r i ts co nnec t ions . Shor t -c ircui ts m ay be caused by the chaf ing of connt ions , accidental shor t ing of the motor terminals or cable seal ing ends or by cafaults:

(d) part ial or com plete collapse of voltage.

1 4 . 2 . 5 A . C . a n d d . c. m o t o r p r o t e c t i o n



Protec tion o f m otors, reactors, boosters and capacitors 165

w hethe r the m oto r is dr iven f rom an a .c . or a d .c . source . In som e ins tances , example , the thermal over load re lay, a modif ied s ingle-phase vers ion is appl iedthe pro tec t ion o f d .c. m otors . An y dangerous o r po ten t ia l ly dangerous con di t ione i ther an a .c . o r a d .c . motor, i t s con t ro l o r connec t ions , mus t be de tec ted aac t ion taken au tom at ica l ly to d i sconnec t the a ffec ted equ ipm ent . Such condi t iare c lassi fied broa dly as low o r fa l ling supp ly vol tage and over loading be yo nd a pdeterm ined safe value for an excess ive time. To these cond i t ions m ust be add ed open-ci rcui ting of one phase o f a three-phase a .c . m o to r and a shor t -c i rcui t in e itan a .c . or d .c . motor.

I 0 0O

¢b

~ - 8 0~ =

E~ 6 0 -

~, 4 o -

2 0 - -

o -

7

4 -

3 -

F i g . 1 4 . 2 . 5 A

S p e e d

C u r r e n t

5 I 0 1 5 2 0

Ti m e i n s e c o n d s

Typica l s ta r t ing charac te ri s ti c fo r d i rec t -on s ta r ted indu c t ion m oto r

M any m otors d raw a s ta r ting cu r ren t f rom the supply of severa l times thnormal ful l - load current , and i t i s essent ia l that the protect ion should be unresposive to th is s tar t ing surge provided that the motor current re turns to i t s runniva lue wi th in the t im e de te rmined by the des ign of the m otor. On the o ther hanthe pro tect ion m ust no t be given a se t ting great ly in excess o f fu ll load or it willunable to safeguard the m oto r agains t overc urrent con di t ions . Figs . 14.2 .5A a14.2 .5B show typical s tar t ing character is t ics for an a .c . 'd i rect -on ' induct ion mot

and for a d.c . shunt -w oun d m oto r, r espec tive ly.

Thermal relays: The two oppos ing requ i rements re fe r red to a re met in a re lahaving an inhe rent t ime- lag character is tic , an e xam ple o f which is sho w n Fig . 14.2 .5C. The essence o f the therm al re lay is the specia l ly des igned e lem ewhich s imulates as c losely as i t can the changing thermal condi t ions in the mota l lowing the motor to be re ta ined in se rv ice up to the po in t beyond which damaw ould pro ba bly be caused. Essentia lly, the re lay consis ts o f three s ingle-pha

e lements , each e lement compr i s ing a hea te r and an assoc ia ted b imeta l ac tua tm ovem ent . The th ree b imeta l sp iral e lem ents a re m ou nted ax ia lly in l ine an




t3¢>

Or}

00

0 -

1¢=¢ )i .i .

12

I SpeedIII

l i l \I L. . . .

. . . . . I - T II I II III I

. . . . . ~ . ,= , - - . . . m . r

I I II I II I II I

I II I II I II I II I II I II I I

0 t I t 2 t 3 t4

T i m e

C u r r e n t

F ig . 14 .2 .5B Ty p i c a l s t a r t in g c h a r a c t e r is t ic f o r a d .c . s h u n t - w o u n d m o t o r

l . o a d ind ica t ing A d j u s t a b l e

s c a le a n , I p o i n t e r o v e r l o a dcon tac t

H e a t s h i e l dI

Over l o ad s e t t i ngsca le and po in te r

A m h i e n t c o m p e n s a t i on b i m e ta l



Pro tect ion of m otors , reactors , boosters an d capaci tors 167

of the con tac t a ssembly (F ig . 14 .2 .5D) . A change in am bien t t em pera tu re ro tathe two ou te r ends o f an e lem ent th rou gh the same ang le w i tho u t caus ing mom ent o f the con tac t a rm. Charac ter i st ic curves fo r a se t t ing o f 125% und' s t a r t ing f rom co ld ' and ' runn ing ' cond i t ions a re shown in F ig . 14 .2 .5E .

P h a s e - u n b a l a n c e ~ , '

c o n t a c t s

/ , %

f

O v e r l o a d v "

c o n t a c t s , ~

f

B I

F i g . 1 4 . 2 . 5 D

I y T r i p

t / .Con tac t a r rangeme nt o f typ ica l therm al re lay

2 8

1 8

1 4

• . lO

= 8E

7.E

6E

•7. 5~o 4

•7. 3

~. 2O

o v ~ d o ~ Ji ' L ' - s e t t i n g [ 2 5 % } " '

\ ~ ~, S t a r t i n ~

R u n n i n g ~ " "

| J l

2 3 4 5 6

M ~ t o r c u r r e n t a s m u l t i p l e o f r a t e d c u r r e n t

Charac te r i s t i c o f typ ica l thermal re lay for motor p ro tec t ionF i g . 1 4 . 2 . 5 E

1

Single-phasing protection :This i s necessary in v iew o f the abi l i ty o f a three-phasmotor to con t inue to run wi th an open-c i rcu i t in one phase , a l though i t may wsu ffe r dam a g e a s a consequence. T he a dv en t o f m o to r s d e si gned on a m ax imcon t inuo us ra ting (m .c . r. ) basi s p rov id ing very l imi ted con t inuo us over locapac i ty, has em phas i sed the im por tan ce o f p ro tec t io n aga ins t s ing le -phase runn iThe s ing le -phase fea tu re i s conven ien t ly incorpora ted in the th ree -phase the rms imula t ing re lay the o pera t ing t ime- lag depend ing to som e ex ten t on the lo



168 Pro tect ion o f motors, reactors , boosters and capacitors

load the opera t ing time is usual ly 20 - 30 s w i th correspo nding ly longer timessm aller loads. Th e single-phasing conta cts w il l close also if the out-of-balanbe tween any two phase cur ren ts exceeds ab ou t 12% a t fu ll load .

Short-circuit protection:Protect ion agains t shor t c i rcui ts in the motor windingor the conn ec t ions to i t , is o f ten prov ided by incorpora t ing in the therma l recase separate h igh-set , ins tantaneous overcurrent or ear th-faul t e lements or boThe a t trac ted-a rm ature type e lem ent opera tes aga ins t the ac t ion of a re tu rn sprand in one des ign res ts agains t a leaf spr ing to reduce the effects of v ibra t ion ashock. In an oth er des ign these effects are guarded agains t by us ing a s ta t icap ivo ted beam , the a rm ature be ing a t t ached to one end of the beam and the mov

con tacts to the oth er end. Typ ical se tt ings are 4 to 8 or 8 to 16 t im es full load the ins tantane ous ove rcurrent e lem ents and 0 .2 to 0 .4 t im es ful l load for tins tan taneous e a r th -fau l t e lem ent . Three such overcur ren t e lemen ts o r two ovcur ren t and one ea r th - fau lt e lem ent can be incorpo ra ted in the s tan dard th ree-phthermal relay case.

I t wi l l be apprecia ted that when the motor supply is ear thed via a res is tor, tear th-faul t current may be less than that needed to operate posi t ively the ovcurre nt e leme nts conn ecte d in the phases . In such cases i t is necessary to em pthe res idual ly connected ear th-faul t e lement .

A frequent cause of s ingle-phasing is a 'b lown' fuse and care is needed in choice of fuse rat ings . D uring s tar ting, the curre nt app roach es fus ing currecausing the fuses to ru n ho t for an appreciable t ime . This m ay give r ise to oxit ion an d de te r io ra t ion of the fuses and u l t im ate ly a fuse m ay b low dur ing s ta rt iThe m oto r will con t inue to run wi th con s iderab ly increased c ur ren t f lowing in othe r tw o phases which, i f a l lowed to pers is t , wi ll damage the windings .

The use o f h igh-breaking-capaci ty fuses for shor t -c i rcui t prote ct ion has the mtha t the c i rcui t is ope ned dur ing the f irs t quar ter-cycle of faul t current thpreven t ing the cur ren t f rom a t ta in ing i t s theore t ica l maximum or ' p rospec t iassym etr ica l peak value which w ould f low in the absence o f the fuses . The curra t w hich the fuse 'b lows ' is kno w n as the ' cu t -o ff cur ren t '. The fusing t im e m ayex t r eme ly sho r t, f o r example be tween 0 .001 and 0 .002 s com pared wi thm in im um opera t ing t ime of an over load tr ip o f ab ou t 0 .1s. Th us the fuse is w

able to co m ple te ly in te r rup t the fau l t cur ren t before the over load t rip can opera t

Smil ing re lays:The s ta l l ing re lay is des igned for use in conjunct ion wi th ththerm al ov erload an d single-phasing relay. Basically the stal l ing relay consists ocont ro l con tac tor and a thermal over load un i t f i tt ed in the same case. The thermover load uni t i s energised via the contactor which c loses only dur ing the s tar tper iod or, i f the m ot or s ta lls, while s tar t ing or runn ing (see Fig . 14.2 .5F ) . D ura normal heal thy s tar t the contactor c loses , swi tching the thermal uni t in to c i rcu

but the motor cur ren t fa l l s to normal before the thermal un i t opera tes and co nta cto r o pens , de-energis ing the therm al uni t . An opt ion al ext ra feature



P r o t e c t i o n o f m o t o r s , r ea c to r s , b o o s t e r s a n d c a p a c it o r s 1 6 9

C o n t r o lr e s i s t o r

c

C o n t r o l

c o n t a c t o r I

7 iu n i t

T r ip s u p p l y

F i g . 1 4 . 2 . 5 F P a n d B t y p e L I A s t a l l in g r e l a y

C o n t r o lr e s i s t o r

C o n t r o lc o n t a c t o r

A l a r m

A l a r mc o m m o n

To c . b .t r i p c o i l

I . i m i t i n g

r e s i s t o r

Tr i p s u p p l y

T h e r m a lu n i t

F i g . 1 4 . 2 . 5 G P a n d B t y p e L 1 A s t a l l in g r e la y i n c o r p o r a t in g a t w i c e- i n f e a tu r e

feature. This permits on e a t tem pte d restart of a m oto r after a s tal l but is soconstructed that i t la tches in on the second s ta l l , thus prevent ing repeated a t temptsto res tar t a defect ive motor.

In one design the ' tw ice . in ' feature con sis ts of a shu nt-c onn ected a t t racted-armature re lay w i th a fo l low er-p in a t tached to the top o f the a rmature , and movingin a labyr inth s lot in a hinged m etal p la te (see Fig. 14.2 .5H ). As the armaturemoves in and out on successive s ta l ls the pin works through the labyr inth s lot ,a l low ing the hing ed pla te to fa ll unt i l , on the seco nd s ta l l, the pin is t rapped at the

end of the s lot , thus la tching the arm ature . Flags are incorp orated to indicate thenum b er o f t ime s the m otor has s ta l led s ince the ' twice- in ' feature was las t reset.



170 Prote ct ion o f m otors, reactors , boosters and capacitors

i ' o l o w e r p i n

Hinge ,p l a t e

L a b y r i n t h

slot ,~

C o r eF l ag I

R e d

( ; r e e n

Fig. 14.2.5H

J\

A r m a t u r e

Arrange me nt o f mecha nica l twice- in fea ture o f type L 1A s ta ll ing re lay

Electronic overload relay:A m od ern a l te rna t ive type o f the rm al re lay, o f so lids ta te design ra ther than o f the b im eta ll ic e lemen t des ign p ro tec t s the mo tor agaover load by der iv ing an a .c . inpu t f rom, and re la ted to , the motor supp ly cur reThis current t ransformer der ived input i s rec t i f ied , adjus ted to an appropr ia te leby a cur ren t se t t ing c i rcu i t and then app l ied to the overcur ren t de tec t ion c i rcuWhen the reference level i s exceeded by the input the t ime-set t ing c i rcui t , incpora t ing a res is to r /capac i to r ne tw ork , fo l lowed by an am pl i f ica t ion stage ac losure o f the t r ippin g c i rcui t , is t riggered. The re lay has a character is t ic s imi latha t of a b im eta l lic e lem ent re lay , i .e . an inverse- t ime c haracter is t ic such th at h iglevels of over load cause fas ter d iscon nect ion of the m ot or an d lower levels

to le ra ted fo r longer pe r iods . The b lock d iagram of a re lay o f th is type is show nFig. 14.2.5I .

Pro tect io n agains t an open c i rcui t in one phase is prov ided by a separa te c i rcwhich de tec t s the h igher r ipp le c on ten t o f the full -wave rec t if i ed cur ren t de r if rom the cur ren t t rans form ers . A f te r pass ing th rou gh a waveform and ga ting c i rin to the power c i rcui t i t provides a s ignal for opera t ion of the t r ipping c i rcui t .

The m ore soph is t i ca ted vers ions o f th i s re lay m ay incorpo ra te means to ad jthe model character is t ics so that an analogue c loser to the ac tual temperature o f a m o to r ma y be ob t a ined . Ou tpu t s may be u sed fo r pu rposes o the r t ht r ipping, e.g . regula t ing the m ot or load. The c loser co ntro l provided by sudev ices is pa r t i cu la r ly use fu l in the p ro tec t ion o f max im um cont inuo us ra t(m.c . r.) mo tors and o thers wi th an a rduou s du ty cyc le . Ap prop r ia te mode l l ing overcom e the p rob lems enco unte red wh en a mo tor is r equ i red to s ta r t aga inss ta l led load , to ' inch ' o r to s t a r t and s top f requ en t ly.

Thermal tn 'ps and e lec t romagnet ic over load pro tec t ion:For the smal ler s ize of



P r o t e c t io n o f m o t o r s , r ea c to r s , b o o s t e r s a n d c a p a c i t o r s 1 71

Ri ( ~ Powerr - - Y source

Tes tb u t t o n

• --- ' ----- -0 C

T °,°r uo c°lac'°r

lO p en p . a s : ] _de tec t ing I -c i rcui t

Waveform , ' 1shap ingcircui t

1

R ec t if i e rc i rcui t

- - 1se t t ing-- circuit

I__ O v e r c u r r e n tq de tec t ing

circui t

Icircuit

V~ltageregu la t ing

circui t

OR circui t

~ o ~r I _ _ _ _ ~ L _ _ _ _upp ly Ampl i f i e rc i rcui t c i rcui t

La t c h ing

Pov~ercircuit

• t rigger

R



172 Protec t ion o f m otors, reactors , boosters and capacitors

techn ica l ly, and a t t rac t ive f rom an econo m ic s tand poin t , to em ploy a s imple r foof p ro tec t ion than the soph is t i ca ted the rmal over load re lay descr ibed a t tbeg inn ing of th i s Sec tion . R ecourse i s then made to e i the r the the rm al over lot r ip ( in essence a much s impl i f i ed fo rm of the the rmal over load re lay)or t

e lec t rom agn et ic over load and associa ted time- lag device . The therm al t r ipopera ted e i the r d i rec t ly o r ind i rec t ly by the hea t ing e ffec t o f the cur ren t onthermal e lement and the e lec t romagne t ic t r ip i s opera ted by the inc rease magne t ic f lux wi th cur ren t .

(a) T he rm al trips: In an ear ly type of thermal t r ip a b imeta l s t r ip was used athe sens ing e lem ent , the m oto r cur ren t pass ing d i rec t ly th rou gh the e lemen t . the thermal s t r ip heated up i t l i f ted and so ra ised, agains t spr ing tens ion,

hor izon ta l t r ip rod pos i t ioned above i t. I f the over load con di t ion pers i s ted , fu r tmovement upwards o f the rod , to a po in t p rede te rmined by the se t t ing , caused mechan ica l t r ip to o pera te to open the assoc ia ted con tac to r. A second ropos i t ioned so tha t the b imeta l s t r ip was be tween the two rods , fo l lowed the moment o f the b imeta l and upper rod un t i l he ld by a s top o f the same mate r ia l atherefo re having the same therm al character is t ics as the therm al s t r ip . A m bit empera tu r e com pensa t ion was t hus ob t a ined , and t he m o to r cu r r en t a t wh ich

re lay ope ra ted was subs tan t ia lly unaffec ted b y t emp era tu re changes. These re lwere appl ied to d .c . , s ingle-phase a .c . and three-phase a .c . m oto rs . W hen used wthe la t ter, inhe rent pro tect io n agains t s ingle-phase runn ing was ob ta ined as aappreciable unbalance in the phase currents caused the two rods to move oppos i te d i rec t ions and so opera te the t r ipp ing mechan ism.

In a l a te r ve rs ion o f th is type o f re lay the b im eta l e lemen ts do no t ca r ry cur rbu t a re opera ted ind i rec t ly by hea te r s . In the case o f small m otors , the hea te r s d i rec t connec ted to ca r ry the motor cur ren t s and a re fed f rom cur ren t t r ans form

wh en used w i th l a rger m otors . Tw o s lo t t ed t r ip ba rs o r s lides a re coupled tog e ta t one end and in normal , hea l thy condi t ions the two bars move toge ther inhor izon ta l d i rec t ion cor respon ding w i th the ba lanced def lec t ion o f the thb imeta l the rm al e lemen ts , as show n in F ig . 14 .2 .5 I . Af te r a p red e te rm ined am ou

Bimetal thermal Upperelements ~ trip bar

L o w e rt r ip bar

A. ,mO.-- - -=-~--- - -

Tr i p s w i t c hcontacts

Fig 14 2 5 1 Simple thermal over load and s ing le -phas ing preve ntor for m oto r pro te c t ion



Protec t ion of m otors, reactors , boosters an d capacitors 173

of m ove m ent dur ing an over load cond i t ion , one o f the bars s t rikes a s top whpreven ts i t fo ll owing t he o the r ba r any fu r the r. Con t inued m ovem en t o f t he o tbar under the ac t ion o f the b imeta l e lements opera tes the mechan ica l t r ip to opthe mo tor c on tac to r. I f o n e phase is open-c i rcu i ted whi le the m oto r is runnthe the rmal e lement in tha t phase wi l l coo l and def lec t in the oppos i te d i rec t ionthose s ti ll carrying curren t . These tw o e lements wil l rem ain def lec ted and di ffert ia l m ove m ent o f the two bars wi ll aga in cause the m echan ica l tr ip to opera te . Pof the co n tac t a r rangem ent o f th is re lay is show n in F ig. 14 .2 .5K.

U p p e rt r ip b a r

Trip s~ 'c o n t a c

B i m e t at h c r m ae l e m e n

L o ~ e rt r i p ba~

Vi e ~ l o o k i n gi n d i r e c t i o no f a r r o w A i nF i g . 1 4 . 2 . 5 J

Fig . 14 .2 .5K C o n t a c t a r r a n g e m e n t o f t h e r m a l o v e r lo a d a n d s in g le -p h a sin g p r e v e n t o r

The s imple the rma l t r ip dev ices jus t desc r ibed should no t be confused wi th m uch mo re soph is ti ca ted and comp rehens ive the rm al re lay desc r ibed a t tbeg inn ing o f Sec t ion 14 .2 .5 . The l a t t e r r e lay is used ex tens ive ly fo r the p ro tec tof th e larger three-phase a .c. m oto rs dr iv ing pow er s ta t ion auxi l iar ies, and a s inelement vers ion of that re lay is avai lable for use wi th s ingle-phase a .c . motors

d .c . motors .

(b) Electromagne tic trips:These over load t r ips a re ins tan taneous in opera t ionand , in o rder to t ake advan tage o f the mo tor ' s pe rmissib le over load capac i ty, usua l ly em ploy ed wi th a time- lag dev ice . The com ple te assembly cons is ts oser ies-wou nd coil surrou ndin g a ver tica l i ron plunger, to fo rm a solenoid , and assoc iated t ime-lag in the form o f an oi l or s i licone f luid f 't lled da shp ot o r air vaThe i r app l ica t ion thus depends on the cur re n t / t ime charac te ri s ti c which can ob ta ined to ensure the necessa ry re ta rd ing ac t ion to p reven t t r ipp ing on toccur rence o f heavy over loads o f shor t dura t ion . The o il dashp ot t ime- lag is t y p e m o s t c o m m o n l y u s e d.

The requ i red t ime- lag is ob ta ined by ad jus tm ent o f the ra te a t w hich o ilpe rm i t t ed to pass f rom the u pper s ide to the low er s ide o f the p i s ton in F ig . 14 .2 .Oi l f low is governe d by a smal l hole B in the ro ta table p la te C and a large hole Dthe p i s ton . H a te C has a nu m ber o f ho les g raded in d iam ete r and by changing

s ize of hole in use a range of t ime- lag se t t ings between, for example , 10 and 3can be ob ta ined . Over load cur re n t ca l ib ra t ions a re m arke d on the ou t s ide o f




~Ovcrload coil

Tr ip p in

Plunger

Plunger easing

Set t ingscrew E

Hole

P l a t e Piston A

Hole D

Fig . 14 .2 .5L Cross - sec t ion o f a typ ica l o i l dashpo t t ime- lag fo r an e lec t romagne t i c over loadt r ip (Al len W est & Co . L td . )

and rai sing o r lowe r ing the dash po t to change the po s i t ion o f the p lunger in re la tto the magnet ic f ie ld of the ser ies over load coi l .

A mo re s oph is t ica ted des ign of o il dash po t i s available in w hich a res t ra indev ice , des igned to opera te on the occur rence o f heavy t rans ien t o r sho r t dura tover loads , p reven t s unnecessa ry t r ipp ing under such cond i t ions . W hen a hea

over loa d occurs the sud den rush o f o i l causes a flap to c lose over the hole B. Theavy cur ren t causes the p lunger to l i f t s ligh t ly, c rea ting a vacuum ben ea th i t , a



P r o t e c t i o n o f m o t o r s , r ea c to r s , b o o s t e rs a n d c a p a c i to r s 1 7 5

hole B, forcing the f lap o ff the hole . The pis ton is then able to funct ion as a t ime-lag device under normal sys tem condi t ions .

The restraining feature has applications with direct-on starters for squirrel cagem otors and where m otors may be sub jec ted to severe peak loads of l imi ted du ra t ion ,enab l ing the overload t r ip to be given a norm al set t ing w itho ut fear of operat ionunder short-durat ion, heavy-current condi t ions.

When an overload condit ion has ceased or the t r ip pin has operated, the plungerand p is ton s ink b ack to the b ot to m of the dashpot tak ing typica l ly some 10 - 30 sto do so: in special designs this resett ing t ime can b e exten ded . This delay shou ldbe considered a necessary feature prevent ing the m otor from being restar tedimm ediately af ter the clearance of an overload con dit ion . Even w ith the special

designs i t is no t general ly possible to exte nd the resett ing t ime be yo nd 10 0 - 150 sAs the reset posi t ion of the plunger is determined b y the p osi t ion o f the d ashp ot ,

the fur ther this is screwed do w n the more current is required throug h the solen oidto raise the plunger and operate the t r ip pin. Thus the posi t ion of the dashpot canbe cal ibrated in terms o f m oto r l ine current and i f one such d evice is used in eachl ine, three-phase protect ion can be provided.

Fig. 14.2.5M show s a typical curren t / t im e curve for a two-rate dashpot thefun ct ion of w hich is to a l low the device to clear a m otor start con di t ion b ut tooperate as intend ed under overload con dit ions. Tim e/current curves vary depending

7 0 0 -

600 -

u~ 500 -

i .

>

4O0 -t , -

O4 . .

~Ji . 3 0 0 -

2 O O

I 0 0

F i g . 1 4 . 2 . 5 M

iI I i i I I I

5 1 0 1 5 2 0 2 5 3 0 3 5

8 0 0 -

T i m e l a g ( s e c o n d s )

T y p i c a l c u r r e n t / t i m e c u r v e f o r a n e l e c t r o m a g n e t i c o v e r l o a d r e l a y w i t h a t w o r a t e



176 Pro tection o f m otors, reactors, boosters an d capacitors

Fig . 14 .2 .5N

4 0 0 0 t3 0 0 0

2 0 0 0

1000 -8 0 0

6 0 0

~ ' 4 0 0

2 300.~.

2 o o¢ d

8 ioo i> 80

6 o

4 0 -

3 0 -

2 0 "

| 0 I - I ' 1 ' I ' I I ' ' ]

0 IO 20 30 40 50 60

Te m p e r a t u re ( ° C )

Var ia tion o f the v iscos i ty o f the dashpot f lu id wi th tempera ture

up on the viscosity o f the dam pening fluid used to fi l l the dash pot. The viscosity mineral off decreases significantly with temperature (Fig. 14.2.5N) and a point mbe reached w hen the t im elag is una cce ptab ly sho rt . The use of sil icone fluid insteof oil considerably im proves the performan ce but the change of t ime delay witemperature will still have some significance (Fig. 14.2.5P).

To ob tain opt im um perform ance and take acco unt of seasonal changes in thviscosity of the f luid the dashpot set t ing should, theoretically, be adjusted to takaccou nt o f tem perature changes betwe en sum m er and w inter working. This obviously diff icult to ensure in a large instal lat ion and the need for i t has to bconsidered a l imitat ion of the device.

I t is not permissible for the dashpot to be f i l led with a f luid having a muchigher viscosity than that for which i t was designed in an at tempt to prevent tdevice from tripping during start ing: such an expe dien t w ould inevitably lead m uch s lower operat ion than is indicated b y the pub l ished cu rrent / t ime curves an

probably resul t in damage to the motor.As the t im e delay is produ ced by util is ing the effect o f a piston moving in



140 -

Fig. 14.2.5P

60 I - - " ' ' ' I ' I

0 I 0 2 0 3 0 4 0 5 0

Te m p e r a t u r e ( ( ~ ( ' )

Change in dash pot t ime delay w ith tempe rature using s il icone f lu id in thedashpot

1 2 0 -

too

~ so

I -

P rotec tion o f m otors, reactors, boo sters an d capacitors 177

app rox im ate m an ne r, the hea t ing and coo l ing charac te ri s ti c s o f the m oto r and l imi t s the degree o f p ro tec t ion which can be a ffo rded the m otor.

C o m p a ri so n o f m a g n e t i c a n d t h e r m a l p r o t e c t i ve d e vi ce s

As descr ibed in the ear lier par ts of th is Sect ion, m oto r pro tect io n fa ll s in to tb road ca tegor ies based on the d e tec t ion o f and response to e i the r e lec t roma gne

or the rmal e ffec ts . Al thoug h s imple r and essen tia lly o f one bas ic type the fo rmclear ly has a number of appl ica t ions , but the range of thermal devices avai lablemore read i ly matched to the motor charac te r i s t i c s and to the condi t ions which lto mo tor fa i lures . The perform anc e com par ison w hich fol lows wi ll serve i l lus t ra te and summarise the essent ia l fea tures of the two categor ies .

Ele c t ro ma gn e ti c t r ip s :

( i) These devices are relat ively insensi tive to small degrees o f ov erloa d w hii f su ff ic ien tly f requ en t an d a l lowed to co n t inue fo r ex tended per iods , cou ld shorthe li fe o f the m oto r insu la t ion . They can not eas ily be se t accura te ly enoug h c lose p ro tec t ion wh ich is a p r ime requ i rem ent fo r m.c . r, motors .

( ii ) The dev ices per fo rm more e ffec t ive ly on m edium to heavy over loapar t icular ly the la t ter when currents approaching s ta l l ing level are reached and

requ i red rap id ope ra t ion under the cond i t ions is ach ieved .




motor and the dev ice the re fore has no the rmal memory.

( iv ) Und er s ing le-phas ing condi t ions the per fo rm ance is com ple te ly dep endupo n the degree o f over load which th is e ffec t p roduces in the two hea l thy l in

If the increase in l ine cu rren t i s on ly smal l the o pe ra t ion of the device is do ub tful

(v) Changes in the v iscosi ty of the dam pen ing f lu id w i th tem pe ratu re wcause variat ion in effect ive overload set t ings.

Thermal overload devices:

( i ) Al l types provide some degree of pro tect io n a t a ll m ot or over loads: tmo re soph is t i ca ted re lay types can be se t mo re accura te ly and a re thus be t t e r su ito the p ro tec t ion o f m.c . r, motors .

( ii ) The re lay types g ive a consis tency of perfo rm anc e wh ich is of ten im po r tan t as inheren t accu racy and a runn ing load sca le can also be incorp ora ted

( ii i) As the op era t ion of the devices is based on therm al effec ts the pro tect i

p rov ided is be t t e r r e la ted to the needs o f the m oto r than tha t g iven by te lec t romagn e t ic type : even so the the rmal t ime co ns tan t o f the re lays is m ushor te r than tha t o f the m otors be ing p ro tec ted .

( iv) Single-phasing perfo rm ance is good and w i th the m ore com prehen stypes of re lay, opera t ion on smal l degrees of load unbalance is achieved.

(v) Bimeta ll ic devices and par t icular ly re lays w i th coi led bim eta ls have appreciable rese t t ing t ime. This is a des i rable fea ture in tha t the m oto r has sot ime in w hich to cool d ow n before a res tar t is poss ib le . Even i f the re lay a l lowres tar t , i t wi l l t r ip again if insuff ic ient cool ing has tak en place .

Thermistors

The fo re ru nne r o f th is typ e o f dev ice as used in m oto r p ro tec t ion was the the rmcouple o r the rmos ta t embedded in the motor end-wind ings and app l icab le wequa l faci li ty to a .c . and d .c . mach ines . Co m m only re fe r red to as 'mo tor overhpro tec t ion ' they p rov ided pro tec t ion aga ins t mos t motor in te rna l condi t ions whgive r ise to excess ive tem pera tures including sus ta ined ov er load; h igh, low unba lanced vo l tages; locked ro to r ; b locked ven t i l a to r ; and sing le -phase runn inga polyphase a .c . motor.

The main advan tage o f th i s type o f p ro tec t ion i s tha t the t empera tu re sens i te lem ents are locate d in the m ot or windings themselves and thus g ive an accurm easure m ent of tem pera tu re a t the po in t s where the overhea t ing is l ike ly to occ



Protection o f m otors , reactors, boo sters an d capacitors 179

permiss ible temperature a t t imes of necess i ty.The m odern vers ion of th i s type of dev ice is the the rm is to r (posi ti

tem pe rature - coeff ic ient res is tor). Several such devices are em be dd ed in bonded to the enamel -covered wi res fo rming the s ta to r wind ings dur ing manuture and conn ec t ions b rou ght ou t to a so lid -s ta te con t ro l un i t and in te rpos ing rem ou n t ed s epa ra te ly fo r sma ll m o to r s and usua lly bu il t i n to t he m o to r t e rmina l of m oto rs above 10 h .p . The re lay is inoperat ive unt i l the therm is tors indicate the w ind ing tem pera ture and the cur ren t f lowing in an y phase exceed ths t ipula ted l im i ts . High winding tem pe rature and excessive current are thus neebefore t r ipping takes place and t r ipping is avoided on s tar t ing and on occur rence of the rmal over loads o f shor t du ra t ion .

The thermis to r need be l i t t l e l a rger than a match head wi th a cor respondinlow therm al iner t ia . As these sm al l sensors can to lera te only a few m ill iam pethe con t ro l un i t inc ludes an ampl i f i e r which , in con junc t ion wi th the re lay (nfrequ ent ly sol id-s ta te) w ould in it ia te t r ipping and, i f required, an a larm. The uis designed to fail to sa fety, i .e . to the tr ipping c on dit io n. The cha racterist ics t es ting requ i remen ts a re la id do w n in BS 49 9 9 which dea ls wi th the therm al p rot ion of e lect r ic motors .

Th erm is tor charac te r is ti cs a re now am enable to c lose m anufac tur ing con t ro l any appropr ia te va lue o f re fe rence tempera ture may be se lec ted f rom the nom110 - 160 °C range of m otor- t r ip reference tem pera tures . The use o f posi titem perature -coeff ic ient (p . t .c .) therm is tors g ives a res is tanc e/ tem pera ture chater is t ic as shown in Fig . 14.2 .5Q, f rom which i t can be seen that over a re la t ivnarro w cr i t ica l - temp erature range the res istance increases very rapidly. Ty picathi s inc rease is f rom ab ou t 100£Z to over 10 0 0 ~ over a t em pera ture range of oabou t 20°C in t he band be tween 100 and 200 °C .

Fig. 15.2.5Q

3 0 0 0 "

2 5 0 0 "

E2 0 0 0 "

u I 5 0 0 "C

•~ 0 0 0 "

5 o o "

p . t . c . 1 3 0

1 4 0

1 6 0

. _ . ~ - ,

0 2 0 8 0 I 0 0 1 2 0 1 4 0 1 5 0 1 6 0

Te m p e r a t u r e ( c J t ' )

Resis tance/ tem perature character is tics o f p osi~j 've- tem perature-coeffic ientthermis tors fo r opera t ing tempera tures o f 1 3 0 , 140° and 160°C

The character is t ics are s table an d the s teep ra te-of-change of res is tance cont inuwell be yo nd the 1 0 0 0 ~ level. The character is t ics are con trol lable dur ing m anufture a nd the re fe rence temp era ture (def ined in BS 49 9 9 as ' the nom in



180 Pro tection o f m otors , reactors, boosters an d capac itors

pro tec t ion sys tem to opera te ' ) can be se lec ted to p rov ide a warn ing or a t r ipp is ignal a t any temp era ture requ ired. In pract ice the required tem per atur e wgenera lly lie be twe en the l l0 °C recom m end ed in BS 49 9 9 as the ea r ly warnreference for C lass E windings an d the 16 0°C t r ip reference for Class F insula t i

The app l ica tion o f the rmis to rs has been ex ten ded ce r ta in ly up to m otooperat ing a t 3 .3 kV and they are now genera l ly accepted as a useful , re la t ivinexpens ive fo rm of c lose p ro tec t ion o f pa r ti cu la r va lue fo r moto rs runn ing a tnear fu ll outp ut . A disadvantage is tha t th ey can only be ins ta lled a t the manuftur ing s tage an d la ter repla cem ent is d i ff icul t. Their response t ime is ra ther s lowtha t the re is a t end enc y fo r them to lag beh ind the t em pera tu re o f the wind ing wthe conseq uen t r isk tha t they m ay o pera te on ly a f te r the insu la tion has been ov

hea ted .A fu tu re deve lopment may be tha t a a un i t incorpora t ing d i rec t the rm

pro tec t ion fo r a t t ach ing to a motor a f t e r manufac tu re thus moving towards tuniversa l appl ica t ion of thermis tor type devices .

Undervoltage protect ion: Th ech ie f reason fo r em ploy ing undervo l tage p ro tec t ionof e i ther a .c . or d .c . motors i s to ensure that the motor c i rcui t -breakers

con tac to rs a re t r ipped on a comple te loss o f supp ly, so tha t when the supp ly res tored i t is not over load ed by the s im ul taneo us s tar ting of a ll the moto rs . Thipar t i cu la r ly impor tan t wi th a .c . motors in a power s ta t ion where the s imul taneos ta r t ing o f a l a rge bank of motors would p robab ly over load , and resu l t in tt r ipping of the feeding t ransfo rm er. I t is necessary a lso to avoid danger opera to rs when the supp ly i s r es to red wi thou t the i r p r io r knowledge and , fo r thomotors which are not d i rec t -on- l ine s tar ted , to prevent fu l l vol tage being appl ieda s ta t ionary machine . Und ervo l tage p ro tec t ion m ay take the fo rm of :

O)

(b)

(c)

(a)

a pla in undervol tage re lease coi l f i t ted wi th an oi l -dashpot t ime- lag anopera t ing d i rec t on to the c i rcu i t -b reaker t r ip ba r o r con tac to r mechan isman undervo l tage no-c lose re lay wi th con tac t s in te r locked wi th the mots tar ter and connected in the d .c . t r ip coi l c i rcui t , the t ime- lag beingobta ined by o i l -dashpot ;an underv ol tage re lay wi th an au xi l iary t ime- lag re lay; or

a s imple co nta cto r w i th e lec t r ica lly held- in coil .

The re leases or re lays are of the s ingle-pole type connected between the twpoles o f a d .c . supp ly or betw een tw o phases of a three-phase a .c . supp ly, tvol tage an d t ime-set tings being var iable . To reduce the r isk of un w an ted t r ipp iof w hole g roups o f mo tors , fo r exam ple , on the occur rence o f a fau l t on the t ramiss ion sy s tem , it is necessary to ensure tha t the re lay wi ll no t opera te whvol tage depress ions o f sho r t du ra t io n are exp er ienc ed. A suff ic ient ly low vol tase t t ing and an adequa te t ime-se t t ing mus t the re fore be app l ied to th i s p ro tec t ional low it to rem ain inoperat ive dur ing trans ien t dis turbance s In pract ice this mea



Pro tection o f m otors, reactors, boosters an d capac itors 181

required, the se t tings chosen being co m pa t ible wi th the character is t ics o f associated plan t . In the part icular case of pulverised-fuel-boilers, special considat ion should be given to the t ime set ting of the undervol tage p rotec t ion to guagainst the r isk of bo i ler ex plosion.

The e lect r ical ly held- in contactor has an inherent undervol tage feature in twhen the coil is energised the contactor closes but remains closed only as longthe coil remains energised. Face-plate type starters, part icularly those used with small d .c . m otors , in corp orate an undervol tage re lease coil in the s tar ting bo x. spr ing loaded moving-contact arm is held in the ' fu l ly on ' posi t ion by the e lecm agnet ic coil which is energised w hen the co ntrol con tactor is c losed. I f voltage falls to a level at w hich the streng th of the electrom agn etic f ield

insuff ic ient to hold the arm, the spr ing re turns the contact arm to the 'off ' posi t id i sconnec ting the m oto r f rom the supply.

14 .3 R eac tors

1 4 . 3 . 1 T h e p l a ce o f r e a c to r s in a p o w e r s y s t e m

The reactors which may be used in a power system can be general ly c lass i f ied tw o main groups"

( a )(b)

series reactors for short-circuit current l imitat ionshunt reactors for react ive compensat ion.

The purpose of the ser ies reactor fol lows f rom the fact that the faul t current whflows for a faul t a t any given point in a power system is determ ined by impedance of the pow er sys tem as seen f rom the po in t o f fau l t. The m aximfaul t current can, therefore , be l imi ted to an acceptable value by the provis ionseries reactors o f ap pro priate value at suitable po ints in the p ow er system . prov ision of such fa ult- l im it ing reactors can avoid the necessi ty o f provid ing laor special ly braced con du ctors or c i rcuit breakers of h igher ra t ing, capable o f w

standing the sh or t -c i rcui t currents which wo uld otherw ise occur. The use of sereactors may thus show appreciable advantages in capi ta l cost and space requments , a d isadvantage being the increased regulat ion of the system, par t iculwh en supp lying low pow er-factor loads . How ever, th is increased regulat ion cacom pen sated by the use of voltage regulators (see Sect ion 14.4) where necess

The second type of reactor referred to , nam ely the shu nt reactor, f indsappl icat ion in the co m pen sat ion of capacit ive reactance, the lagging curren t taby the shunt reactor being used to reduce or cancel the leading current taken by

shu nt capacit ive reactance of the system at the poin t conc erned. Thus, shreactors are commonly used to compensate for the large capaci tance currents w



182 Protec t ion o f m otors, reactors , boosters an d capacitors

14 .3 .2 Type s o f reac tor

Series reactors m ay be c lass if ied accord ing to the i r con st ruct ion :

( a )

( b )

air- insulated, cast- in-concrete or concrete clad types, as in Fig. 14.3.2Aandoff-im m ersed, as in Fig. 14 .3.2B.

Fig. 14.3.2A 50 Hz single-phase air insulator reactor GEC Transformers Ltd. )

They may be c lass i f ied a lso according to thei r appl icat ion, namely generabusbar or feeder reactor.

Air-insulated reactor: In th i s type , the wind ing i s wound on suppor t s and then sin concrete so that the ver tica l and hor izo nta l spacers are solid conc rete , or wind ing is da m pe d f irmly be twe en concre te end-r ings . A cast -in -concre te reac toinev i tab ly bu lky, and because of the s t rong m agne t ic f ie ld sur rou nding i t, i t mu s

s ited wel l away f rom m eta l work and wi th adequa te space fo r hea t d i s sipa tion . use o f a t ank i s thus ru led ou t making i t necessary to p rov ide some o ther fo rm



Protection of motors, reactors,b o o s t e r s a n d capacitors 183

Fig. 14.3.2B 50 Hz three-phase oil- immersed reactor GEC Transformers Ltd. )

adjoin in g, the m ain bui ld ing . This type of reac tor is used genera l ly a t voltages upand inc lud ing 33 kV.

Oil - im m ersed reac to r (m agn e t ica l ly sh ie lded).In the o i l - immersed reac to r thed i ff icu l ty o f the ex te rn a l m agne t i c f ie ld is overco m e by p rov id ing a l am ina ted ipa th a round the co i l th rough which the f lux can pass eas i ly and wi thou t s ign i f i closses . Thu s the re i s no app rec iable exte rnal f ie ld to cause hea t ing of ne arb y stw ork o r a ffec t ad jacen t e lec t ri ca l m easur ing eq u ipm ent . Th i s type the re fo re l ei tse l f to bo th ind oo r and ou td oo r se rv ice . The mag ne t i c sh ie ld , a s show n F ig . 14 .3 .2C, cons i s ts o f a 'b i rd cage ' o f iron l am ina t ion s b u i l t up in the fo rm ob a r re l , th e e n d c l a m p i n g b ar s a n d t h e h o o p s a r o u n d t h e b o d y h a v in g i n s u l a te d j ot o p r e v e n t t h e f o r m a t i o n o f s h o r t -c i r c u it e d t u r n s in w h i c h t h e f lu x w o u l d o t h e r w

se t up a c ir cu la ting cur ren t . The p resence o f the m agne t i c sh ie ld reduces r e lu c t a n c e o f th e m a g n e t ic p a t h c a us in g t h e r e a c ta n c e u n d e r n o r m a l o p e r a t



184 Protect ion o f motors, reactors , boosters an d capacitors

F ig. 14.3.2C Co il assembly o f 1980 k VA r, 11 k V, 50 Hz three~ohase oi l - im m erse d reac tor(GEC Transformers L td . )

Dur ing the shor t c i r cu i t , s a tu ra t ion o f the i ron occurs and the r eac to r behav

subs tan t i a l ly the same as an a i r-co red reac to r. Ho w ever, the r a t ing is based on tshor t c i r cu i t reac tan ce , the inc rease in r eac tance a t no rm al load cu r ren t s be ing o f



Pro tectio n o f m otors, reactors, boosters an d capa citors 185

Oi l -imm ersed r eac to r (non m agne t ica lly sh ie lded) : T hea l t e rn a t i ve o f no nmagn e t i csh ie ld ing wi th copper, o r somet imes a lumin ium, shee t s ben t to fo rm a cy l indeof ten used . Typ ica l sh ie lds a re show n in F ig . 14 .3 .2D. The am pere - tu rns induin the sh ie ld have the e ffec t o f r educ ing the reac tance by an amount equa l to

pe rcen tage o f the co i l am pere - tu rns indu ced in the sh ield . The l arge r the d iam eof the shie ld the sm al ler wi ll be the 12 R loss , but th is advan tage has to be weigagains t the d isadvantages of a la rger tan k a nd a grea ter volum e o f o il .

The c i rcu la t ing cur ren t s induced in the sh ie ld resu l t in a coun te r m.m. f , whconf ines the f lux to a

i.,.~:.i, i iI ~ ~i i:.

h be tw een the w ind ing and the sh ie ld .

/ . i

~ ~.......

Fig. 14.3.2D A l u m i n i u m s h i e l d s

Since no i ron is in t ro du ced in to the f lux pa th , the nonm agn et ica l ly shie ldreacto r can be c onsidered as being of a con stan t ohm ic reactanc e over very wl im i ts o f cur ren t .

Th i s i s the im po r tan t advan tage o f the copper- sh ie lded reac to r bu t aga ins t ti s the re la t ive ly h igh loss in the shie lds when compared wi th the magnet icash ielded ty pe . Aga in , com pared w i th the magne t ica l ly sh ielded reac to r, the copp

shie lded typ e g ives a lower reactance for the same s ize of coil and requires a lac learance b etwe en coil and shie lds , wi th th e resul t th a t i t is usual ly b igger thanequivalent reactor wi th magnet ic sh ie ld ing.

14 .3 .3 Rea c to r r a ting

The overcu r ren t f ac to r o f a r eac to r is de f ined as the ra t io o f the sym m et r ica l r.m

through - fau l t cu r ren t to the ra ted cur ren t . W hen th i s r a t io is smal l the the rmra ti ng o f t he r e ac to r is de t e rmined b y t h e no rm a l t h rou gh p u t c on d i t io ns , c a l cu l a



1 8 6 P r o t e c t i o n o f m o t o r s , r e a c to r s , b o o s t e r s a n d c ap a c i t o r s

conductors giving a less s teep gradient between oi l and winding temperatures andpermit t ing the use of a somewhat higher current densi ty.

When the overcurrent factor is large, the current densi ty is determined by theshort circuit con dition s. There is no B rit ish Standard exclu sive to reactors b ut

B.S. 171 19 70 shou ld b e used where i t appl ies . I t specif ies that reactors shall bedesigned to withstand, without damage, the electromagnet ic forces due to a currenthaving a peak value equal to 2.55 t imes the r.m.s, value o f the rated currentmult ipl ied by the overcurrent factor ; and also the thermal effects corresponding tothe specif ied overcurrent con di t ions. Table 13 o f that Standard lays dow n them axim um current densi ties depending upon the d ura t ion o f the overcurrent.

=

S E R I E S C U R R E N T - L I M I T I N GR E A C T O RT O B R I T I S H S T A N D A R l ) 171 • 1 9 7 0

R e a c t i v e k VAS y s t e m v o l t a g eR a t e d ~ u r r e n t A m pO v e r c u r r e n t f a c t o rO v e r c u r r e n t t i m e s e cN u m b e r o f p h a s e sI ) i a g r a m d rg . N o .M a k e r 's ser ia l N o .

T y p e ~f" c o o l i n gF r e q u e n c yR e a c t a n c e / p h a s e o h mOilW ei gh t ~ )f ( i llt ' ( ) r e & w d g s .T ( ) t a l ~ ' e i g h tYe a r o f m a n u f a c t u r e

u _

m

F i g . 1 4 . 3 . 3 A R a t i n g p l a t e f o r s er ie s c u r r e n - l i m i t in g r e a c t o r

A 2 B 2 C 2

I I 1 lA I B 1 C I

F i g . 1 4 . 3 . 3 B Te r m i n a l m a r k i n g p l a t e f o r s er ie s c u r r e n t - l i m i t i n g r e a c t o r

A typical rating plate and terminal marking plate for a series current-l imitingreactor are show n in Figs . 14.3 .3 A and 14.3.3 B.

14.3.4 Reactor appl icat ion

The m os t com m on ly used posi t ions for the appl icat ion of current- limit ing reactorsare in the generator connect ions to the circui t breaker, between adjacent sect ions

o f b usba r, and in series w ith the feeders. Their effective ness as regards the main ten-ance of busbar vol tage and the safeguarding of switchgear under faul t condi t ions is



Pro tect ion of m otors, reactors , boosters and capacitors 187

G e n e r a t o r r e a c t o r s : M ost mo de rn genera to r s a re designed and bu i l t w i th su ff i c ieinhere n t r eac tance to enab le them to wi ths tand a sym m et r ica l shor t c i rcu i t ac rthe i r t e rmina l s . Th i s was no t a lways so and in o lde r s t a t ions i t is no t unusu a lf ind a cu r ren t - limi t ing reac to r con nec te d in se ri es w i th each genera to r. The reacl imi t s the cur ren t which can f low to a genera to r f au l t f rom the o the r mach incon nec ted to the same sec t ion o f busbar, and in do ing so reduces the damasus ta ined by the fau l ty genera to r a s we ll a s e ffec tive ly reduc ing the shor t c i rcM VA to be hand led by the sw i tchgear. Thus the e ffec t o f the reac to r is to confthe d i s tu rbance and p rov ide re l ie f to hea l thy appara tus and feeders . Ge nerareactors are effec t ive whenever the machine i s running and therefore involve a smbu t con t inuou s energy loss und er runn ing cond i t ions . F ig. 14 .3 .4A shows a typ i

a r r ang emen t .l"eeders

A

v ,wBusbar

( ; e l l e r a t o r r e a c t ( ) r s

( ; e n e r a t o r s

A

2

2

I,'ceders

F i g . 1 4 . 3 . 4 A Connection o f generator reactors

B u s b a r r e a ct or s. ( a ) S e r i e s c o n n e c t e d : A typ i ca l e xa m p le o f t h i s me t hod o fcon nec t ion is shown in F ig. 14 .3 .4B and i t wi ll be seen tha t the reac to r is inse rd i rec t ly in the busb ar run . P re fe rab ly the c onn ec t ion o f the feeders to the va r iosec t ions o f busbar shou ld be such tha t a t fu ll load the cu r ren t c ircu la ting be tw ethe sec t ions is a m in im um . Ab so lu te ba lance be tw een sec t ions is un l ike ly to ob ta ined bu t i f th i s idea l can be approached undes i rab le losses and vo l t age d roin the reactors wi l l be ke pt to re la t ive ly ins igni f icant va lues. T hey di ffer f ro

Feeders I"eeders

Busbar BusbarReact or 1/2 section 2 React or 2/3 sectio n 3

v ,-

( ; e n e r a t o r s I 2 3

B u s b a rs e c t i o n I



188 Protect ion o f motors, reactors, boosters and capacitors

genera to r r eac to r s in tha t such un i t s incur losses whenever the mach ine i s connecto the sys tem w he the r i t is opera t ing on l igh t load o r fu l l load .

Th i s m e th od d ivides the s t a t ion capa c i ty in to severa l sec t ions so tha t nom inagene ra to r 1 supp l ie s the load on busbar sec t ion 1 , gene ra to r 2 the load on bussec t ion 2 , and so on . W hen a f eeder f au l t occurs , fo r exa m ple on one o f sec t ion 2 feeders, r eac to r s 1 /2 and 2 /3 have the e ffec t o f l imi t ing the cur ren t f ling to the fau l t f rom genera to r s 1 and 3 .

Bu sba r r e acto r s. ( b ) Ti e ba r con n ec t e d :Tw o m e t h o d s a re c o m m o n l y a d o p t e d a n dm ay be c lass if ied as

( i )( i i ) the s t a r connec t ion , a s i l lus t ra ted by F ig . 14 .3 .4C andthe r ing con nec t ion , a s i l lus t ra ted b y F ig . 14 .3 .4D .

F e e d e r s

B u s b a r

G e n e r a t o r s 1

v

2

2

Fig . 14 .3 .4C

l v

.=

3

3

R e a c t o r s

[ Tie-bar

Star connection of tie-bar busbar reactd

Fig . 14 .3 .4D Ring connection of tie-bar busbar reactors

R e a c t o r s

v 1 . . .

( ; e n e r a t o r s 1 2 3

T i e - b a r, • , , , , , ,

In ( i ) each sec t ion o f busbar i s connec ted v ia a r eac to r to a common s ta r po in t , i f the feeders and genera to r s a re su i t ab ly a r ranged l i t t l e o r no cur ren t need f lth rou gh the reac to r s. I f one sec t ion o f busbar is ou t o f se rv ice the o th e r sec t ir ema in in para ll el th roug h the reac to r s . Th i s m e th od has the obv ious d i sadvan t

tha t an add i t iona l b usbar, the t ie -ba r busbar, is r equ i red . I t w i ll be no ted tha t tha re tw o reac to r s in ser ie s be tw een sec t ions and the o hm ic value o f each reac to r w



Pro tection of m otors, reactors, boo sters an d capacitors 189

In ( i i) the reactcrs are con nec ted in series betw een sect ions of busbar and c loby a t ie-bar to form a ring. The r ing is bro ken w hen one sect ion is ou t of servand there wi l l be considerable reactance between remaining sect ions resul t ingpo or v ol tage regula t ion. As in the s tar co nne ct ion an addi t iona l busba r is requir

F e e d e r r e a cto r s: T h efunct ion of the feeder reactor i s to local ise the vol tage droto the feeder on which the faul t has occurred and a lso to enable smal ler acheape r feeder c i rcui t breakers to be used. Co m pared w i th gen erator reactors , advantages of connect ing a reactor in ser ies wi th each feeder are that a feeder fawil l not ser iously affect the busbar vol tage wi th consequent ly less tendency for gene rators to lose synch ronism and the effect of the faul t wi ll be localised. Tcon nec t ion o f feeder reactors is sho w n in Fig. 14.3 .4E.

F e e d e r r e a c t o r s

Bus har - -


v

F i g . 1 4 . 3 . 4 E C o n n e c t i o n o f f e e de r r e a ct o rs

( ; r o u p f e e d e r

r e a c t o r s

B u s b a r : '


t t t _ _ l t t




The d i sadvan tages a re tha t a con t inu ous pow er loss and adverse e ffec t on vo l t aregu la t ion m us t be accep ted , and the reac to r a ffo rds l i tt l e r e l ie f aga ins t busbfau l ts . I f the to ta l genera ting capac i ty o f the s t a t ion is inc reased i t m ay necessary to increase the s ize of the feeder reactors to l imi t the h igher shor t c i rc

cur ren t , bu t th i s inheren t r eac tance wi l l o f course have a s imi la r e ffec t on sys tregu la t ion as the de l ibe ra te in t rodu c t ion o f a r eac to r un i t .

Group f ee de r r eac to rs : Ana l te rna ti ve t o t he a r r an ge m en t w i th a re a c to r co n nec t edin each feeder i s sho w n in Fig . 14 .3 .4F . Such uni ts are ca l led grou p feeder reacts ince severa l f eeders are g rouped on to a s ing le reac to r. Ec on om y in the n um ba nd t he r e fo r e t he cos t o f re ac to r s is th e o n ly m er i t o f th i s me tho d o f co nnec t i

and i t has the d i sadvan tage tha t the d i sconn ec t ion o f the reac to r fo r e i the r fainves t iga t ion o r m ain tena nce purposes deprives the s ys tem of no t one feeder bth ree . I t i s used occas iona l ly a t vo lt ages up to and inc lud ing 33 kV .

14.3.5 Reactor pro tec t ion

G e n e r a l : Althoug h a reac to r is o f t en s imi la r in appearance to a t r ans fo rm er as imi la r wind ing m etho ds a re em plo yed , they d i ffe r in several o the r r espec t s . Ch

am ong these is the s ing le wind ing o f the reac to r and the absence o f m agne t i sin rush cu r ren t wh en the un i t i s sw i tched in . I t is thus unne cessa ry, exc ep t fo r i ron-cored reac to r, to go to the expense and com pl ica t ion o f b iased sys tem s wthe resu l t tha t r eac to r p ro tec t ion i s usua l ly s imple r than t r ans former p ro tec t ion .

In some cases the reac to r may be inc luded in the p ro tec ted zone o f ano thsys tem e lem ent , such as a f eeder, and l i t t le , i f any , add i t iona l p ro tec t ion w ouldnecessa ry. Exam ples o f th i s a re a r eac to r con nec ted in a busb ar run and em bracby the busbar p ro tec t ion , o r a r eac to r connec ted in se r i es wi th a feeder aem braced b y t he f eede r p ro t ec t i on . Th e add i t iona l p ro t e c t i o n fo r a ny i m p o r to ff- imm ersed reac to r wo uld inc lude a B uchholz g as - and o ff -actua ted re lay anwind ing t empera tu re ind ica to r, bo th o f which a re desc r ibed l a te r in th i s Sec t ion .

The types o f f au l t w hich m ay occur inc lude f la shover o f ex te rna l bush inear th fau l ts on the wind ings o r co nnec t ions (phase fau l t s i f the th ree phases m ou n te d i n on e t ank ) , co r e f au lt s , i n t e r t u rn f au lt s and o ve rh ea t in g o f t he w indresul t ing f rom excess ive loading or fa i lure of the cooler eq uip m en t . I t is usua

cons idered unn ecessa ry to p ro te c t aga ins t in te r tu rn fau l ts since exper ience show n th a t such fau l ts ve ry qu ick ly sp read to ea r th and are then de tec ted the ea r th - fau l t p ro tec t ion .

Ser ies -con ne c ted reactors: overa ll d i ffe re n t ia l p ro tec t io n : Indec id ing upon the p ro-tec t ion requ i red fo r a g iven reac to r, the size and im po r tanc e o f the un i t m us tt aken i n to accou n t and , a s w ill be s een l a te r, t h e a r r an ge m en t and nu m be r

t e rmina l s b r o u gh t ou t f r om the w i nd ing ha s a b ea r ing on t h e t yp e o f p ro t ec tw hich can be app l ied .




brou ght ou t to t e rm ina l s , an overa ll sys tem of p ro tec t io n is jus t if i ed , an d , as a l rem ent ion ed , no b ias ing is r equ i red fo r th i s app l ica t ion . The emph as is on the numof ends b rought ou t s t ems f rom the need fo r an overa l l sys tem to be d r iven f rtwo se ts o f cur ren t t r ans form ers , one se t on each side o f the reac to r. The cur rt rans formers a re connec ted by p i lo t wi res and the re lay connec ted d i ffe ren t ia l ly,show n in Fig . 14.3 .5A, g iving pro tect io n agains t phase and ear th faul ts . Th e cur ren t t r ans form ers have iden t ica l r a t ios and m ay be ho used in the reacbushings , in the case of h igh-vol tage reactors , or a l ternat ively , in the associaswi tchgear. In reactors fo r lower voltages, bushings m ay no t be f i t ted and the eof the wind ings a re b rou ght ou t to a te rmina l cham ber on each s ide l arge enou ghaccommoda te t he cu r r en t t r ans fo rmer s .

I I C .T .Three-phasereactor

Relays

C.'F.s

Fig . 1 4 .3 ,5 A Ove ra l l d i ff e ren t i a l p ro tec t ion o f a th ree -phase se rie s r ea c to r

The m odern re lay used fo r the d i ffe ren t ia l p ro tec t ion is a th ree -po le re lay th ree s ing le -po le re lays ) o f the ins tan taneous , h igh impedance , a t t r ac ted a rmattyp e. A tapp ed plug-bridge enables the se t t ing to be var ied , and in v iew of i t s himpedance i t i s necessary to connect a non- l inear res is tor in para l le l wi th the reand i ts associa ted co m po ne nts to l im i t the p i lo t voltage to a safe value durin terna l faul ts .

The ope ra t ion o f th is ba lanced sys tem of p ro tec t ion is based upon the well t rand re li ab le M erz-Pr ice p r inc ip le o f c i rcu la t ing cu r ren t p ro tec t ion . I t com pares

cur ren t s f lowing in on one s ide o f the p ro tec ted equ ipment wi th those f lowing on the o ther. In such a sys tem a fau l t occur r ing wi th in the p ro tec ted zon e wcause p r imary fau l t cur ren t to f low th rough bo th se t s o f c . t . s , in the d i rec t ioshown by the heavy a r rows in F ig . 14 .3 .5B, the resu l t ing secondary cur rec i rcu la ting as ind ica ted by the l igh t a r rows , the sum m ated cur ren t s f lowing th routhe d i ffe ren t ia lly con nec ted re lay to cause opera t ion . W hen a fau l t occurs ou t sthe p ro tec ted zone , p r imary fau l t cur ren t f lows in the same d i rec t ion th rough bsets of c . t . s , as shown in Fig . 14.3 .5C, and the secondary currents c i rcula te d i rec t ions such that there i s negl ig ible current through the re lay and no reop era t io n. Neglig ible cur ren t and not zero cu rren t , s ince in a balanced sys tem




F ig . 1 4 .3 .5 B

C . T. C . T.

~ D

w

, i, ,,• ~ ~~ ~lay

Overall differen tial protection: operation on internal fau lt

F ' a u l t ~

.,j,

C ~ . T.( . . ' . " r .

< - - -. ("V'%

A

, . A

I Relay, , ,, _ ,, _ |

F ig . 1 4 .3 .5 COverall di fferential protec tion : non-operation on external fau lt

in the c. t.s , at the tw o ends and , despite the use of pi lot com pen sating resistosom e m ismatch o f the pi lo t burdens betw een the po int a t wh ich the re lay

con nected and the c . t. s, a t e i ther s ide . Tests m ust be done d ur ing comm issioningcheck that the spil l current is indeed negligible, s ince i ts presence in any significquant i ty i s t end ing towards ins tab i l i ty o f the pro tec t ion under normal runningexternal faul t condi t ions .

Overcurrent and earth-faul t protect ion:The system of overall-differentialc i rcula t ing-current protect ion jus t descr ibed provides protect ion of the reacagainst both phase- to-phase and phase- to-ear th faul ts , but fa i lure to operate woprobably have ser ious consequences and i t i s prudent to add a back-up featureguard against such an ev entual i ty. This usual ly takes the form o f a conv ent ioinverse def in i te min imum t ime ( i .d .m. t . )overcur ren t re lay dr iven f rom separc. t .s , and co nn ected as show n in Fig. 14.3.5D . Such relays have already bedescribed in detai l in Chapter 6.

A var ia tion o f th is form of back-up pro tect ion to include an ear th-faul t featis i l lustrated in Fig. 14.3.5E, and consists of residually connecting the cen

elem ent and ap plying a som ew hat lower ear th-faul t se t t ing to it . This e lem envir tual ly ident ical wi th the ove rcurrent e lem ent b ut w ould have plug bridge se t ti



Pro tection of mo tors, reactors, boosters and ca pa citors 193

On smal l reactors , where an overal l d i fferent ia l sys tem is not jus t i f ied , tr eac tor m ay be pro tec ted by separa te o vercur ren t and ear th - fau l t sys tems , and ti s deal t wi th more ful ly in the fol lowing sect ion on shunt reactor protect ion.

R Y B T h r e e - p h a s er e a c t o r

C.T.s

O v e r c u r r e n t r e l a y

JL I.I

C . T. s f o r o v e r a l ld i f f e r e n t i a l p r ~ t e c t i o n

Fig. 14 .3 .5D Overcurrent protection as back-up to overall differential protection of three-phase series reactor

R Y B

Overcurrent re la '

T h r e e , p h a s er ~ a c t o r

C . T. s

. J ' i . " ~ - - - - B

• C . T . s f o r o v e r a l l

d i f f e r e n t i a l p r o t e c t i o n

E a r t h - f a u l t r e l a y

Fig. 14 .3 .5E Overcurrent and earth-fault protection as back-up to overall differentialpro tec tion of three~hase series reactor

T h e B u c h h o l z r e la y: A noi l- imm ersed reac tor is no t com ple te ly p ro tec ted un less is f i t ted w i th a gas an d oi l opera ted re lay. This is because a very s lowly developcore fau l t o r inc ip ien t fau l t wi th in the reac tor t ank w i l l no t be de tec ted by to ther fo rms of p ro tec t ion . I t is we ll kn ow n tha t an in te rna l reac tor fau l t is accopar tied by gases which the h eat l ibera tes f ro m the oil . This ph en om en on is u t il iin the Buchholz re lay which is fi t ted in the run o f the p ipew ork f rom the tankthe oi l conse rvator. Fig . 14.3 .5F show s a typica l re lay.

With an incipient faul t producing gas the upper f loat , or in the i l lus t ra t ion tupp er buck e t , opera tes wh en a spec if ied vo lume of gas has been co l lec ted acauses an alarm to operate W ith a heavier faul t requir ing im m ediate disco nne ct



194 Protec t ion o f m otors, reactors, boosters and capacitors

]ampling cock

To o i lconservator

Alarm

(closed)

T~~'ipcircuit(open)

Fro m

transformer

Mercury Drainswitch plug

Fig. 14.3.5F Doub le f lo at gas- and oi l -actuated relay (GEC Measurements Ltd )

surge f loat t o com plete the tr ipp ing circuit. Fall ing oil level also is de tecte d b y relay giving f irst an alarm and, if the loss continues, ini t iat ing the disconnectbefore damage can occur.

One modern re lay consis ts of a cas t housing containing two pivoted buckets ecou nterbalan ced by a weight. Each assembly incorp orates a m ercu ry switch whowing to the weight d i s t r ibu t ion of bucke t and coun te rweight , is norm al ly in t

open posi t ion. W hen an incipient faul t occurs smal l bubb les of gas wi ll be genated and these , in a t tem ptin g to pass to the conservator, wi ll be t rapp ed in the rehou sing. As the gas accu m ulates the o il level in the relay will fall an d ev entua lly top bu ck et wi ll be lef t fu ll of o i l. The buck et wi ll no t then be ful ly imm ersed the weight of the oil left behind wil l cause the whole assembly to t i l t , closing mercury switch and complet ing the a larm ci rcui t .

W ith a heavier faul t the gas is generated m uc h m ore rapidly and the displaoi l surges throu gh the re lay causing the lower bu cke t assembly to t i lt clos ingm ercury switch and com plet ing the t r ipping c i rcui ts to the h .v. and 1 .v. c i rcubreakers.




bu ck ets will be left ful l of oi l. This will cause f irst the alarm elem en t an d then surge e lement to operate , complet ing the a larm and t r ipping c i rcui ts .

The re lay is m ou nte d in a s t ra igh t run of p ipew ork s lop ing up a t abo ut 10 ° f rt ank to conserva tor, a s show n in F ig . 14 .3 .5G. The a r row on the re lay m us t poin the same direct ion as the oil flow to the conse rvator o r the re lay wi ll nfunc t ion p roper ly. I t m us t be app rec ia ted tha t the Buchholz re lay is no t a h ispeed re lay com pared w i th , say, a m od em a t t rac ted-a rma ture re lay. The surge f lm ay take as long as 0 .5 s to op erate i f the m agn i tude o f the fau l t current is l imi tand on a l l but the smal les t uni ts i t fu l f i l s a ro le supplementary to the other prott ion arrangements a l ready descr ibed.

I t should be no ted tha t a l though the re lay descr ibed is o f the do uble e lem e

pat tern a s ingle-e lement pat tern is avai lable , and in th is the contacts would con nec ted to g ive a gas a la rm only.

Winding tem pera ture protec t ion:A reac tor wind ing , l ike tha t o f a trans form er, canw i ths tand sho r t pe r iods o f overload ing w i thout damage bu t overhea t ing causedprolonged over loading or fa i lure of cooler equipment wi l l , i f a l lowed to pers iresul t in prem ature deter io ra t ion of the insula t ion a nd so sho r ten the useful l ifethe reac tor. To ob ta in a warn ing of overhea t ing of the wind ing , a ho t - sp o t t em peture ind icator i s f i t ted in the m an ner s how n in Fig . 14.3 .5H . This device indicathe t em pera ture o f the top o il and takes in to a cco unt a lso the t em pera ture o f reac tor w inding . The f i rs t is ob ta ine d by the the rm om eter imm ersed in the topthe tank , and the second by feed ing a hea te r in the same pocke t f rom a cur rt rans form er loca ted in the reac tor wind ing . To ob ta in m axim um advantage f rthe re acto r ' s abi l ity to to lera te reasonable ove r loading for sho r t per iods , tthe rmal t ime-cons tan t o f the thermometer i s made to match as c lose ly as poss i

the therm al t ime-co ns tan t o f the reac tor wind ing .In add i t ion to se rv ing as a w ind ing ho t - spo t ind ica tor the ins t rum ent incorpo

a tes m ercu ry switches : one to in i ti a te an a la rm i f the t em pera ture should reacprede te rm ined f igure , 100 °C is typ ica l, and the o ther to co m ple te the reac tor tc i rcu i t i f the t em pera ture should increase by a fu r ther f ixed am ou nt , say 20°C .

Shu nt-con ne cted reactors : ove ra l l d i fferent ia l protec t ion .If al l s ix ends of theshunt reac tor wind ing a re b rought ou t an overa l l d i ffe ren t ia l sys tem of p ro tec t ican be appl ied us ing an unbiased c i rcula ting-current sys tem as for the ser iconnec ted un i t .

Restn 'cted earth-faul t protect ion:Ina shunt reactor i t i s not s t r ic t ly necessary tobr ing ou t a ll s ix ends , and , fo r reasons o f ec on om y, the s ta r conn ec t ion i s somt im es m ade in te rna l ly and on ly four connec t ions b rou ght ou t . I t is no t thposs ible to app ly an overall d i fferent ia l sys tem of the typ e referred to above a

recourse is m ade to a sys tem requir ing only fou r c . t. s. This is kn ow n as a res t r icear th - fau l t sys tem and , as the name impl ies , p ro tec t s the equ ipment aga ins t ea



196 Protec tion of motors , reactors, boosters and capacitors

B u c h h o l zr e l ay

..=:=. /

Fig. 14.3.5G

Reactortank

S e c o n d a r y w i r i n g /t o t r i p a n d a l a r m c i r c u i t s

Buchholz re lay m ounted in p ipew ork

L rt~

as such, th is i s no t such a ser ious draw bac k as i t ma y seem at f i rs t , and phase faucan be catered for w i th a separate re lay.The appl icat ion of res t r ic ted ear th-faul t protect ion is i l lus t ra ted in Fig . 14.3

and em ploy s a s ingle p ole , h igh speed, a t t rac ted arm ature re lay of the tydescr ibed in Chap ter 6 . I t is a balanced curre nt sys tem res t r ic ted to ope rat ion faul ts wi thin the zone spanned by the c . t . s , and therefore unresponsive to exterfaul ts . The se t t ing appl ied shou ld be the lowest poss ible com pat ible w i th s tabiand should have regard to the need to protect as much of the winding as poss ible

Overcurrent pro tec t ion: Phase faul ts on reactor ins ta l la t ions are a considerabllower r isk than ear th faul ts and can be protec ted ag ains t by us ing a three-pi .d .m. t , overcurrent re lay of the type descr ibed in Chapter 6 , and connected show n in Fig . 14.3 .5I . This re lay is by i ts nature re la t ively s low in op erat ion andm us t be se t to d i sc rimina te w i th o the r p ro tec t ive e qu ipm ents .

Buchho lz and ove r- t empera tu re p ro tec t ion :A shunt -connec ted o i l - immersedreac tor wou ld qua l i fy fo r the f it ting of a Buchholz re lay and a wind ing tem pera tu



P r o t e c t io n o f m o t o r s , r ea c to r s , b o o s t e r s a n d c a p a c i t o r s 1 9 7

( ' . T. I

//

R e a c t o rw i n d i n g

-"1

R e a c t o r t a n

Wi n t l i l l g h~t - sp~ t- = - . .

ten1pcra t u re intlic~,t~r

Oi l

. 'actor

t o p p l a t e I[

L I I

_~ Therm°meterll

Uapi l | a ryt u b e

Tr i p

M ere ur.v sxvitches

M e r c u r y s t i t c h c a rr ie r

Bt~urdt~n tubei l l e c h a nism

Fig. 14.3.5H W i n d i n g h o t - s p o t t e m p e r a t u r e i n d i c a t o r f o r o i l -i m m e r s e d r e a c t o r

l n t e r t r i p p i n g : As the name impl ies , in te r t r ipp ing i s a means o f e ffec t ing thet r ipp ing o f a c i rcu i t b reaker a t a p o in t rem ote f ro m tha t a t w hich a fau l t hasoccu r r ed . T o d o th is i t is neces sa ry t o tr ansmi t i n t e ll igence f rom the po in t a t w h ichthe f au l t ha s been de t ec t ed t o t he o the r end o r ends o f t he c i r cu i t and t o a r r angefor i t s recep t ion to in i t i a te the t r ipp ing of the appropr ia te c i rcu i t b reaker o rbreakers w i tho ut re fe rence to any o ther con di t io ns p reva i ling a t the t im e . In te r-t r ipp ing f inds i ts m a in app l i ca t ion a s pa rt o f t he p ro t ec t i on s che m e o f a tr ans fo rmer

con ne c t ed d i r ect ly t o t he e nd o f a f eede r, t ha t is w i thou t an h .v. c ir cu it b r eake rloca l to the t rans form er, and as par t o f the p ro tec t ion o f a reac tor s imi la r ly l ack ing



1 9 8 P r o t e c t i o n o f m o t o r s , r ea c to r s , b o o s t e r s a n d c a p a c it o r s

Three - pllasereactor

C. T.

( ) v e rc u r r e n t , i ca r th-t'au l tr e l a y ( i . d . m . t . ) r r e l a y

Fig. 14.3.51 R e s t r ic t e d e a r t h - f a u l t a n d b a c k - u p o v e r c u r r e n t p r o t e c t i o n o f a th re e -p h a s e s h u n t

r e a c t o r

consis ts of sending a s ignal , in i t ia ted by the reactor protect ion, over a rented PostOffice p ilot cha nn el, or privately o w n ed pilot ch ann el if available. A carrier signalt ransmi t ted over the pow er sys tem condu ctors may be used in certa in c ircum-stances . Fig. 14.3.5J i l lus t ra tes a typical appl icat ion.

O v e r c u r r e n t O v e r c u r r e n t

r e l a y r e l a y

Pilot w i r e f e e d e r p r o t e c t io n

' I' b b ;INT _~ , . . . . INT

I £ - s1 3 2 k V I , I

' - f D "-11 O v e r a l l p r o t e c t io n r e l a y

I Shun t II ~ a c t o r ( ~ . ~I "~ I Buch holzrelay

I f _ Ii _ _ I

Fig 14 3 5J

II "=" - - I__~______~~L W indingt e m p e r a t u r e indicat~r

INT IntertripR receive r e l a y

1 3 2 kV

INT l ln ter t r ip

S se nd relay

D ia g ra m o f p r o t e c t i o n a n d i n t e r t r i p p i n g f o r a 1 3 2 k V f e e de r a n d o i l im m e r s e d

Next Page





Pro tection o f m otors, reactors, boosters and capacitors 199

For fur ther considerat ion of in ter t r ipping and pro tect io n s ignal ling facil i tiestheir appl icat ion reference should be made to Chapter 7 .

To obtain a f igure for the tota l t ime required to isola te a faul t f rom succircui t i t i s of course necessary to add the inter t r ipping t ime to the protect ion

circui t bre aker operat ing t imes. Thus the absence of a c i rcui t break er has the eof appreciably increasing the length of t ime a faul t remains on the system wconseq uent increase of dam age a t the po in t o f fau l t and a grea te r t ende ncy towsystem instabi l i ty.

An al ternat ive to convent ional in ter t r ipping techniques for t ransformerrea cto r faults uses a single-pole fault- thro w ing switch. O pera tion of the ovdifferent ia l protect ion or the Buchholz protect ion t r ips the local c i rcui t breaker

ini t ia tes the c losing of the faul t - throwing switch to apply a del iberate system betw een one phase and ear th . The substant ia l faul t curre nt flowing unde r condi t ion i s de tec ted by the pro tec t ion a t the remote end and t r ipp ing takes pI t is s imple and robus t and funct ions inde pen den t ly of s ignall ing channels and eqment but has disadvantages in that i t s operat ion wil l subject the power systemanother, possibly more severe , faul t condi t ion than the one i t i s about to c lDesigns have been developed sui table for appl icat ion on systems havinthree-phase faul t level of up to 2500 MV A at 132 kV.

14.4 Boosters

14.4.1 The place of boosters in a pow er system

It is the responsibi l ity of the Sup ply Ind ustry to m aintain w ithin declared l imthe voltage at every consumer 's premises. This is no small task, but , as wil l be

la ter, i t i s one which is tackled in var ious ways depending upon the requirementhe system concerned. The pract ice has been to concentra te an increasing prot ion of generat ing s ta t ions , w i th the e xce pt ion of nuclear s ta t ions , in a few producing areas, result ing in long transmission l ines and the inevitable voltage dbetween the generat ion areas and the load centres . Vol tage adjustments are usum ade at t ransform ing s ta t ions , b ut in a scat tered dis t r ibut ion area suppl ied one supply po in t ad jus tme nts m ay be m ade a t the rem ote ends of the d is t r ibulines.

In this chap ter, the term 'boo ste r ' is no t res t r ic ted to the boos ter t ransfo rbut em braces othe r forms o f vol tage regulat ing equ ipm ent . These include t rform er tap-changing, the moving coi l regulator and the induct io n regulator,together wi th the booster t ransformer these are discussed in more deta i l infol lowing S ect ions. I t should be no ted a lso tha t a l though the term boo ster is commonly used, these equipments provide faci l i t ies for reducing or 'buckingwell as for boosting the voltage.

A t ransformer with tap-changing gear performs a double funct ion; volt ransformat ion and vol tage boost ing or bucking by varying the set t ing of the

Previous Page





20 0 Protection o f motors, reactors, boosters and capacitors

in size and cost and can be installed at any point in the system where volregulat ion is required. On the other han d, the lossses in the boo ster make combined t ransformer and boos te r a r rangement less e ff ic ien t than the t ransforw ith integral tap-changing. Features of the m oving coi l regulator and the indu c

regulator are the con t inuo us regulat ion ob tainable and the absence o f switchesmo ving contac ts, thus reducing the a m ou nt o f main tenance requi red .

14.4.2 Trans form er tap-changing

In this , the most obvious method, the t ransformer secondary vol tage is var iedchanging taps , that is by varying the n um ber of effective tu rns in one of the wings, thereby changing by a smal l amount , and without phase-shif t , the t ransfot ion ra t io .

This is done by means of selector switches, and i t is essential that there isbreak in the winding circui t whi le the selector is passing from one tapping tonext. Since this means that there will be a short period when the selectoconnected to two adjacent tappings, i t i s necessary to int roduce res is tors , or sot imes reactors , to l imit the f low of short c i rcui t curren t dur ing the changeperiod. One phase of the winding and tap-changing arrangements of a typ

27 5/13 2 kV a utotra nsfo rm er is show n diagramm atically in Fig. 14.4.2A.Transformer tap-changing gear does not form par t of this chapter and

mentioned here only for the sake of completeness . The tap-changing equipmeincluded in the zone em braced by the t ransfo rm er p rotec t ion and no sepapro tect ion is required, a l tho ugh the presence of the tap-changer does inf luencedesign of the protect ion appl ied to the t ransformer uni t as a whole .

Ser ie s t ' ~mm~n

275 kV -

/ Boo,st /b uck s~ i tch

Diver terswi t ch

132 RV - - - ' ' - - - - ' - ~ Trans fe r ] ~res is tors Tap winding

Fig. 14.4.2AW inding arrangement of one phase of 2 75 /13 2 k V three-phase autotra nsfo rm er

14.4 .3 Boos ter t ransformers




essent ia l ly of an auxi l iary t ransformer with i ts secondary winding connecteseries w ith the m ain t ransfo rm er, so tha t the ou tpu t is the resul tant of secondary vol tages of the two t ransformers . The secondary winding of the auxit ransformer is provided with a number of tappings to give a var iable vol tage ol imited range. Thus the boost ing vol tage is governed by the number of seconturns actually in circuit , and the regulation is adjustable in steps, rather tcont inuously as with the moving coi l regulator and the induct ion regulator. kV A ra ting of a boos te r m ay be def ined as the produ ct of the max imu m chanvol tage it can produ ce and the load cu rrent i t can carry.

A s imple form of boo ster for s ingle-phase c i rcui ts consis ts of an autotran sfow ith tappings, the tap posi t ion being selected b y a switch of the face-plate typ

vol tages above, say, 3 .3 kV and for high ou tpu ts on- load tap-changing geincorpora ted .

Tapped, ,

F i g . 1 4 . 4 . 3 A Diagrammatic arrangement of simple v o l ta g e b o o s t e r

I f the equipment were connected as shown in Fig. 14.4.3A the tap-change would have to carry the ful l l ine current . Apart f rom other considerat ions wou ld make the cos t p rohib it ive , and the usua l m etho d i s to em ploy an untapboost ing winding with i ts secondary connected in the l ine and i ts pr imary energ

from a tapped regulating or ' teaser ' w inding, as sho w n in Fig. 14.4.3B and mfully in Fig. 14.4.3C. I t wil l be seen from these diagrams that the on-load changing is performed in a circuit remote from the l ine whose voltage is bcon trolled . Bu cking facili ties also are available w ith this equ ipm en t, a nd are ob tasimply by reversing the conn ect ions to the pr imaries of the regulating t ransforusing the switch provided (not shown in diagram). The boost ing and regulat ransformers usua l ly form a combined uni t mounted in a common tank .

For l ines having high power factor loads, for example dis t r ibut ion ci rcui tdomest ic consumers , a s impler and cheaper arrangement can be appl ied. I t con

Boos t ing t rans former

|,

Regula t ingt ransformer



2 0 2 P r o t e c t ~ o n f m o t o r s , r e a c to r s , b o o s t e r s a n d c a p a c it o r s

Y i •

B o o s t i n g w i n d i n g

O u t p u t

R e g u l a t i n gwin d i ng

F i g . 1 4 . 4 . 3 C C o n n e c t io n s o f t h r e e -p h a s e b o o s t e r w i t h r e g u l a t i n g w i n d i n g

of an au to t ransform er wi th one tapping only and a reac tor con nec ted in ser ies wi ththe incoming l ine , as shown in the s ing le-phase d iagram Fig . 14 .4 .3D. Reference F ig . 1 4 .4 .3 E wi ll sho w tha t under no- load con di t ions the no- load cur ren t Io , tha t the exc i t ing cur ren t o f the au to t ransform er, l ags near ly 9 0 ° behind the incom ing

F i g . 1 4 . 4 . 3 0

R e a c t o r. ~ ,,

V

TA u t o 4 ra nsform e r V 2

VII I

S in g l e -p h a s e b o o s t e r f i x e d b o o s t )

V

V~ o< : V 2

V x

10

0



P r o t e c t i o n o f m o t o r s , re a c to r s, b o o s te r s a n d c a p a c i to r s 2 0 3

voltage VI. The small reactor voltage dropVx i s in quadrature with Io , and theresu ltant p rim ary voltage V~' is directly p rop ortio na l to II2.

V X

V

V t I o c V 2

V x 0

F i g . 1 4 . 4 . 3 F Phasor diagram o f single-phase booster on high p ow er-factor load

Fig. 14.4.3 F il lus t ra tes the con di t ions w ith load p ow er factor ap proach ing uThe pr imary current 1is now almost in phase with the incoming vol tage V, andvol tage drop Vx across the reactor will be increased. With the increase in powfactor this drop wil l be a lmost in quadrature with VI; the resul tant pr imary volV, ' being considerably higher than before but again proport ional to V2. Thusjudiciously choosing the turns ra t io and reactor impedance the increase in voltage 112 can be made equal to any required value. This method provides uabou t 6% boo s t and f inds ap pl ica t ion in d is t r ibu t ion sys tems supply ing h igh pfactor domest ic loads.

In-phase boost ing. Th eboosters so far described inject a voltage in phase with, in phase opposi t ion to , the l ine- to-ear th vol tage, that is they add or subtravoltage in phase with the l ine voltage. This can be i l lustrated by consideringpoints A and B in an intercon nec ted netw ork , the vol tages a t these tw o points bVA and VB . If , as shown in Fig. 14.4.3G(a), the voltages are equal and in phase,feeder b etween A and B will carry no current , bu t i f an in phase vol tage bo os

V A V B VB

i

(a) (h)





P r o t e c t i o n o f m o t o r s , r e a c t o r s , b o o s t e r s a n d c a p a c i t o r s 2 0 5

i] . . . . . .

B

R e g u l a t i n gt r a n s f o r m e r

1• i

B ( ) o s t e r t r a n s f o r m e r- - ~ . . . .

. . , , J " Yv ' v ' ~ _ ,

F i g . 1 4 . 4 . 3 1 C o n n e c t io n s o f t h r e e -p h a s e q u a d r a t u r e b o o s t e r

1 4 . 4 . 4 T h e m o v i n g - c o i l r e g u la t or

I n t r o d u c e d m a n y y e a r s ag o b y F e r r a n t i L t d . , th e m o v i n g - c o i l r e g u la t o r h a s it s

w i n d i n g s w o u n d o n a t w o - l i m b e d c o r e m u c h a s in s t an d a r d tr a n s f o r m e r p r a c t ic e .

T h e l a m i n a t e d i r o n c o r e c a r r ie s t h e f i x e d c o i l s a a n d b , o n e a t e i th e r e n d o f o nl i m b o f t h e c o r e , a n d a s h o r t - c i r c u i t e d m o v i n g c o i l s w h i c h is f r ee t o m o v e u p

d o w n t h e l im b , o v e r t h e f ix e d c o i l s, as s h o w n in F i g. 1 4 . 4 . 4 A . T h e m o v i n g c o ii s o l a t e d e l e c t r i c a ll y, t h a t is s h o r t - c i r c u i t e d o n i t se l f, a n d s o d o e s n o t r e q u ir e c o n n e c

t ions or s l ip r ings .

I



206 Protection o f mo tors, reactors, boosters and capacitors

The e ffec t ive imp edan ce of co il s a and b is governed b y the pos i t ion of co il coi l s is m ove d c loser to coi l a the low er the effect ive im ped anc e of coi l abecome un t i l co i l s i s in the pos i t ion shown in F ig . 14 .4 .4B, when the e ffecimpedanc e o f co il a w ill be a m in im um and tha t o f co il b a m ax im um .

If a vol tage is app l ied across coils a and b in ser ies , the g reater pa r t of th is vowil l appear across coi l b , as shown in Fig . 14.4 .4B. Conversely, wi th coi l s inpos i t ion show n in F ig . 14 .4 .4C, the impe danc e o f co i l a wi ll be a m ax im um andb a m in im um , and the g rea te r vo ltage wi ll appe ar ac ross co il a .

F i g . 1 4 . 4 . 4 B Moving coi l in upper posi t ion

F i g . 1 4 . 4 . 4 C

E

IiI I1

Moving coi l in Iower pos i t ion

tI 9 5

al V~Jlts

+.' l i - I Vo l t s

Since the core i ron is run a t a very low f lux densi ty the i ron loss i s smal l , anorder to reduce the cur ren t in the moving co i l , co i l s a and b a re connec teopp os i t ion , so tha t the cur ren t s the y induce in the m oving co il a re to a l arge exneut ra l i sed . The resu l tan t fo rce on the mo ving co i l is the re fo re very smal l anno- load is ze ro fo r an y p os i t ion of the regu la tor. The cur re n t in th i s co il w henregula tor i s on- load i s p ropor t iona l to the load and remains independent o fregu la tor pos i t ion . Thus the power requ i red to opera te the regu la tor in very sand even la rge regu la tors can be opera ted by hand .

The vol tage values quoted in the f igures are for i l lus t ra t ion purposes , and forin a p rac t ica l sys tem a vo l tage var ia t ion of 25% would be adequa te in mos t c



Pro tection of m otors, reactors, boosters and capacitors 20 7

t app ed t rans fo rm er, as in F ig . 14 .4 .4D , o r by add ing w indings on the regu la toin Fig . 14.4 .4E and Fig . 14.4 .4F.

" ' 2

4

9

qlq

q l

, q

. ._./

1

I1() V I M(~vingc()il

II _l

h 20vlI

, IliOV91 V I ;)tltpUt

" - Ld

q

lr ; i i1s I '() r 11e r

~ov["" i

iI

III'DI .

I. 1p90V

- - - - i

IIa 2 0 V I

l

1

lt~O V

.J out ut

F i g . .1 4 . 4 . 4 D Movh~g~:oil regulator connections when used with fixed-tap transformer

o f0 a 90 a

I00 Voutput1O0 V 90 V

100 b 1 s l 0 b

, w v

(a) (b)

1 0 0 Vo u t p u t

F i g .1 4 . 4 . 4 E Connect ions of m oving-coi l regulator fo r boost ing

In the fi rs t case the regu la tor is conn ec te d ac ross par t o f the t rans fo rm er w in

and by a ca re fu l cho ice o f t app ing the requ i red vo l tage var ia t ion can be ob taThe range shown g ives a var ia t ion f rom 90 V to 110 V for one tapp ing pos i tan the r t app ing w ould g ive a d i ffe ren t range.

The add i t iona l t rans fo rm er is undes irab le and the more usua l m eth od i s tad di t iona l co i ls on the regula tor. In Figs . 14.4 .4E (a) an d (b) , coi l r has badded a t the top of the l imb over co i l a , and we wi l l a ssume tha t co i l a has t im es as man y tu rns as co il r. I t wil l be seen tha t the ou tp u t vo l tage is m ain taa t a cons tan t va lue o f 100 V when the inpu t vo l tage var ies be tween 90 V appl it he max imum boos t pos i t i on and 100 V app l i ed i n t he o the r ex t r eme pos i tReversing co il r w ould have the effec t of sub t ra c t ing it s seco nda ry vol tage f rom



20 8 Protection of motors, reactors, boosters and capacitors

I

1 0 0 Vut

I I O V

(a)

E

9 0 V

i o

: a

100 Vo u t p u t

b )

F i g . 1 4 . 4 . 4 F Connections of m oving-coil regulator fo r (a) buck ing and (b) boosting

To provide bo th buck and boos t , another co il I is m oun ted a t the bo t t om ofl imb beneath coi l b , as shown in Figs . 14.4 .4F(a) and (b) . Assuming that coi l wound with e leven t imes as many turns as coi l 1 , then, wi th 110 V input and moving co i l in the min imum boos t (maximum buck) pos i t ion , the ou tput vo lwill be 110 - 10 = 10 0 V (Fig. 14 .4.4F (a)) . If the m oving coil is m ove d tom axim um boos t pos i t ion , the input o f 90 V will be boos ted to g ive an ou tpu100 V (Fig. 14.4 .4 F(b)) . Thus w ith the turns ra t io q uote d for coils r and l,

ou tpu t voltage can rema in co nstan t a t 100 V over inp ut vol tage var ia tions of -+One s ignif icant advantage of the moving coi l regulator over the induc

regulator is tha t by choosing ap prop ria te turns ra t ios for coils r and I any valubuc k or boost , including all boos t and no b uck , can be obta ined . The indu cregulator provides facil it ies for equal values of buck and boost .

One appl icat ion of the moving coi l regulator is as an a l ternat ive to on- ltap-changing gear on a dis t r ibut ion t ransformer, par t ly because i t i s economicpar t ly because it is m ore effect ive to apply the regulat ion a t som e p oint a longdis t r ibutor. W hen used for th is p urpose i t is usual ly auto m at ic in opera t ionvol tage operated re lay or contact-making vol tmeter is used to ini t ia te operat iothe regulator in the correct d i rect ion whe n the vol tage varies by, say, +-1% fromrequired value.

A rela t ively rece nt deve lopm ent of the moving coil regulator for control of cess ing work incorporates the means of re leasing the moving coi l ins tant ly, soducing the vol tage to zero in under a second. Thus loads can be switched on o

without recourse to switches or contactors .Figs. 14.4 .4G, 14.4 .4H and 14.4.4I show views of typical s ingle-phase and th



P r o t e c t io n o f m o t o r s , r e a c t o r s , b o o s t e r s a n d c a p a c i t o r s 2 0 9

o

F i g . 1 4 . 4 , 4 G Core ~nd co i l a s sembly F i g . 1 4 . 4 . 4 H Core and co i l a s sembly o fo f lO Ok VA , s in gle -p ha se , 4 1 5 - 3 7 3 / 4 5 V 1 0 0 0 0 k VA , th re e- ph as e, 11 0 0 0 / 1 0 3 4 5 - 11 6 0 5 Vau tom a t i c mov ing -cc~ i l vo lt age r egu la to r au tom a t i c mo v ing -co i l vo l tage r egu la to r(Fe r ran t i L td . ) (Fe r r an t i L td )



2 I0 Protect ion o f m otors , reactors , boosters and capaci tors

F i g . 1 4 . 4 . 4 1 Core and co i l a s sembly o f pa r t o f 14 O0 0kV A, 13 O00V mo ving~ :o i l r eac to rbank fo r h .v. cab le t e s ting equ ipm ent (Fer ran t i L td . )

14.4 .5 Ti le ind uc t ionregulator

An indu c t ion regu la tor is des igned in m uch the same way as a th ree-phase , woro to r i ndu c t ion m o to r , w i th t he excep t ion t ha t t he ro to r is l ocked to p reveturn ing under the e lec t romagne t ic fo rces ac t ing on i t . The angula r pos i t ion oro tor wi th respec t to the s ta to r can be var ied by hand . F ig . 14 .4 .5A showsm eth od of conne c t ion , f rom wh ich i t will be seen tha t the s ta to r o r p r imwinding is conn ec ted ac ross the supply and the r o to r o r seco nda ry wind ing ser ies wi th the l ine whose vol tage is being control led .

W hen the s ta to r is con nec ted to the supply a ro ta t ing m agne t ic f ie ld is sew hich induces an e .m. f , in each phase o f the ro tor. Th is induc ed e .m. f , is o f co nm agni tude and , s ince i t depends only on the s t ren g th an d speed o f the ro ta t ing




Statl)r x~ llding_., ~ ~ ~ _ Rc:t~Jr, ~ nd illg

l " x

F i g . 1 4 . 4 . 5 A

I n p i tA ~ . , ,

1 , , , , , , , , _ _ _ _

Connections of three-phase induction regulator

( )utput

V

V I/

( )

F i g . 1 4 . 4 . 5 B Phasor diagram o f one phase o f three-phase in du ction regu lator

affect the strength or speed of the rotat ing field i t does affect the phase abetween the rotor induced e .m.f , and the s ta tor vol tage.

The connectior, of the secondary windings in series with the l ine results to ta l l ine vol tage equal to the vector sum of the incoming vol tage and the volindu ced in the sec ond ary. This is i l lustrated in Fig. 14.4.5B, in w hich V1 represthe supp ly voltage, V2 the boo sting voltage, and V the resulta nt voltage. Altethe posi t ion of the rotor has the effect of rota t ing the phasor112,which remains ofcon stant m agni tude wh atever the posi t ion of the rotor. A phase-shift is in t rod ube tw een the su pply voltage V1 and the resu ltant voltage V for all posit ions exwhen the boosting voltage II2 is exactly in phase withV, t h e m a x i m u m a m o u n t o fphase-sh i ft depending upon t il e m axim um boos t o f the regula tor concerned . FFig. 14.4 .5B i t wi ll be seen that , w hen the ro tor posi t ion is such that the seconvoltage is in phase with a nd in the sam e d irection as the supp ly voltage, the resuvoltage is equal to the sum of the voltage VI and I/ '2. W ith the roto r mov ed tooppos i te p osi t ion, so that the secon dary vol tage is in ant iphase with the su

voltage, the resu ltant is equal to the difference o f the voltages VI and V2. Im ediate posi t ions o f the roto r wi ll , of course , g ive interm ediate resul tant volt



21 2 Pro tection o f mo tors, reactors, boosters and capacitors

The phase-shif t o f the l ine vol tage is no t of great pract ical im porta nce wh enind uc tion regulator is associated w ith a line feeding an isolated section ofsys tem, b ut is an emb arrassm ent i f the line forms par t o f a ne tw ork in te rconn ew ith a no ther source of su pply. This is because the phase-shift gives r ise to

exchange of current between the sources and to differences in the magni tude ofresul tant voltages . Th u s the two sources o f supply w hich norm ally have eoperating voltages will not exchange current as long as these voltages remaiphase, bu t will do so if a phase-shift is intro du ced , for exa m ple, by the installaof an induct ion regulator. In such cases a double regulator with two rotors is ueach uni t c ontr ibu t ing on e half of the tota l vol tage added to each phase. O f tequal boost ing vol tages one is arranged to lead and one to lag so that the comb

phase-shift is zero and the resultant voltage is always in phase with the suvol tage. Fig. 14.4.5C shows the main connect ions for this equipment Fig. 1 4.4.5D the op erating principle in phas or fo rm . VI is again the sup ply voand I:2 a nd V3 are the individual boos ting voltages. M echanical cou pling o ftwo rotors ensures that I :2 and V3 are inclined at the same angle to V1 andbo os ted vo ltage V is always in phase w ith V I.

S t a t o r R ~ ) t o r S t a t o r R o t o r

w i n d i n g i w i n d i n g I ~ v i n d i n g 2 w i n d i n g 2

I n p u t m - , J i e Y V ' v ' ~ , , ~ - O u t p u t

~ ,

I C " " ' -

F i g . 1 4 . 4 . 5 C

Reg u la to r I R e g u l a to r 2

Connec t ions o f do ub le th ree~ohase indu c t ion regu la to r

v

V s (

0




1 4 . 4 . 6 P r o t e c t i o n o f b o o s t e r s

I t was m ent io ne d in Sec t ion 14 .4 .1 tha t the t e rm 'boo s te r ' is no t l imi ted to boos te r t r ans former bu t inc ludes a l so t rans former t ap-changing , the moving-

regu la to r and the induc t ion regu la to r. Of these the t ap-changing equ ipmeninvar iably included in the zone covered by the t ransformer overa l l d i fferent ia l t ec t ion , and fo r th i s re fe rence shou ld be m ade to Ch apte r 12 . The p ro tec trequ i rem ents o f the remain ing typ es o f boo s te r can bes t be desc ribed by consing a typ ica l scheme of p ro tec t ion fo r a boos te r t r ans former.

Le t us cons ider a boo s te r ins ta l led a t on e end o f a 132 kV overhead l ine fo rpurpose o f m od i fy ing the phase-ang le o f the l ine and im prov ing the loa d sha

be tw een th is and o the r pa ra l l e l-connec ted feeders . I f the l ine is a ssum ed to haphase-angle of 70 ° lagging, the boo ster wi ll have been des igned to have a phangle of 70 ° leading, so enab l ing th e ove rhead l ine to carry i ts fu ll w at t fu l loatyp ica l r a ting o f such a boos te r wo uld be 90 M VA a t 132 kV, and the p r imcon nec t ion s wo uld be as show n in F ig. 14 .4 .6A. In F ig . 14 .4 .6B the loca t ion onecessa ry cur ren t t r ans formers i s shown in , o r ad jacen t to , the loca l 132 kV c ib reaker, on the line side o f the boo s te r and in the boos te r ea r th con nec tAdd i ti ona l cu r r en l t r ans fo rmer s a re shown fo r f eeder d is tance p ro t ec t ion (Z )the back -up ove rcu r r en t p ro t ec t i on .

The pro tec t ion o f the boos te r m ay cons i s t o f res t r ic ted ea r th - fau l t p ro tecdr iven f rom current t ransformers in the c i rcui t breaker, in the l ine and in the econ nec t i t . a . The res t r i c ted ea r th - fau l t r e lay wo uld be o f the h igh speed , a t t r aa rmature type desc r ibed in Chapte r 6 . Wi th such pro tec t ion , sens i t ive to ea r th fonly, i t i s necessary to add protect ion agains t phase faul ts . This can bes t

70 ~

L=J L . . J

132 k V I 32 kV

F i g . 1 4 . 4 . 6 A C o n n e c t i o n o f a b o o s t e r t r a n s f o r m e r

prov ided as shown, by u t i l i s ing the overcur ren t re lay which would be requ i reany case , fo r feeder back-up pro tec t ion . Th is re lay wi l l the re fore p rov ide bacto the h igh-speed d i s tance feeder p ro tec t ion and wi l l a ffo rd p ro tec t ion agaphase- fau lt s on the bo os te r o r on the conn ec t ions be tw een the c i rcu i t-b reakerthe boos te r.

When a booster i s ear thed via a neutra l ear th ing res is tor i t i s necessary to pro



214 Protec tion of motors, reactors, boosters and capacitors

I _ L . L I I I

II I

L _ I

II

132 kV I

IIIIII

II II

I 32 k V

I - - IL . . . . . . . . J

F i g . 1 4 . 4 . 6 B D i ag r am o f p r o t e c t i o n a n d i n t e r t r i p p i n g f o r a 1 3 2 k V f e e d e r a n d o i l - im m e r s e db o o s t e r t r a n s fo r m e r

This is convenient ly and adequately achieved by a s tandby ear th-faul t re lay drfrom a curren t t ra nsform er loca ted in the res is tor ear thing con nec t ion, as show

Fig. 14.4.6B.As in the case of the off-Idled reactor and, of course, the conventional tr

form er, the boo ster requires pro tect io n capable of detect ing faul ts wi thin the tpreferably before they develop to the s tage in which severe damage wil l be cauThe o nly re lay capable of detect ing inc ipient boo ster in terna l faul ts o f th is tythe Buch holz re lay of w hich one typ e is descr ibed in Sect ion 14.3 .5 o f th is chaand anothe r type in Chapter 12 .

Again, as wi th reactors and t ransform ers , a winding hot-sp ot indicator w oulf i t ted to provide a warning of any overheat ing of the booster windings. The mercury switches incorporated are arranged to provide f i rs t , an a larm to indithat the winding temperature has r isen to a cer ta in value, and second, a s ignaini t ia te t r ipping of the c i rcui t -breakers i f the temperature r ises by a fur ther famount . The ins t rument is descr ibed in Sect ion 14.3 .5 and shown in Fig. 14.of th is c hapter.

Intertripping: Since the booster is connected sol idly to the feeder, faul ts wi thin booster can only be c leared by opening the local c i rcui t breaker and the c i r




boos te r p ro tec t ion but in te l l igence has to be sen t f rom the boos te r p ro tec t io' ins t ruct ' the remote end to t r ip . As explained in Sect ion 14.3 .5 , th is is know'intertr ipping' and is usually effected by signall ing over a Post Office l ine rentethe purpose. The appl icat ion of in ter t r ipping is shown in Fig. 14.4 .6B, the res t r

ear th faul t , B uchholz and dis tance prote ct ion a ll being con nec ted to ini t ia tesending of an inter t r ip s ignal in the event of the operat ion of any one of them.

14.5 Capaci tors

14.5.1 C apac itors in ani n t e r c o n n e c t e d p o w e r sys t em

The t ransmiss ion sys tem in England and Wales i s a c lose ly in te rconnec ted provid ing fac i li t ies for the bu lk t ransfer o f pow er f rom cent res o f genera t ion incoa l p roducing a reas to o ther la rge load cen t res , fo r in te rconnec t ion be twcent res wi th bo th genera t ion and load , and for the supply of power in bu lk tod is t r ibu t ion companies for d i s t r ibu t ion to the i r consumers . Al though an unloatransmission or dis t r ibut ion l ine is capaci t ive in character a ful ly loaded l ineinheren t induc t ive and res i s t ive charac te r i s t ics . On main t ransmiss ion l iopera t ing a t near un i ty power fac tor, the power which can be t ransmi t ted

de te rmined la rge ly by the sys tem s tab i l i ty l imi t ( the sys tem wi l l remain s tprovided the phase angle be tween the vo l tages a t the sending and rece iv ing edoes no t excee d a cer ta in c r it i ca l va lue) . On d is t r ibu t ion ne tw orks , espec ia l ly topera t ing a t low pow er fac tors , the induc t ive rea c tance m ake s a m ajor cont r ibuto the vo l tage drop , and i t i s vo l tage drop cons idera t ions which f requent ly d icthe amount of power which can be d i s t r ibu ted .

The effect of ser ies induc tance in a pow er n etw ork is to cause

( a )

( b )( c )

a phase sh if t be twe en the d i ffe ren t par ts o f the ne tw ork tending towainstabi l i ty a t an undesirably low power level ,excessive voltage drops be twe en the ends o f feeders , andunequal shar ing of the load between paral le l feeders , thus l imit ing the tpower which can be t ransmi t ted .

Of these the s tabi l i ty problem affects pr imari ly the t ransmiss ion system, andregulat ion problem the dis t r ibut ion system. Inequal i t ies in load shar ing of parfeeders pose problem s for b oth system s. W hen such problem s ar ise i t i s som etpossible to p ostpo ne cost ly system reinforce m ent b y ins ta l l ing reactive pocompensat ion equipment , a l though i t i s necessary to s tudy each case on i ts meri

Overal l im prove m ents in operat ing condi t ions will be broug ht a bo ut i f m eare int roduced of reducing the system reactance or of reducing the phase abetween the system current and vol tage. This can be done by ins ta l l ing s ta t ic p

namely se r ies or shunt connec ted capac i tors , shunt connec ted reac tors or ro taplant in the form of large sync hron ous m achines capable of being over-exci te



216 Protection of motors, reactors, boosters andcapacitors

as shunt capac i tors or shunt reac tors respec t ive ly, depending upon the sysrequi rements a t the t ime. These machines a re opera ted as capac i tors (overexc idur ing p er iods of heav y ac t ive load t ransfer, and as reac tors (underexc i ted) d uperio ds of l ight load t ransfer.

With a s ta t ic capaci tor or reactor bank the react ive power can be var ied onlswitching sect ions of the insta l la t ion in or out as required, thus a l ter ing capaci tance in s teps . With a synchronous machine immediate var ia t ion is obtaby automatic var ia t ion of the exci ta t ion, but disadvantages are complexi ty and the la t ter being app rox im ately three t im es that of s ta t ic plant of equivalent raThe remainder of this Sect ion is devoted to considerat ion of the appl icat ios ta t ic se r ies and shunt capac i tors and synchronous shunt compensa tors togewith measures necessary to protect them.

14.5.2 Series-connec ted capac itors

The simp lified equ ivalent circuit o f a transm ission l ine w ith series co nn ecapaci tor is show n in Fig. 14 .5.2 A and the associated phasor diagram in 14.5.2B. Fig. 14.5.2C shows how the vol tage drop increases with the dis tance the sending end an d h ow the vol tage rise is con cen trated at the capaci tor.

XC1 RL XL IIo ; = _ . . . . . . . , , . . . . . + - ,

F i g . 1 4 . 5 . 2 A S imp lif ied equivalent c i rcu i t of t ransmission Ih~e with series capaci tor connectedat receiving end

IX L

IR L

Voltage rise /: V C sin 0 j j ~ / ~ / V s

IX C - V C

V r



Pro tection o f m otors, reactors, boosters and capacitors 21 7

F i g .14.5.2C

I . . . . . l . ) i s t a t i c c | l _~ a d

i i

Diagram show ing voltage dro p with distance and effect o f series cap acitor

In the F ig u re s , /~ and XL are the to ta l series res is tance and induct ive reactin ohms, X c is the ohmic reactance of a series capacitor,Vs and Vr' are the sendingand receiving end l ine voltages, ~ is the voltage on the load side of the capacand I is the load current .

Neglect ing the effects of l ine charging current , or shunt capaci tance, consider ing lumped values for the l ine constants , as shown in Fig . 14.5 .2A,pha se-to-ne utral w)ltage dro p b etw een I/s and I/r can be expressed as

voltage d rop = I [RL cos 0 +(XI . - Xc)sin 0 ]

The condi t ionsXL < Xc, XL = Xc andXL > Xc are referred to as overcompensat ionfu l l compensa t ion and undercompensa t ion , respec t ive ly.

Overcompensat ion is not used in t ransmiss ion systems, but appl icat ions of type a re som et imes found in d i s t ribu t ion sys tems . Exam ina t ion of the regulaform ula above shows that by varying the qu an t i ty X the regulat ion can

increased or d ecreased at wil l, prov ided th at s in $ :# 0. As $ is the phase anglecondi t ion s in $ = 0 represents uni ty load power factor.

The co m pen sat ion of the l ine reactance ob ta ined wh en a ser ies capaci toappl ied has the effect of(a) increasing the l ine carrying capaci ty,(b) imp roving the load shar ing of two or m ore paralle l con nec ted feeders ,(c) imp roving vol tage regulat ion chief ly a t the po int of ins ta l la t ion.The vol tage change obta ined takes the form of a sudden r ise a t the capacterm inals so tha t i ts beneficial effects are fel t on the load side of the capacTh us the load voltage l/r will be larger than the received voltage ~ ' . Since vol tage r ise is depe nd en t on the load c urre nt and pow er factor ( that i s vol tageVC sin 0 = IXc sin 0) a change in I produces a change inVc and the capaci tor actsauto m at ical ly as a voltage regulator.

14.5.3 Shu nt-conn ected capacitors

The simpl i f ied equivalent c i rcui t of a t ransmiss ion l ine wi th shu nt con nec



2 1 8 P r o t e c t i o n o f m o t o r s , r e a c t o r s , b o o s t e r s a n d c a p a c i to r s

1 4.5 .3 B. F ig. 1 4 .5 .3 C shows tha t, unlike the effec t wi th a ser ies capacitor, tvol tage r ise is dis tr ibuted uniformly along the length of the l ine.

RI. X L IL

XC Vr

F i g .14.5 .3A Sim plif ied equivalent circui to f t r a n s m i s s i o n l i n e w i t h s h u n tcapacitor atreceiving e n d

v. . IC_X L

IL'R 1.

I(,

Vr

i rXL

Voltage drop duet o load current

- - I

F i g . 1 4 . 5 . 3 B

F i g . 1 4 . 5 . 3 C

Phasor diagram o f equivalent circu it

LoadCapacitor ~ p o i n t

voltage rise ~ . ~ . , J

I

I .

J

0 - ~ , ~ voltage drop eI

. . . .o tag.edrop due/ "~ _ Ito h)adcurrent

Diagram show ing voltage drop with distance an de f f e c t o f s h u n t capacitor

The r educ t ion o f the phase angle betwe en the v ol tage an d the current , wh ich is



Prote ction of m otors, reactors, boosters and capacitors 21 9

a )(b)

( c )(d)

reducing l ine curren t losses owing to the generat ion o f reactive powreducing the t ransmiss ion l ine current to a value less than the current inloa d (Fig. 14.5.3 B),improving the power fac tor o f the t ransmi t ted power, andreducing the vol tage drop uniform ly a long the length of the l ine (F14.5.3C). This should be com pared with the s tep-funct ion vol tage r ise acthe terminals of the series capacitor.

The response to voltage dips is not as rapid as with series capacitors sinceswitching of sect ions of the bank is in i t ia ted by the change in vol tage, and at the regulatio n is in steps.

14.5.4 Series ors h u n t c o n n e c t i o n

Fro m wh at has a l ready been said , it wi ll be apprecia ted th at series and shcapaci tors serve a broadly s imilar purpose in improving operat ing condi t ions ,ser ies connect ion by compensat ing for l ine reactance, that i s reducing i t , andshun t c onn ect ion by com pensa t ing the phase angle , tha t i s reducing the pdiplacement between system current and vol tage. Clear ly, there are several fac

governing the choice of connect ion and these may be summarised as fol lows.With a ser ies capaci tor the regulat ion, or reduct ion in vol tage drop, achie

depends main ly on the reac t ive power of the load , and i t fo l lows tha t th i s typcap acito r is of l i t t le use unless the con dit ion s are such tha t reactive po w econsumed by the load. With a shunt capaci tor the regulat ion achieved depemainly on the reactance of the system, and a useful increase in vol tage wilobta ined only i f the reactance is substant ia l .

I f con t inuo us and au tom at ic vol tage regulat ion is the m ain object ive , a scapa ci tor ins ta l la t ion is m ost l ikely to sa t isfy the nee d. O n the oth er ha nd a scapac i tor bank wi th means of au tom at ica l ly increas ing or decreas ing the num bsect ions of the bank in service wil l provide a measure of regulat ion, a lbei t in sand wi th some de lay.

A capaci tor connected in ser ies wi th a l ine must have a current ra t ing equivato tha t o f the l ine , the ou tp ut o f reac tance power f rom the capac i tor bedependent upon the l ine cur ren t

P X c( o ut p ut = - - - - - k V A r p er p hase ).

10 3

Wi th a sh un t connec ted capac ito r t he ou tpu t o f t he bank is i ndependen t o fcur ren t , com pensa t ion be ing de te rm ined by the appl ied vol tage .

On transmission circuits there are technical advantages in si t ing a series capaat some point in the l ine route , but th is in t roduces the need to acquire a sui t

s it e and problems o f m ain tenance . A shunt capac i tor, however, can convenien t lconnected to the low vol tage s ide of a gr id t ransformer s i tuated a t a substa t ion



22 0 Protection of motors, reactors, boosters and capacitors

either series or shunt capacitors varies l i t t le over a wide range of capacity, recent f igures for losses indicate the negl igibly low level o f 3 W per kV A r.

In the mid 1960s , the CEGB e m barke d on a program m e of provid ing reaccom pensa t ion a t ap propr ia te p o in ts th ro ug ho ut the t ransmiss ion sys tem an

the sho r t t e rm thi s t o ok the fo rm o f hydrogen-coo led synchrono us mach ines oto 60 MVA r ra t ed ou tpu t a t 13 kV , opera t ing as re fe rred to in Sec t ion 14 .5 .1th is chapter. As an imm edia te m easure , a few such m achines ra ted a t 4 0 Mwere commiss ioned a t 275 kV subs ta t ions to be fo l lowed soon a f te r by shcon nec ted s ta tic capaci tors a t o the r 275 kV si tes (see Fig. 14.5 .4A for a typexam ple o f the s ta tic shu nt capa c i tor a r rangem ent ) . An im po r tan t fea ture of tequipments , whether ro ta t ing or s ta t ic , was the i r t ranspor tab i l i ty and the va lu

th is fac i l i ty was demons t ra ted when i t subsequent ly became necessary to dethem at other s i tes as dic ta ted by the operat ing requirements of the system. Stha t t im e a ll new au to t ransform ers have been suppl ied w i th 13 kV te r t ia ry windsui tab ly ra ted for the poss ib le connec t ion of compensa t ion equipment of up60 M VAr.

2 7 5 k Va

[

1 8 0 M VA2 7 5 / I 3 2 RVa u t o - t r a n s f o r m e r

P ~

E a r t h i n gs w i t c h

.¢.

2 0 M VA r shuntcapac i to r

I 32 kV

F i g . 1 4 . 5 . 4 A Primary arrangement of typical shu nt capacitor

Several ser ies connected capaci tors have been ins ta l led in Area Board 11circuits for the purpo se of reduc ing ' f l icke r ' , par t icular ly in the vic ini ty of sworks and arc furnaces . The duty cycle dur ing the mel t a t an arc furnacepar t icular ly one rous, and i t is in such areas tha t a ppl icat ions fo r smal l secapac i tors a re m os t l ike ly to be fou nd in th i s coun t ry.

Ab road , no tab ly in Sw eden and the U SA, i t is the prac tice to ins ta l l l arge bof ser ies capaci tors in the very long, high vol tage t ransmission l ines commo



Pro tection o f m otors, reactors, boosters an d capacitors 221

14 .5.5 The capa c i to r un i t

A capa c i to r un i t (See F ig. 14 .5 .5A for i l lus t ra t ion o f typ ica l un i t )c on s i s t s nu m be r o f smal l cap aci tor e lem ents in a single case sui table for assem bl ing

o ther s imi la r un i t s to fo rm a capac i to r bank , o r t ank- type capac i to r comple te t e rmina l bush ings and pro tec t ion .

Ini t ia lly i t was the pract ice in Great Br i ta in to ma ke capac i tors of the tank tand exam ples o f these a re to be found a t Kenf ig (950 kVA r, 33 kV, ins ta l led1942) an d U ane l ly (75 00 kV Ar, 33 kV, ins ta l led in 1951) . Fo l lowing the estabment of a need for large high-vol tage capaci tors on the t ransmiss ion sys tem,uni t type bu i l t up to fo rm a capa c i to r bank has found favour and such ins ta ll a t

are in service a t Fleet , West Weybridge and Els t ree . These are a l l shunt connein t he 132kV s ide o f 275 /132 kV au to t r ans fo rm er s and ar e o f 40 , 20 and 20 MVrespect ively.

Oil impreg na ted Hermet i ca l lypaper sea led cont a ine r

Meta l foil Metal fi~ilconnec~,ed to ct~nnect ed t~con ta ine r t e rmina l

F i g . 1 4 . 5 . 5 A A r r a n g e m e n t o f t y p i c a l A S E A f o i l c o o l e d c a p a c it o r u n i t

Each e lement consis ts of t i le capaci tor p la tes or fo i ls wi th in ter leaving t i ssuespecia l paper, the whole wound up to form a cyl indr ical ro l l . In a tank- tcapac i to r a numbe r o f such e l emen t s a re m oun ted on w ooden suppor t s co nn ec ted in ser ies-paralle l to g ive the required kV A r ou tp ut . The tank is Fdled w i th an imp regn at ing f lu id to ensure th at a ll the in ters t ices of the pt issues are fi lled and m ois ture ex clude d. U ni t type capa ci tors are usual lrec tangula r meta l -box cons t ruc t ion and employ s imi la r e lements in f l a t fo rmrela t ively smal l number being housed in each uni t .

One in te res t ing des ign o f the un i t type is made by the ASEA C om pan ySw eden . T he co nta in er is c i rcular abo ut 1 f t in d iam eter and 6 inches d




F i g . 1 4 . 5 . 5 B Genera l v iew of 20 M V A r, 13 2kV cap aci ty ban k (Br i t ish Insula ted Cal /endersCables Ltd.)

tha t i s , s ix a r ranged a round one cen t re e lement . The impregnan t used i s a mineo i l , under a low pressure , to improve the d ie lec t r i c s t r eng th o f the impregnapaper.

The un i t c ons t ruc t io n has the advan tage o f g rea te r f l ex ib i l ity, in tha t un i t s be assembled on op en- typ e o u td oo r con s t ruc t ion as in F ig. 14 .5 .5B, and to as ize required .

14.5 .6 Pr otec t ion of cap aci tors

The need fo r adequa te p ro tec t ion aga ins t damage i s ev iden t when cons ide ra t ion

g iven to the vu lnerab i l i ty o f such equ ipm ents to sys tem fau l t s, fo r exam ple l igh tnf la shover s , and t he lo ss o f co m pen sa t i on wh ic h wo u ld f o l l ow th e r e m ova l o




re turn to the very cond i t ions i t was in ten ded to prevent .The p ro t ec t ion m ay be looked upon as comp r is ing two pa r ts : p ro t ec t ion o f

ind iv idua l capac i to r e lements wi th in a un i t o r t ank ( ' in te rna l ' p ro tec t ion) p ro tec t ion to p reven t excessive s t ressing of the d ie lec t r ic by sys tem d is tu rba( ' ex t e rna l ' p ro t ec t ion ) .

In t e rna l p ro t ec t ion consi st s o f d e t ec ting any abnorm a l unba lance o f nom ina l ly ba lanced imp edances of the sec t ions o f each phase ban k . It wi lapprec ia ted tha t because of manufac tur ing to le rances in the capac i to r e lemensmal l s tand ing uba lance i s inev i tab le , bu t th i s i s kep t to an abso lu te min imumcarefu l se lec t ion and match ing of e lements and , where re lays a re used , i s a l lofor in se t ting the re lay. Requ i rem ents o f the pro tec t ion a re:

( a )

( b )( c )

disconnec t ion of a fau l ty e lement before damage or case rup ture occurs , w i t h a m i n i m u m lo ss o f k VA r,ind ica tion o f t he l oca t ion o f a f au lty e l emen t , andno n-op era t ion wh en the capac i to r is sub jec ted to non- in jur ious sysdis turbances .

Exte rna l p ro tec t ion is necessary to p ro tec t aga inst the heavy cur ren ts which f low when a shor t c i rcu i t occurs on the capac i to r i t se l f , o r i t s connec t ions , otha t p ar t o f the sys tem to w hich i t is con nec ted . The e ffec t o f such cur ren ts p rodu ce cor respondingly h igh vo ltages across the capac i to r wh ich , if a l low epers is t , wi l l overs t ress the die lect r ic . External protect ion may be provided in sevdi ffe r ing forms depending upon the s ize and connec t ion of the capac i to r.

For smal l se r ies capac i to rs in d i s t r ibu t ion ne tworks , spark gaps in one forman oth er are general ly used an d are descr ibed in m ore deta i l la ter in th is Sect

I t m ight be supposed tha t a non- l inear res is to r wo uld f ind an appl ica t ion as a s ip ro tec tor fo r a se r ies capac i to r, bu t the thermal charac te r i s t i cs o f the res i s tamater ia l used in i t s manufac ture a re such tha t the res i s to r would have to be la rge , and there fore cos t ly, to be ab le to ca r ry the fau l t cur ren t fo r t imes oforder o f seconds .

1 4.5.6.1 Series capac itor internal pro tection: Thechoice res t s be tw een the use of

an ind iv idua l fuse for each e lem ent w i th in the un i t o r t ank , o r som et ime s one for a g rou p of e lem ents , and the use of cur ren t o r vo ltage t ransform er dr iven rconn ec ted to de t ec t any s ign i fi can t ou t o f bal ance be tween the two a s socisec t ions . So m et imes a com bina t io n of fuses and ins t ru m ent t rans form er drre lays is considere d desi rable .

The prac t ice of us ing fuses for e lem ent p ro tec t ion was ado pted m an y yago in shunt connec ted capac i to rs and i s now used ex tens ive ly in se r ies capac ia l so . The fuses a re o f the h igh break ing-capac i ty type incorpora t ing a p in wh

pro jec t s when the fuse has b lown, o r the expuls ion type , f rom which a t a i l hadow n to ind ica te a b low n fuse ; in e i ther case the e lem ent is d i scon nec




change in the capac i tance of the ban k and therefore a small ou t o f ba lance, oorde r o f 1%, betw een tha t sect ion and the oth er sect ions . This in i t se l f i s re la tiins ignif icant, bu t assum es im porta nce i f i t is fol lowe d by the fa ilure o f fur ther fthus increasing the s t ress of the remaining e lements . A capaci tor is designed sothe fa ilure o f , say, one e lem ent w i l l no t resul t in the res t of the e lemen ts bovers t ressed, tha t i s the vol tage across the rem ainder w i l l be w ithin the con t inrat ing o f the e lemen ts . Care mu st , of course , be taken to ensure tha t the fremain in tac t when the ex te rna l p ro tec t ion opera tes .

The in t roduc t ion of cur ren t o r vo l tage t ransformers br ings wi th i t the needdivide each phase bank into two equal paral le l halves , so that the detect ion eqm ent can be connec ted across the midpo in ts . U nder normal condi t ions no cu

f lows in the re lay (o ther than the very small am ou nt du e to the m inor d i ffe renindividual e lements) , but the fa i lure of , say, two elements would be arranged toan a la rm and the fa ilure of any fur ther e lem ents to t r ip the capac i tor f romsys tem.

The cost of providing current or vol tage t ransformers , and the space requto acc om m oda te the m , renders thi s m etho d o f pro te c t ion m os t su itab le wi th ins ta l la tions where a case can be m ade o ut for providing a cap aci tor shor t -c i rcuswitch.

1 4.5.6.2 Ser ies capacitor external protection: Asalready ment ioned, smal l ser iescapaci tors such as are used in dis t r ibut ion c i rcui ts for vol tage regulat ion purpare usua l ly provided wi th spark-gap pro tec tors to prevent damage to the capacon the occur rence of fau l ts on the capac i tor o r the ne tw ork to which icon nec ted. There is a wide var ie ty o f gaps available em bracing the ext inguishand nonext inguishable , the t r iggered and the nont r iggered and those employ

electronic technique s . De tai led co nside rat ion w ill now be given to a se lect iothese g aps:

(a) Nonextinguishing, nontriggered ga ps: Theelectrodes of these gaps are ofcopper or s ta inless s teel and their useful l i fe can be extended by adding a switau tom at ica l ly sh or t c i rcu i t them im m edia te ly a f te r opera t ion . One exam ple ofa gap is the Engl ish Electr ic T ype HH device w hich c onsis ts o f a spark gap c onn e

in paral le l wi th a normal ly open spr ing-operated isola t ing switch (Fig. 14.5 .6 .The cur ren t t ransformer opera ted inverse- t ime re lay de te rmines the t ime for wthe gap is perm it ted to arc , and i f th is is exce eded the isola t ing sw itch is c losshort c i rcui t the gap and the capaci tor. They wil l remain shor t -c i rcui ted unt i li sola ting switches on a l l three phases are re-set by m eans o f an insulated pole .

Since these s imple gaps do nothing to ass is t in ext inguishing the arc theyunl ikely to survive m ore than one o r tw o o pera t ions , an d so their use is l imitesmal l ser ies capaci tors in places w here the throug h-faul t incidence is low. The

sometimes appl ied to such capaci tors in low-vol tage cable networks .



Protec tion of m otors, reactors, boosters and capacitors 22 5

I n v e r s et i m e r e l a y

F i g . 1 4 . 5 . 6 . 2 A

_ _ _ I Z "1 I 1

//J

I t --J

C . T.

/

I

I

S p r i n g l o a d e d~h(Jrt-circuit i t lg~ i t c h c ( ~ n t r ~ l l c db yillw,'r,~c t i m e r c l a ). /

/

l - Ia n tlZ~ rese t t ing

lever

.-\d.iustab Ic, , p a r k g~lp

Type H H serie s capac i to r p r o t ec to r

classif icat ion and these may be considered as the op en typ e and the sealed typ e. Adesirable feature of any self-restor ing gap is that the vol tage at which i t wil l f laover after resetting shall be sensibly the s am e as the orig ina l sett ing , and it ic la imed tha t bo th the gaps descr ibed ap p roach th is requi remen t .

(i) Open type: Designed and f i rs t appl ied by Elect r ic i t6 de France on tAyergues Val ley 10 kV ne tw ork , th i s type o f gap cons i st s o f two c onc en t r i c coelect rodes , the inner one carr ied a t the top of a ver t ica l copper spindle (

14 .5 .6 .2B) . The e lec t rodes a re so shaped th a t the space be tw een them is in the of an ann ular gap s l ight ly larger a t the top than a t the base . The s ize of th isde term ines the se t t ing an d is adjus ted by ra is ing or lowe r ing the spindle . Awound round the ou te r e lec t rode ca r r i es the shor t -c i rcu i t cur ren t . When sub jeto a th rough- fau l t , the e lec t rodes f l ashover a t the base , the magne t ic e ffec t o fcoil causing the arc to rotate. As i t does so i t cl imbs in a spiral to the top ofe lec t rodes and cont inues to burn unt i l i t ext inguishes i t se l f when the faul t i s c le

The advan tage o f ro ta t ing the a rc and moving i t away f rom the po in t a t wf lashover occurs l i e s in reduc ing the amount o f burn ing a t tha t po in t and he lp inmainta in the se t t ing . In service these gaps have wi ths tood up to 50 opera twi thout requir ing pol ishing or re-se t t ing.

(ii) Sealed type: Anexample of the sealed type of se l f - res tor ing gap is odes igned by BICC Ltd. and developed by the then AEI Ltd (Fig . 14.5 .6 .Esse ntial ly i t c on sists o f a B1 CC gap sealed in a glass env elope f 't lled w ith hyd r

a t low pressure . A dju s tm en t o f the gap is ob ta ine d by vary ing e i the r the spac ithe e lec t rod es or the gas pressure . A coil is w ou nd rou nd the glass enve lope



2 2 6 Protection o f m otors, reactors, boosters and capacitors

E l e c t r o d e s

I n s u l a t i o n

F i g . 1 4 . 5 . 6 . 2 B O pe n type, self-extinguishing spark gap for series capac itor pro tec tion

k gap

loca t ing co i l

H y d r o g e n f i l l e dg l a s s e n v e l o p e

Ser ies r e s i s to r

Insu la to r







Protect ion o f mo tors , reactors, boosters and capaci tors 22 9

0-3 s to pe rm it the use of h igh speed autorec ios ing facil it ies . The purpo se ofbypass c i rcui t breaker i s to shor t c i rcui t the ser ies capaci tor and spark-gap inevent of a faul t in the bank or an inadver tent f lashover of the gap.

L i n e . . . . . -

/ / I/

capaci t , J r

I ) a m p i n g [ ~ ~ ] D a m p i n gc i r c u i t ~ ( . l ~ s e c i r c u i t

q

By I)a.ss , ( ) p e n ic i rcui t [ " ~ " = " ~ [ I l 1b r e a k e r [ I

l ' r ~ l c c t i w :c a p a c i t ~ b r

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( ) / 1 . ( ) ~ , e rc 'u r re t ~ r e J a ~ U / C I _ ; n d e r c u r r e n t r e la . ~

F i g . 1 4 . 5 . 6 . 2 F P r o t e c t i o n o f /a r g eseries capacitor for Swedish State P o w e r B o a r donly one phase shown)

Faul ts in the ca pac i tor ins ta l la t ion causing unba lance are detec ted by a re laeach phase dr iven f rom a cur ren t t rans fo rm er co nne c ted ac ross equ ipo ten t ia l poO pera t ion of the re lay in i t ia tes , v ia a t iming re lay, the c los ing of the bypass c ibrea ker, the t iming re lay ha ving a se t t ing of 1 sec . This delay is necessary to entha t ope ra t ion of the re lay does n o t cause c los ing of the bypass b reaker onoccur rence of fau l ts in the com pen sa ted l ine , o r by d ischarge cur ren t on bypathe capac i to r.

The three-phase current t ransforn~ers dr ive t ime- lagged current re lays wini t ia te c los ing of the bypass breake r only i f the spark-gap re- igni tes af tert r ipping, and au tom at ic reclos ing, of the c i rcui t break ers a t the ends of compensated l ine . Timing re lay se t t ings and the wai t ing per iod or 'dead tbefore the l ine breakers reclose are careful ly chosen to prev ent the bypass brebeing c losed when the l ine faul t occurs and the spark-gap i lashes over the f i rs t A cont inua t ion of the au tomat ic sequence reopens the bypass b reaker a f te r seseconds and i f the spark-gap re- igni tes again the breaker recloses and rem

closed.



23 0 Protection o f mo tors, reactors, boosters and capacitors

gas-fi l led spark-gap and two ignitrons, and is shown in Fig. 14.5.6.2G. The igniare connected in opposi t ion across the capaci tor terminals and are t r iggered byco m m on spark-gap. I f a faul t on the load s ide of the capa ci tor is severe enou g

l g n i t r o n

Ign i t e rc i rcu i t

I ~ 1 .. r ~

l g n i t r o n

I F

C I

i To c a p a c i t o rt e r m i n a l s

S p a r k

N

t l1

D i s c h a rg er e s i s t o r

( '2

iFig . 14 .5 .6 .2G Diagramm atic arrangement o f electronic device for series capa citor pro tec tion

cause the spark-gap to f lash over, a pulse of current f lows through the ignici rcui t of one igni t ron or the other, depending upon polar i ty. The igni t ron oper

and carr ies faul t current in addi t ion to a capaci tor discharge current dur ingha lf -cyc le of the fundam enta l power f requency. The sequence is repea ted wi tho ther ign i tron during the next ha lf -cyc le, the capac i tor be ing tem porar i ly re turneservice a t each faul t -current zero and permanent ly re turned to service a t the current-zero af ter c learance of the faul t. The discharge res is tor is provided to the value of the capaci tor discharge current carr ied by the igni t rons , and ignit ing c i rcui t res is tor to l im it the igni t ron igni t ing current .

A device of th is type has been labo ratory tes ted up to 1000 A for 1 s and

service at the Alston 11 kV series-capacitor instal lat ion.

14.5. 6 .3 Sh u n t capaci tor in ternal protect ion : A swith ser ies-connected capaci tors ,the choice res ts between the use of fuses , current t ransformer or vol tage t ransfodr iven re lays , o r b o th , fo r the de tec t ion o f unba lance be twe en the nom ina l ly ecapaci tances of tw o sect ions o f the bank . In the case of large shu nt capac i tors which this section is chiefly concerned, i t is usual to f i t fuses and relays to pro

com plete pro tect io n against the consequences o f e lem ent fa i lure .The fuse will , of course , me rely disconn ect the faul ty e lem ent or grou p



Protection o f motors, reactors, boosters and capacitors 231

ins t rument t ransformer in the midpoin t connec t ion be tween two sec t ions usually be a ' tw o-sta ge' device, the f irst s tage giving an alarm for the loss o f ontwo elements , that i s the blowing of one or two fuses , and the second s tdisconnect ing the capaci tor bank from the system in the event of fur ther fa i lure

Relay protection: Theuse of re lays for the detect ion of capaci tor unbalance br inwith i t the need to provide current or vol tage t ransformers with consequent incin overall cost , part icularly if voltage transformers are used. For this reasonarrangem ents descr ibed below are usual ly appl ied only to the larger capa ci tor b

(a) Star-connected bank:(Fig. 14.5 .6 .3A). A vol tage t ransformer is connecte

across par t of the ser ies capaci tance of each phase of the bank with i ts seconwindings conn ected in broken del ta to a sensi tive re lay. Any redis t r ibut ionvol tage caused by the fa i lure of capaci tor e lements wi l l be detected, but m ethod , appl ied to a bank w i th ear thed neut ra l , is l iab le to be a ffec ted by sydis turbances and by th i rd harm onics .

R _ 1O' I

C I

B I 1I I. i I _

Relay

F i g . 1 4 . 5 . 6 . 3 A P r o t e c t i o n o f s t a r -c o n n e c te d s h u n t c a p a c i to r b a n k w i t h e a r t h e d s ta r p o i n t

(b) D elta-connected bank:(Fig. 14.5 .6 .3B). An arrangement of overvol tagpro tec t ion s imi lar to ( a ) m a y be appl ied to the unea r thed de l ta -connec ted bshow n in Fig. 14.5.6.3 B. Again the single relay will de tec t any abn orm al un balbetween sect ions , but wi l l be unaffected by harmonics .

(c) Split star-connected bank:(Fig. 14.5 .6 .3C). When an unear thed s tar-connectebank is large enoug h to be spli t in to tw o sect ions i t is conv enient to p rote cagainst capaci tance unbalance by conn ect ing a vol tage or curre nt t ransfo r

between the neutra l points of the sect ions . The two sect ions must be designehave ident ical capaci tances or unacceptably high out-of-balance currents wi l l



23 2 Protec tion o f mo tors, reactors, boosters and capacitors

i,t J i l _ I I _ I Io r - ~ I - I

Relay

F i g . 1 4 . 5 . 6 . 3 B Protec t ion of delta~connected shu nt capaci tor bank

iR O '

Y C

B C

I II I

I| _.

I

i l .I I

I,I

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i l l i _

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F i g . 1 4 . 5 . 6 . 3 C Pro tec t ion o f sp l i t st a r. connec ted sh unt capac i to r bank

(d) Star-connected bank ( two limbs per pha se) :(F ig . 14 .5 .6 .3D) . When eachpha se is sp li t i n t o tw o equa l s ec t io ns t he cu r r e n t t r an s f o rm e r s m ay be m ou na t e i the r end o f th e phase conne c t ions , bu t s ince m uch l ess insu la t ion i s r equa t t he n e u t r a l e nd , t he r e i s a c le a r e con o m ic adva n t age i n mou n t in g t h em the r e .

T h i s b a s i c m e t h o d w a s e m p l o y e d a t t h e 1 32 k V s h u n t c a p a c i t o r i n st a l la t i o n

W e s t W e y b r i d g e a n d E l s t r e e , t h e c u r r e n t t r a n s f o r m e r s b e i n g m o u n t e d a t t he n e uend . I t i s ve r y s en s i t i ve and can b e app l i e d t o de l t a o r s t a r con n ec t ed , e a r t



Prote ction of m otors, reactors, boosters and capacitors 23 3

render ing the pro tec t ion inherent ly s tab le u nder unbalanced sys tem condi t ionsassuming equal values of thi rd harmonic in each sect ion, unl ikely to be affectesuch currents.

_ I I _1 tI I I

RC -

M

) 1. . . . ]

YC

II 1 - - - - - I , "

I

' I

II

h

BC

- - - - I I - - - I I

, I - - - - - I |

lI

F i g . 1 4 . 5 . 6 . 3 D Protection of star connected shunt capacitor bank

Ii I'

(e) Star-connected bank ( two limbs per phase):(Fig. 14.5.6.3E). An alternativeto (d) is shown in Fig. 14.5.6 .3E. This method of detect ing unbalance may be w hen each phase is spli t into tw o equal parallel l im bs, each l imb consist ingnu m be r o f units in series. T he c urren t tra nsf orm er driving the relay is connected across equipotent ia l points of the two l imbs. I t has the advantage (d) in requi t ing only one current t ransformer per phase.

It wil l be apparent that al l the methods described above suffer from the divantage tha t the y re ly on the com parison o f the capaci tance of one sect ion o

bank with that of another. Thus, i f e lements fa i l s imultaneously in the sectbe ing compared , symmetry of capac i tance wi l l be main ta ined and the pro tec



2 3 4 P r o t e c ti o n o f m o t o r s , re a c to r s, b o o s t e rs a n d c a p a c it o rs

R¢

I I

I I ' TA

I

BC

, , l r1 1 v R e ~ l a

,UF i g . 1 4 . 5 . 6 . 3 EP r o t e c t i o n o f s t a r , c o n n e c t e d s h u n t c a p a c i t o r b a n k

I I

T h e r m a l o v e r l o a d p r o t e c t i o n :A two-stage feature per phase is used to achievprote ct ion against over loads in excess of the level a l lowed in BS 165 0:195 5. Tover loads may ar ise f rom system condi t ions producing:

( i) increased vol tage (kV Ar loading is prop ort ion al to voltage squared) .( ii ) increased f reque ncy (kVA r loading is prop ort ion al to f requ ency ) ,

and(ii i) harm onics , tha t is f requencies othe r than the pow er f reque ncy .

The f i rs t s tage consists of a therm al e lem ent having a m inim um set t ing of 1

nomina l bank cur ren t and an inverse t ime charac te r i s t ic match ing tha t o f capaci tor up to 300%, and is arranged to sound an a larm. The second s tage use



Protection o f mo tors, reactors, boosters and capacitors 23 5

exceeds 300%. The two-e lement re lay i s opera ted f rom a cur ren t t ransforlocated in the 13 kV capaci tor c i rcui t breaker.

1 4.5 .6 .4 Sh un t capacitor external protect ion: T heear ly shunt capaci tors in the

Bri tish t ransm iss ion system were direct ly co nne cted to the 132 kV side of275/132 kV auto t ransformers bu t i t i s now usua l p rac t ice to connec t them, their ow n circui t breakers , to the 13 kV ter t iary windings of the autotransformThus in the ear ly ins ta l la t ions the capaci tors were control led by the autotransfoh.v. and 1.v. c i rcui t breakers so that a capaci tor faul t resul ted in the tem podisconnect ion of i t s associated t ransformer. However, there is c lear ly some merproviding pro tect io n which enables operat ion al s taff to determ ine, f rom roperat ions and indicat ions , whether the faul t i s on the capaci tor, the t ransforor the connect ions between them. To this end subsequent ins ta l la t ions have bprovided with discrete pro tect io n systems cover ing each o f these main e lem enthat i f i t i s es tabl ished that the capaci tor a lone is faul ty, i t can be isola ted andtransformer re turned to service .

The rem ainder of th is Sect ion is devoted to a descr ipt ion of the prote ctarrangements usual ly appl ied to s tar connected s ta t ic capaci tor ins ta l la t isuppl ied f rom the a utotra nsfo rm er 13 kV ter t iary windings. A diagram showin

these arrangements is given in Fig. 14.5.6.4A, and Fig. 14.5.6.4B shows the c i rcui t ry associated with them.

1 3 k V connec t ion pro tect ion

The con nec t ions be twe en the t ransform er te r t ia ry winding bushings and the 13capaci tor c i rcui t breakers are protected by a di fferent ia l c i rculat ing current sysThis takes the form of ins tantaneous high- impedance re lays in the differen

circui t form ed by b alancing, phase b y phase, a se t of three current t ransformerthe tran sform er bushings a gainst similar sets of c. t .s o f identica l turns rat io in capaci tor c i rcui t breaker or i t s associated c . t . housing. The set t ing of the protecshould be capable of a djustm ent in excess of the full load curren t of the c i rc

Capacitor circulating current p rote ction

The individual capaci tor banks are each protected by a separate di fferent ia l sys

This consis ts of tw o sets of three curren t t ransforme rs one set located in capaci tor bank ci rcui t breaker or i t s associated c . t . housing and the other se t incapaci tor neutra l end connect ions , a l l the c . t . turns ra t ios being the same. se t t ing of the high impedance c i rculat ing current re lays is of the order of 20%capac i tor ba nk ful l -load current . Op erat ion of th is capac i tor di fferent ia l prote cwould t r ip only the 13 kV capaci tor c i rcui t breaker associated with the faul ty bleaving the t ransformer and the remainder of the capaci tor bank in service .

Res tr ic ted ear th-faul t protect ion



23 6 Prote ction o f m otors, reactors, boosters and capacitors

t ie the 13 kV eq uipm ent dow n to ea r th and this is teed to the 13 kV conn ectbetween the ter t iary winding and capaci tor c i rcui t breaker. A different ia l systeres t r ic ted ear th faul t pro tect io n consist ing of a single pole high imp edance dr iven f rom c. t. s located in the bushings or associated c . t . ho using of each 1capaci tor c i rcui t breaker, and in the ear thing t ransformer neutra l connect ionappl ied to cater for faults producing very low values of ear th-faul t cu rrent inear th ing t ransformer or au to t ransform er te r t ia ry w inding. The re lay is conn eto t r ip the auto transfo rm er h .v. and 1.v. c i rcui t breakers .

1 3 k V standby earth-fault pro tection

The neutra l ear thing res is tor is designed to l imit the ear th faul t current to 30and i t i s im po rtan t to ensure that i t is not loaded bey on d i ts ra t ing. A s ingle of s tandby ear th-faul t protect ion is therefore provided and this a lso affords efaul t prote ct ion to the capaci tor banks and back-up protec t ion for 13 kV efaults . A relay with an i .d.m.t , characterist ic is used and is supplied from a loca ted in the connec t ion be tween the ear th ing t ransformer and the ear thres is tor. I t i s se t to operate a t 15% of the ra ted ear thing res istor curren t w i tope rating t ime of 2 s at this set t ing. T he tr ip circuit is designed to open

auto transfo rm er h .v. and l .v. c i rcui t breakers .

Overcurrent pro tection

To cater for the possible fa i lure of the main different ia l protect ion an i .d .m. t , bup overcurrent re lay is provided dr iven f rom c. t . s in the t ransformer ter twinding bushings. The relay is connected to tr ip al l circuit breakers capablproviding fault current infeeds.

Overvoltage pro tectio n

U nder l ight system loading condi t ions the capaci tor banks ma y be subjectesymmetr ical three-phase overvol tages . The value of the maximum permissovervol tage and i ts a l lowable durat ion wil l depend upon the overvol tage charais t ic of the capaci tors and upon the system operat ing requirements .

Overvoltage prote ct ion is provided by two overvoltage re lays with i.d .mcharacter is t ics one connected for a larm and the other for t r ip and both suppfrom the same s ingle-phase 13 kV vol tage t ransformer. The t r ip contacts arranged to tr ip sequent ia l ly the capa ci tor 13 kV circui t breakers a t in tervals of Subject to system operat ing requirements and to the character is t ics of the capacbanks, the overvol tage a larm relay is se t to op erate a t 130% and the overvoltagerelay to operate a t 140% of capaci tor ra ted vol tage. Both re lays would haveoperat ing t im e o f abou t 30 s a t their respect ive set tings .

Other protection



Protection o f mo tors, reactors, boosters and capacitors 23 7

frequency , or f rom the effects of harm onic currents . Two-stage theroverload protect ion for each capaci tor bank is provided by a two-element rsuppl ied from c. t .s located in the 13 kV capaci tor bank circui t breaker, wsett ings as described in Sec tion 14.5.6.3. The first i .d.m .t , stage is set to alarm

and the second, instantaneous element is connected to t r ip i ts associated 13circuit breaker.

Protect ion to detect capaci tor out .of-balance resul t ing from the fai lureindividual capacitor elements or the blowing of their fuses, takes the form of aand relay scheme, the choice depend ing upon the s ize and con figurat ion ofcapaci tor bank. A number of typical arrangements are descr ibed and i l lustrateSect ion 14.5.6.3.

The ear thing t ransform er is provided with a do uble f loat gas and oi l ope rBuchholz relay general ly of the type descr ibed and i l lustrated in Sect ion 14The relay has alarm and t r ip funct ion s, the t r ip con tact being con nected to t rcircuit breakers capable of supplying fault current infeeds.

The ter t iary winding of the autotransformer can withstand short per iodsoverloading bu t prolonged ov erheat ing w il l hasten insulat ion deter iorat ion eventual ly shorten the l i fe of the t ransformer. To prevent this happening a wintemperature indicator (w.t . i . ) , previously described in Section 14.3.5. , is f i tAp propriate a larm and t r ip set tings are appl ied and the t r ip con tact is con nec tetrip the 13 kV m ain oil circuit breaker.

14.5. 6 .5 P rotection o f synchronous shun t compensators :Synchronouscompensators l ike the modern versions of their s ta t ic capaci tor counterparts conn ected to the 13 kV ter t iary windings of the autotran sform ers . In add i t ion m ain 13 kV circuit-breaker tw o similar circuit breakers, 's tar t ' and 'ru n' required in the synchronous compensator s tar t ing and control c i rcui ts . Agdiscrete protect ion systems are provided for the machine, the 13 kV connect ibetween i t and the autotransformer, and for the t ransformer i tself . Each of thsystems is descr ibed below together with the appropriate back-up protectfeatures and these com posi te arrangem ents are sho wn diagramm atical lyFig. 14.5.6.5A. The d.c. circuitry associated with them is shown in Fig. 14.5.6.5

1 3 k V connect ion p rotect ion

The co nne ct ions betw een the t ransform er ter t iary w inding bushings and the 13main oil circuit breaker are protected by a differential circulating current sysThis takes the form of a set of current t ransformers in the t ransformer bushbalanced phase by phase against another set in the main oil circuit breaker o

associated c.t . housing. High-impedance relays are used and the sett ing of pro tect io n should be capable of adjustm ent to above the full load current of





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Protec t ion of m otors , reactors, boosters and capaci tors 241

Machine c i rcula thzg current protect ion

The c on n e c t i on s be tween t he mach in e neu t r al , th e s t a rt i ng t r an s fo rm er neu t ra lthe 13 kV main o i l c i rcui t breaker are protec ted by the i r own di fferen

ci rcula t ing current sys tem. This consis ts of a se t of c . t . s in the 13 kV mainci rcui t br eake r, or i ts associa ted c . t. housing, ba lanced phase by phase agains tfu r the r se t s o f c . t . ' ; l oca ted one each in the ne tu ra l connec t ions o f the macand the s t a r t ing t r an s for m er, r e spec tive ly. High- impedance re lays a re used he reand the d i ffe ren t i a l p ro tec t ion se t t ing shou ld be appro x im ate ly 20% of fu ll current . Again , a l l the c . t . turns ra t io must be the same.

Re str ic te d ear th-faul t pro tec t ion

As the mach ine i s unear thed , an ea r th ing t r ans former and res i s to r a re requ i ret ie the 13 kV equipment to ear th and, as shown i l l F ig . 14 .5 .6 .5A, th is i s teethe co nn ec t ion s be tw een the t e r t i a ry wind ing and the mach ine main o il c i rb reaker. A d i ffe ren t ia l sy s tem of res t r i c ted ea r th - fau l t p ro te c t ion is appconsis t ing of a se t of c . t . s in the machine main c i rcui t breaker, or associa ted hous ing , and o ne c . t. in the ea r th ing t r ans form er n eu t ra l conne c t ion , i t is des ito de te c t low leve l ea r th - fau l t cu r ren t s in the ea r th ing t r a ns fo rm er o r au to t rfo rm er ter t ia ry wind ing. T he d .c . c i rcui t of the h ig la-impedance re lay usecon nec ted to t r ip tl~e au to t r an s fo rm cr h .v. and l.v. ci rcu it b reakers .

13 k V s tand by ear tlz- fault pro tect io n

Tw o-s tage s t a ndb y ea r th - fau l t p ro tec t ion is p rov ided . S tage 1 a ffo rds ea r th - fp ro tec t io n fo r the mach ine and s tage 2 p rov ides back-up p ro tec t io n fo r unc le13 kV ea r th fau l ts and ensures tha t the neu t ra l ea r th ing res i st3 r is no t loa

be yo nd i ts ra t ing . Both re lays are suppl ied f rom the same c .t . loca ted in con nec t ion be tw een the ea r th ing t r an s form er and the ea r th ing res is to r. The s tare lay is ins tan tane ou s in op era t ion , usua lly has a se t t ing o f 10% and is con neto t r ip the 13 kV m ain o il c i rcui t bre aker only . The s tage 2 re lay which t r ipthe h .v. and l .v. c i rcui t breakers associa ted wi th the au to t ra ns fo rm er as welthe 13 kV mach ine main o i l c i rcu i t -b reaker i s o f the induc t ion type wi th an i .dcharac te r i s t i c . Th i s re lay shou ld have a se t ting o f 5% and an o pera t ing t ime o f a2 s a t twice the se t t ing .

Overcur ren t p ro tec t ion

To guard aga ins t t i r e poss ib le fa i lu re o f the 13 kV connec t ion p ro tec t ion an i .dback-up overcur ren t r e lay i s p rov ided supp l ied f rom c . t . s loca ted in the bush ingthe 13 kV m ain o i l c i rcui t b reak er or i ts associa ted c . t . housing. Ti le re laconnec ted to t r ip the 13 kV main o i l c i rcu i t b reaker on ly.

Reve r se pow er p ro t ec t i on







244 Protection of motors, reactors, boosters and capacitors

circui t breakers remained closed, subsequent res torat ion of supply could resulthe m achine being re-energised out o f synch ronism . A lthough this s i tuat iounl ikely to occur a t busbar type s ta t ions i t i s a possibi l i ty a t s ta t ions with a l imnum ber of in feeds.

To cater for th is eventual i ty, which could resul t in ser ious damage to machine, reverse power protect ion is usual ly considered necessary a t s ta t ions othan those of the busbar type. The t r ip c i rcui t i s arranged to t r ip the 13 kV moil circuit breaker only.

The low internal losses of the sy nch rono us m achines necessi ta te the use highly sensit ive re lay for the dete ct ion of loss of power fol lowing an interrup t iosupply. The vol tage supply to these re lays m ust therefore be of high accurac

malfunct ioning is to be avoided. The phase angle errors resul t ing f rom the interegulat ion of the ear thing t ransformer preclude i ts use for th is purpose i f therany l ikel ihood of i ts being used to sup ply varying loads , such as m oto r s tarloads. In such cases a separate voltage tran sform er o f high acc urac y should be u

The re lay is , of course , designed to measure power and thus requires a currenwell as the voltage sup ply just described . This cu rren t source is provide d by a sc . t . s located in the neutra l end connect ions of the machine.

ther protection

To prevent the m achine suffer ing dam age from excessive the rma l effects i t is upract ice to provide a therm al over load re lay suppl ied f rom a c. t. in one phase omachine neutra l end connect ions . Ideal ly, the character is t ic curve of the r

should match the thermal character is t ic of the machine over i t s whole rangepractice this may not be possible but i t is essential that i t does so over the worpar t of it . The re lay set ting ado pte d will thus be based on the the rm al cha racter iof the machine and the re lay contacts are connected in the machine control c iso that fol lowing re lay operat ion the machine exci ta t ion is reduced.

Temperature indicators wi th faci l i t ies for providing a larm and t r ipping functare provided on the machine bear ings . The t r ip contact i s connected to t r ip13 kV main oil circuit breaker.The ear thing t ransformer is provided with a double f loat gas-and off-operBuchholz re lay general ly of the type descr ibed and i l lus t ra ted in Sect ion 14The re lay has a larm an d t r ip funct ions , the t r ip co ntac t being con nec ted to al l circuit breakers capable of supplying fault current infeeds.

The te r t ia ry winding of the au to t ransformer can wi ths tand shor t per iodsover loading but prolonged overheat ing wil l hasten insulat ion deter iorat ion eventual ly shor ten the l i fe of the t ransformer. To prevent th is happening a win

tem pera ture indicator (w. t . i .) , previously d escr ibed in Sect ion 14 .3 .5 . , is f i tAp propria te a larm an d t r ip se t t ings are appl ied and the t rip con tact i s conn ecte



Pro tect ion of m otors , reactors , boosters an d capaci tors 24 5

14.6 Bibliography

Br i t i sh S tandards

BS.587:1957, 'Motor s tar ters and control lers 'B S.4 941 :197 3, 'M oto r starters for voltages up to and including 1000 V a.c.

1200 V d.c. 'BS .4999:Part 72 (197 2) 'Thermal protect ion for e lectric moto rs rated at 6

a.c. and belowBS.5000:1973, 'Rotating electrical machines of particular types for partic

appl icat ions '

BS.171 : 1970, 'Pow er transfo rm ers 'BS.1650:1971, 'Capaci tors for connect ion to power frequency systems'

B o o k s

M otor selection a nd applicationby C Libby Charles (McGraw-Hill)The protect ive gear han dbo okby F E WeU man (Pi tm an)

Automatic protect ion ofa .c , c i rcui tsby G W Stubbings (Chapm an & Hall)P rotective relays. their theory an d practiceby A R Van C W arrington (Chapm an &

Hall)Applicat ion o f pow er capacitors(BICC Ltd. Publ.)

Papers and ar t ic les

'Sh ort circuit rating and testing of reactors ' by J G W ellings and R V W h(J. lE E, 1942 , Pt. 2, p. 47 3)

'Technical and economic aspects of the supply of reactive power in EnglandW ales' by W Casson and t t J Sheppard(Proc.ll;'l:', 1961, 108, Part A)

Re po rt on the use of series capac itors in pow er system s (ACE R epo rt No. 3, 1'The protect ion o f shunt capaci tor banks: a cri tical review of publ ished info rm a

by J A Nicholson (ERA Repo rt No. Q/T 136 , 1953)'Protection of series capacitors against external disturbances and internal faultR Pellissier (BE Translation No. 391)

'380 kV ser ies capaci tors in S we den' by G Jan cke, K S Sm edsfelt and P t t jer(CIGRE Paper 322, 1954)

'Self-extinguishing gaps in larg e series capa citor sta tions ' by 1. A hlg ren anGrundm ark (CIGRE Paper 317. 1956)

'The use of series capacitors on hig h voltage transm ission system s' by S Lala

and L Norl in (CIGRE Pape r 330. 1958)'Series capac itors for d istribution ne tw ork s' by K S Sm edsfelt and P Hjert



24 6 Protection o f mo tors, reactors, boosters and capacitors

'Series capacitors for high power transmission' by I Herlitz (ASEA Paper 72Repo rt on the protection of series capacitors (up to 33 kV ) (Protection

Com m ittee M2, ad hoe P anel on C apacitor Protection , 1961)



Chapter 15T h e ap p l i ca t ion o f p ro tec t ion to ru ra l

d i s tr ib i j ti on sys t em sby J. H arris

15 .1 In t roduc t ion

The growth in demand for electr ici ty supplies in rural areas has led to rapidincreases in the extent of rural high voltage networks, and consumers in these areahave come to demand suppl ies comparable in re l iabi l i ty to those provided in urbandis tr ic ts . In v iew of the typ e of cons t ruc t io n e m ploy ed for rura l e lec t r i fica t ion , th ide m an d has cal led for co nsiderable skil l and ingen uity on the p art of electr ici tysupply engineers .

Because of the savings achieved thereby, rural feeders are almost ent irely of overhead cons t ruc t ion , backbone l ines radia t ing f rom s tep down t ransformer subs ta t ions fed f rom the pr imary sys tem and f rom these tappings are made to affordsupplies to small groups of consumers. A typical system is i l lustrated in Fig. 15.1AIn order to take advantage of the impulse s t rength of the wood poles , these over

head l ines are general ly of 'unea r thed con s t ru c t ion ' , tha t is the pole top s tee lwork iear thed only a t t ransformer and sec t ion swi tch pos i t ions and where pole type cablbox es are ins ta lled a t jun ct ion s b e tw een l ines and cables . The perform ance o f l inewi th th is type of cons t ruc t ion i s improved under cer ta in condi t ions of faul t l iab i l i tycom pared w i th the o lder typ e w i th a ll s tee lwork ear thed, but ov erhead linegeneral ly are susceptible to faul ts due to lightning and othe r cl imatic co nd it ion sbirds, cat t le and a variety of addit ional causes. A large proport ion of these faul tsamount ing to approx imate ly 80% are , however, o f a t r ans ien t na tu re and b r ingabou t the opera t ion o f the p ro tec t ive dev ice wi thou t caus ing pe rmanen t damage tothe system. As wil l be seen later, this fact enables automatic restorat ion of supplieto be em ployed wi th con sequen t im provem ent in the con t inu i ty o f suppl ie s to consumers fed from such l ines.

M anual res tora t ion of suppl ies in rura l a reas af ter in ter rup t ion is of ten rendereddi ff icul t by inc lem ent w eather and rou gh ter ra in and f req uen t ly involves jou rne ysof cons iderable length .

The faul t p ow er level on a rura l ne tw ork may be as h igh as 250 MV A at thes o u r c e subs ta t ion . Such a con di t ion ma y cause burning th rou gh of small sec t ion



248 The applicat ion o f pro tect ion to rural distr ibu tion systems

Primary transmission l ine

Primary substat ions

Addi t iona lfeeders

Addi t iona lfeeders

'

Sect ion

i n t ( ~

ily-openswitch

Sec t ion po in t

Sect ion poin t

F ig . 15 .1A Diagram of typ ica l ru ra l system

to restr ict the fault level by the isolat ion o f infeeding transfo rm ers to discretsect ions of switchgear. In such cases automatic sequence closing of busbar sect ioci rcui t breakers m ay be em ploye d to res tore suppl ies in the event of a t ransform efailure.

Due to the com parat ively high reactance of overhead l ines , the a t tenua t ion o

fault power along the l ine is high and the current values result ing from faults apoints rem ote f rom the substa t ion are considerably reduced as wi ll be seen f rom



T h e a p p l i c a t i o n o f p r o t e c t i o n t o r u r a l d i s t r ib u t i o n s y s t e m s 2 4 9

2 5 0

C ~ n d u c t o r, ~ - - 3 2 m m 2 C ~ p p e r

1 5 0

<>

-C

- - I 0 0

5 0

F i g . 1 5 . 1 B

l' t

i , ,

, 2 ; , ;R q ) u t e l e n g t h f r ~ i n s~urce - k m

In the case of ear th faul ts , a t 'ur ther reduct ion may occur due to high res is tancescaused by d i ff i cu l t ea r th ing condi t ions . Pro tec t ive dev ices fo r ru ra l l ines mus tthere fore be capab te o f w i ths tan d in g the th ermal e ffec t s o f heavy curren t w hi l s tbe ing su ff ic ien t ly sens i tive to opera te on rest r ic ted cur ren t s. Fu r therm ore ,p ro tec t ive eq u ip m en t , such as fuses o r c i rcu i t b reakers , w hich is ins ta ll ed on l inesuppor t s mus t per form re l i ab ly a f te r p ro longed exposure to vary ing c l imat ic con-

d i t ions . I t is the p rov is ion of d ev ices m ee t ing these requ i rem ents , a t a cos t inaccord wi th the overa l l eco n om ics o f ru ra l e lec t r i f i ca t ion , tha t poses the p rob lem s



25 0 The appl ica t ion o f prote ct ion to rura l d is t r ibut ion sys tems

1 5 . 2 F u s e s

15.2.1 Typese m p l o y e d

One of the oldest and simplest m etho ds of protec t ing rural systems is the use ofuses. E arly designs consisted o f li tt le m ore tha n an enclose d wire and their peformance was somewhat errat ic . A large amount of research and development hahowever, been carr ied out and the modern fuse is a much more rel iable device.

Tw o ty pes of high-voltage fuse are now general ly available for the pro tect io n orural sys tems, nam ely

( i )( i i )

Expuls ion typ ePowder f i l led (high breaking capacity) type.

The con struct io n and characteris t ics of these fuses have already been discussed Chapter 5 .

The l iq ui d f 't lled fuse was once w idely used but t roubles w ere expe rienced duto breakage of the glass and leakage of l iquid. Furthermore, i ts applicat ion wasom ewh at restr icted by its limited break ing ca pacity as system fault levels increaseThe expulsion type is cheaper in capital cost and fusel ink replacement and witavailable ratings being suitable for the fault power levels normally encountered irural netw ork s, i ts use has now superseded tha t o f the liquid tidied fuse.

Fo r fault levels in excess of 150 M VA at 11 kV , an ada pta t ion o f the expu lsiofuse carr ier to incorp orate a cartr idge type powder-fi l led fuse l ink m ay be em ploy ed

The con st ruct ion of the expuls ion ty pe fuse readily permits the removal of thfuse l ink to be used as a means of isolat ing the protected equipment.

15.2.2 Applicat ion

Early applicat ions tended to fuse each spur l ine and individual t ransformer sepaately , as in Fig. 15.2.2A (a), the fuse rat ing being related to the ful l load currenrat ing of the plant protected. Whilst fuse blowing due to transformer fai lure wainfrequ ent , surges during l ightning storms o ften resulted in widespread fuse ope rt ion wi thou t perm anen t dam age to app ara tus . This led to the co nce pt of 'groufusing' , that is the protect ion of a number of t ransformers on a spur l ine by a s inglset of fuses at the spur tapp ing po int , as shown in Fig. 15.2.2A (b). This safeguardthe main l ine from disturbances on the spur and is the principle general ly employetoday. I t is usually accepted that h.v. fuses be employed for short circuit rathethan overload protect ion, and i t is usual , therefore, to instal l 'group fuses ' ocomparat ively high rat ing to prevent unwanted operat ion due to switching surgeand l .v. faults . This , however, has the disadvantage that low current faults , such a

t ransformer in ter turn fa i lures and cer ta in broken conductor faul ts , may not causthe fuse to operate. The number of such incidents is , however, negligible in com



T h e a p p l ic a t i o n o f p r o t e c t i o n t o r u r a l d i s t r i b u t i o n s y s t e m s 2 5 1

dis t r ibut ion t ransformers and present day overhead l ine design, the pr inciple ofh.v. group fusing is undoubtedly jus t i f ied. Overload protect ion of individual t rans-form ers is provid ed by 1.v. fuses.

Current f lowing to a fault on the l .v. side of a transformer is supplied via the h.v.

group fuse and, under these condi t ions , the l .v. fuse protect ing the faul ty c i rcui tshould opera te f irst in order that the h.v. supp ly to hea lthy transform ers is not

s u b s t a t i o n

1, i

Sour~ ~ lu b s t a t i , ~ n ]

l , II ]I ll I.' k I~ ,, I1 . '

I - l i n e

(l>---

( b )

S p u r l in c s

I i a c k h , l l l v

l i r i c



2 5 2 T h e a p p l i c a t /o n o f p r o t e c t i o n t o r u r a l d i s t r i b u t /o n s y s t e m s

affected. T he use o f h igh breaking cap aci ty pow der f i lled fuses, w ith their inherenthigh speed of op erat ion, on the 1.v. s ide, com bin ed w ith the compa rat ively highcurrent rating of the h.v. group fuse, materially assists in obtaining thisdiscr iminat ion.

This is shown in Fig. 15.2.2B where the h.v. and l .v. fuse protect ion of three-phase 11 0 0 0 /4 4 0 V transformers is i l lustrated. A 10 0 k V A transformer havingan impedance of 4-75 per cent and connected to an h.v. system having a faul t

l 0

1.0

u~

• 0.5O

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E. , . , ,p.,

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,

3 0 0 a m p H . B . C 2 Sa m p f a s t

I .v. fu se / b l ow in g h .v.f u s e

\\

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i . . . •

5 0 0 1 0 0 0 S O 0 0 1 0 0 0 0 $ 0 0 0 0

L . V. c u r r e n t - A m p s



T h e a p p l ic a t i o n o f p r o t e c t io n t o ru ra l d i s t r i b u t io n sys tems 25 3

level of 15 0 M V A w il l pass a m ax im um l .v. fault current of 2 7 6 0 A. F or a l l valueof faul t current up to th is f igure discr iminat ion wil l be obtained between the h .v.expuls ion type fuse of 25 A rat ing and the high breaking capaci ty l .v. fuse of 300 Arat ing, to lerances on fuse character ist ics being neglected . T he ra t io be tw een l .v. and

h.v. fuse ra t ings is 12 1 , a l though the t ransformat ion ra tio is ap prox im ately 25 1 ,discr imin at ion w ith fuse ra tings of th is ra t io being m ade possible by the high speedof the f i l led type of fuse re la t ive to that of the expuls ion type.

An l .v. fuse of 400 A rat ing wil l not discr iminate wi th the 25 A h.v. fuse a t faul tcurrents below the m ax im um that can occur on the l.v. s ide of the 10 0 kV A trans-

2 . 0

1 . 0

0 . 5

0 . 4

0 . 3

. ~ 0 . 2

¢ . ¢ q

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_ L1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0

S y m m e t r i c a l c u r r e n t - A m p s



254 The app l ica t ion o f p ro tec t ion to ru ra l d i s t r ibu t ionsystems

former but wi l l do so a t h igher va lues of faul t cur rent which may be exper ienced o lthe l .v. s ide of larger t ransformers.

In order to check the d iscr imina t ion be t w een fuses in series, i t is necessary tp lo t the to ta l c lear ing- t ime charac ter i s t ic of the fuse rem ote f ro m the source an

the prearc ing- t ime charac ter i s t ic of the fuse nearer the source to a common currenbase . In the case of t rans form er h .v. and l .v. fuses, the current va lues m ust beconver ted to a com m on vol tage base . Discr iminat ion wi l l resu l t if the charac ter i s ticare separa ted by an in terva l of a t leas t the sum o f the pe rm i t ted charac ter i s t ito lerances for the fuses in que s t ion . Pr ior to the advent of BS 269 2, to lerances o

1 0 0 0

500

100

50

0

I

.E

1.0

0 .5

0 . I 0

O.OS

0 . 0 1 -

I

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5

5 10

Ill

5 0 I 0 0 $00 1000 5000 100 00

C u r r e n t - A m p s



The app l ica t ion o f pro tec t io n to rura l d is t r ibu t ion systems 25 5

the t im e/ cu rren t charac ter is t ics of high-voltage fuses we re not specif ied and s imple ru le-of -thumb m etho d of ensuring d iscr iminat ion b e tw een h .v. fuses was temploy a major fuse having a current rat ing twice that of the minor fuse. Thtoleran ce on the chara cter is t ics of fuses to BS 2692 is + 20% and in the absenc

of mo re specif ic inform at ion f rom the fuse m anu fac tu rer, th is to lerance must ba l lowed wh en com par ing fuse charac ter i s tics . W ith cer tain des igns of expuls ion fushow eve r, greater ac cura cy is possible and a discr im inat io n fa ctor, that is m ino r fusrat in g/m ajor fuse rat ing, of 0-75 is ob tain ab le. Fig. 15.2.2C shows thdiscr imina t ion l imi ts for three c om m on ly used s izes of expuls ion fuse . The l ines Areprese nt for each rat ing the published cha racter is t ic w hils t the l ines BB have beeplo t ted wi th the current ord ina tes reduced to 0 .75 of the va lues represented by AAThese fuses wil l discr iminate s ince the shaded area between the l ines tbr any rat indoes no t over lap tha t of the o the r ra tings. Fuses of in terme dia te ra ting wi ll nodiscr iminate i f over lapping of the i r charac ter i s t ics , p lo t ted as above , occurs .

In some instances, the use of fuse l inks with a s lower operat ing character is t ic athe h igher currents may be requi red to obta in adequate d iscr iminat ion . Fuse l inkwith such character is t ics are avai lable in the l iquid f i l led and expulsion types buare not avai lable with the powd er-f i lled h.b.c , typ e w hich is inhe ren t ly fast iope ra t ion . A com par ison of t ile rela tive prearc ing t imes of fas t and s low blowin

h.v. fuse l inks is i l lustrated in Fig. 15.2.2D.Some types of faul t on the low-vol tage s ide of t ransformers employing cer ta i

winding connect ions wil l give r ise to unbalanced currents on the high vol tage s idand currents in excess of those car r ied by a ba lanced three-phase faul t may occurI t i s essent ia l therefore to ensure tha t adequate d iscr iminat ion can be obta ined wi tthese part icular faul t condit ions.

Ex per ience has indica ted tha t the t ime/ curre nt ch arac ter i s tics of overhead linh.v. fuses change af ter a length of t ime in service and maldiscr iminat ion may occubetween fuses in ser ies , even though correc t d iscr iminat ion i s indica ted by a compar ison of the i r publ i shed charac ter i s t ics .

For th is reason and because of the wide to lerances on the charac ter i s t icssomet imes encountered , the use of such fuses in ser ies i s not recommended.

1-5.3 A uto m at ic c i rcui t rec los ing

15.3.1 Principle

As men t ioned in the In t roduc t ion , appro x im ate ly 80% of the f au lt s occur ring onrural sys tems are of a t rans ient na ture and cause no p erm ane nt dam age to lines oplant . Such faul t s , however, requi re the opera t ion of a pro tec t ive device , forexam ple a fuse or c i rcuit b reak er, for c learance , and an in ter rupt ion of supplyresul ts unt i l the fuse has been replaced or the circui t breaker reclosed. In rural areaa long jou rney m ay be involved of ten at an inconven ient t ime and in inc lem en



25 6 The applicat ion o f pro tect ion to rural distr ibution systems

wil l be ap pa ren t the re fore tha t an improvement in the con t inu i ty o f supp ly tconsum ers and a reduction in operat ing costs will result i f the supply can brestored au tom atica l ly. A utom atic circuit reclosing is now extensively applied trural networks and has resulted in a high standard of securi ty of supplies to rur

consumers .Of the factors bearing on successful automatic reclosing, the most s ignif ican

is the speed of operat io n of the circuit breaking device. If the op ening o f the circuis delaye d m ore tha n a few cycles, the the rm al and oth er effects of the fault cu rrem ay cause perm ane nt dam age at the fault posi t ion th us creat ing a persistent faulpreventing im m ediate supply restorat ion . H igh speed fault clearance, how ever, wiin most cases, prevent such damage and permit successful restorat ion of supplies.

The t ime interval between tr ipping and subsequent reclosure is usually referreto as the 'dead t ime' and may vary over a wide range. I t is influenced by thcharacteris t ics of the circuit-breaking device, the nature o f the faults l ikely to benco untered and the ty pe of load suppl ied . In th is la t ter conne ct ion, d i ffering typof m otors have conf lic ting requ irements , synchrono us machines requir ing a deat ime sufficiently long to ensure ope rat ion o f their undervo ltage protec t ion whilfor induct ion motors the per iod needs to be shor t to enable them to coas t througthe open-circuit period. The dead t ime must in al l cases however, be of suff icien

dura t ion to ensure deionisat ion of the fault path and stabilisat ion of the break emechanism. In pract ice t imes ranging from 0-4-120s are employed but evidencfrom field tr ials suggests that a t ime of 10-15s affords the best performance undetypical sys tem c ondi t ions .

An important characteris t ic of reclosing schemes is that known as reclaim oresott ing t ime, i .e . the t ime fol lowing a successful reclosing operat ion after whicthe scheme wil l provide a ful l operat ing sequence in the event of subsequent faultIt is of particular significance under repetitive fault conditions, e.g. l ightning stormor conductor clashing in high winds, when rechim t imes in excess of the intervabetw een the incidence o f successive faults ma y cause unnecessa ry lock out aninterruption of supply. Service experience indicates that good results are obtaineby the use of reclaim t imes o f the order of 5s, al though in the case of spring actua temechanisms i t m ay be governed and e xtended by the spring winding requireme nt .

The use of s ingle phase inter ruptio n and reclosing produce s a ten de nc y for thvoltage on th e faulty phase to b e m aintained by the sound phases via the mag net

circuits of three phase transformers connected to the system. Whilst this may assiinduct ion m otors to cont inue to run, i t ma y delay the d isconnect ion of synchronoumachines . Since ind uct ion m otors are the more com m only used in rural areahowever, the scheme has been used with considerable success.

In a large pro por t ion of non-pers istent faults , es timated to be some 80% of thtotal , the circuit may be successful ly re-energised at the f irs t at tempt, but with thremainder a second or th ird rec losure m ay be necessary. Considerable improve me nin the cont inui ty of supply can therefore be obta ined by the use of ' s ingle shotreclos ing, and any fur ther improvement which may be achieved by mul t ishoreclosing mu st be judged against the cost of the addit ional e quip m ent required t



The app l ica t ion o f p ro tec t ion to ru ra l d i s t r ibu t ion sys tems 25 7

15.3.2 R epea ter fuses

Where protect ion of the h.v. system is by fuses, automatic restorat ion of suppliem ay be achieved by the use of rep eater fuses. These are of the dro p out expulsion

type and o ne , two or three res tora tions may be o bta ined by the ins ta l la t ion of thappropriate number of fuses per phase. Only one of these is normally in circui t , andfollowing its operation on fault the energy of the falling carrier in isolating thespent fuse is ut i l ised to actuate a spring loaded changeover contact to bring the nexfuse into circui t. By the in corp orat io n of a t ime lag device this act ion m ay bedelayed to ensure that any faul t arc path has beco m e suff icient ly deionised topermit the circuit to be successfully re-energised.

The d isadvantages of the use of repeater fuses are the amount of equipmentwhich i t i s requi red to accommodate on an overhead l ine suppor t and the numbeof replacement elements required af ter clearing a persis tent faul t .

15 .3 .3 Po le-mou nted auto m at ic c i rcui t reclosers

(a) Construct ion:The repeater fuse perm i ts only a l imi ted num ber of opera t ionsbefore reset t ing becomes necessary and periodical inspect ions are therefore

required to ensure full availabili ty.A form of reclosing circui t breaker which overcame the disadvantage of this

l imitat ion, by vir tue of an operat ing mechanism which is self reset t ing i f the faul t iremoved before the complet ion of i ts operat ing sequence, was developed inAm erica a bou t the mid 1940s . This type of equ ipm ent offered o ther advantages fothe protect ion of rural overhead l ines, and designs are now avai lable in this country

Uni ts of th is type have been commonly termed 'h igh speed rec losers ' but th i

term is something of a misnomer s ince, al though the open circui t interval is reducedto ap pro xim ately one seco nd, their main at t r ib ute is a very high tr ipping speedwhich minimises faul t damage to appara tus and prevents unnecessary opera t ion o

t Fle xible SeriesI c o n n e c t i~ n ~

M Al te rna t ive ~_1 ~'N~,n l inearc , ,n tac t 7 l in k I ] ~ ~ I " resist, ,r

I pos i t ion s ~- ~1 I~ f I '~. I

_ i I' ' I l

I I ,Fixed c , ,n tac t ]



2 5 8 T h e a p p l i c a t i o n o f p r o t e c t io n t o r u r a l d i s t r ib u t io n s y s te m s

associated fuses . F or cer tain app l icat ions, h owev er, som e delayed t r ipping operat ionsmay be required and units with variable tripping characteristics have therefore been

produced .The first recloser of this type produced by a Brit ish switchgear manufacturer was

a single phase per tank design which uti l ised the energy of the fault current to openthe circui t -breaking conta cts , and , a t th e same t im e, to charge springs w hich subse-qu ent ly reclosed the con tacts. I t was essentially a series solenoid dev ice, theeletr ical arrangement being as shown in Fig. 15.3.3A. The thermal character is t ics

I 0

876

" 0¢ :0¢J

¢ )

E..,,

¢ ).,,,.,

I,,.. m

c..)

| m

0 . 9 - -0 . 8 - -

0 . 7 - -

0 . 6 - -

0 . S - -

0 . 4 - -

0 . 3 P - - -

0 . 2 i ' - - - - - - - -

\\ / D e l a y e d t r i p

\

J J J1 1

0 |0 . 0 90 . 0 8 - - - - - - - - -

0 . 0 7 - - - - - - - - - -

0 . 0 6

0 . 0 S - - - - - - - - -

0 . 0 4 - - - - - - - -

0 . 0 3

. . ~ I n s t a n t a n e o u s t r i p

J j j i ll l l l l lI I I I I

0 . 0 2C~ C) C~ C~ C~ C~ C~C~ Q O C~ C~ C~ C~

, m ¢ , q ¢ 1

C u r r e n t ( P e r c e n t a g e o f n o r m a l c u r r e n t r a t i n g o f s e r i e s c o i l )



The appl ication o f protec t ion to rural d is t r ibut ion sys tem s 259

of the solenoid imposed some l imi ta t ion on the maximum breaking capaci typart icularly on units for the lower normal current rat ings.

The t im in g mechanism incorporated an oi l dashp ot w hich provided a fuloperat ing sequence of two high speed and two delayed tr ips , or a lternatively up tfour high speed tr ippin g ope rat ion s. On the d elayed tr ips i t im parted an inverst ime characterist ic modified to improve discrimination with t ime fuses f i t ted to thsource circuit breaker. This characteristic is indicated in Fig. 15.3.3B.

If a fault was cleared before completion of the ful l operat ing sequence, threcloser remained closed and the m echanism reset . W hen, how ever, the faulpersisted for the durat ion of the ful l sequence, the recloser locked in the opeposit ion after the f inal t r ip and then had to be reclosed manually after the faul

had been removed.Following a manual reclosure, the recloser had only one delayed tr ip to locout in order to prevent t r ipping due to inrush currents and to prevent unnecessarreclosing, i f a fault s t i l l existed on the l ine. If no such tr ipping occurred howevethe mechanism reset within a short t ime to provide a ful l operat ing sequence.

This type of recloser was manufactured with rat ings up to 200 A in single phasunits with mechanical interphase coupling. I t was arranged to provide tr ipping anreclosing on the faul ted phase or phases on ly, bu t for locking open or manuaoperat ion a ll three phases were actuated s imul taneously.

The minimum operat ing current was approximately twice the normal currenrating and thus t r ipping o n low values of fault current , such as m ay obta in o n somtypes of earth fault , was not possible. This type of recloser is therefore no longein p rodu ct ion having been superseded by the cu rrent design wh ich affords thlat ter feature.

The design no w available encloses three phase units in on e tank and is arrange

for three-phase tr ipping and reclosure. The energy for charging the reclosing sprinis derived from a mult is troke electromagnet , the coil of which is connected acrotwo of the incoming terminals through auxi l iary swi tches coupled to the maicontacts which ensure that the coil is in circuit only for the t ime necessary tcharge the spring. The electr ical arrangement is as shown in Fig. 15.3.3C. I t shoube noted that the act of manual closing merely closes the coil auxil iary switcheand closing of the main contacts is by the act ion of the electromagnet and closinsprings.

U nder cer tain c i rcumstances , such as wh en maintenance or repair w ork is beincarried o ut , i t may be necessary for the c urrent f low thro ug h a recloser to be indirect ion reverse to that norm al ly em ploye d. W hen operat ing under th is con di t ioa recloser with the closing arrangement described above wil l t r ip once to clear fault occurr ing on the s ide remote f rom the new source of supp ly, bu t c ann ot breclosed, ei ther automatical ly or manually, s ince the reclosing coil is then isolatefrom the supply by the main contacts . To avoid unnecessary in terrupt ions due t

this cause, a ' lock-closed' feature has been in trod uce d which is bro ug ht into act ioby the operat ion of an external handle and which prevents the recloser f rom



26 0 The app licat ion o f pro tect ion to rural distr ibu tion systems

Incoming supply terminals

"--'J ~ transformersurrent'7 - , .

Spring 0 -II

Charging E

solenoid

Earthfaultrelay

sAwUi;cliha~Y " [ / 1

Outgoing supply terminals

Series overcurrent

trip coils

M ain o ntacts

Fig.15.3.3CIntern al con nec t ion diagram o f recloser with ear th fa ul t feature(A Reyrol le & Co. Ltd.)

Ov ercur ren t p ro tec t ion is p rov ided by c on ve nt iona l se r ies t rip -co i ls w idas hp ot type mech anism to gi ve an inverse t ime characterist ic show n in Fi15.3 .3F. For ins tantaneous t r ipping, the dashp ot ac t ion is bypassed by the openinof a valve in a cylinder, the co ntro l of this valve being by m eans of a cam w hicthus provides a ready m eans of sequence con trol .

As a consequence of the f ine mechanical clearances employed in the construct ion of the dash pot , som e increase in operat ing and resett ing t imes occurred in coweather due to the increased viscosi ty of the oi l . Under repeti t ive fault condit ion

e.g. conductor clashing during winter s torms, this had the effect of cumulativprogress ion to the loc kou t posi t ion due to incom plete reset ting betw een consecut ivoperat ions. One method of al leviat ing this s i tuat ion was to use a special lowviscosity insulat ing oil which produced operat ing characterist ics at 0°C approxmat ing to those a t 20°C w ith normal o i l to BS 148.

This cold weather problem is now overcome by the incorporat ion in the des igof a modif ied t iming and sequencing mechanism. In the modif ied t r ipping dashpothe clearances are controlled by a bimetal l ic s tr ip and thus afford delayed overcurrent t r ipping t imes wh ich are com parat ively ind epende nt of temp erature . Th

i h i i l i d id f l l



The appl icat ion o f protect ion to rura l d is t r ibut ion sys tem s 261

have occ urred. The standard reset t im e is 90s, but this t ime can be reduced to som5-10s by dri ll ing a small hole in the contro ll ing dash po t.

Provision is made for an instantaneous earth-fault t r ip coil operated from curretransform ers in turrets at the base o f the termina l bushings. This feature is operat iv

on ear th faul t currents down to 20 amps.

So urce

subs ta t ion

Fig . 15 .3 .3D

Spur line

Recloser

Spur line

Recloser and fuse p ro te c ted l ine

=

A range of tr ip coil ratings u p to 20 0 A is available w ith this design and the fubreaking capacity of the unit is maintained with al l rat ings throughout the rang

Ow ing to the s ingle tan k con st ruct ion , the uni t m ay be m ou nted on a s ingle linpole.

Both types o f rec loser described above have nom inal dead t imes o f one second.

(b) Application:In order to protect as large a sect ion of network as possible, arecloser shou ld b e instal led as near to the source of su pp ly as l imitat ions of normcurrent and breaking capaci ty wi l l permit , the actual locat ion being determinelargely b y these factors. Reclosers in series m ay be just if ied on extensive oimportant networks and in some cases individual spurs may be protected by recloser.

The method of appl icat ion of rec losers based on thei r use in conjunct ion wi th.v. fuses is indicated on Fig. 15.3.3D.The recloser and fuses are so co-ordinatedtha t t ransient faults are cleared b y the high speed ini t ial t r ipping of the recloseThe fuse, or fuses, operate during the delayed tr ipping period to isolate persistenfaults and minimise the sect ion of network without supplies. This requires

recloser wi th an operat ing sequence o f ins tantaneous fol lowed by delayed t r ippinoperat ions , as shown in Fig . 15.3 .3E. The fuse should remain in tact dur ing th



26 2 The appl ica t ion o f pro tect ion to rura l d is t r ibut ion sys tems

performance on subsequent faults m ay be in accordance wi th i ts publ ishecharacteris t ic . On the delayed tr ipping operat ions, however, the fuse should meand isolate the faulty sect ion before lockout of the recloser occurs.

In order to determine the current range over which these requirements may b

achieved i t is necessary to compare the total heat input to the fuse during thre levant por t ion of the rec loser opera t ing sequence wi th the t im e/cu rrent characteist ic of the fuse. Over the port ion of the sequence during which the fuse shoulrema in inta ct , how ever, some m odificat io n o f the published fuse curves is requirein orde r to take into acc oun t the effect of preloading and progressive dete riorat ioof the fuse element with consequent loss of characteris t ic af ter carrying repeatetransient fault curren ts . This is don e by dera t ing the melt ing t ime curve of the fuby the appl ica tion of a fac tor of 0-75, a f igure wh ich m ay be used in the absence specif ic inform ation on this point relat ing to the part icular fuse em ploy ed. On thother hand, some degree of cool ing of the e lement takes p lace dur ing the opecircuit interval of the recloser, and the c orrec t ion t ime to be add ed for this effeis given by the formu la

t = - - - T

whe re t is the co rrect ion t ime to be add ed to the fuse m elt ing t ime characteris t iIm is the minimum melt ing current of fuse, T is the total open-circuit t ime for thpor t ion of rec losure sequence considered, and I / i s the c i rcui t faul t current .

In pract ice the error introduced by neglect ing the cooling effect is so small ato be of l it tle conseque nce.

F a u l ti n c e p t i o n

/

C l o s e d ~ ~ -- , _

O p e n

_ _ . . _ j¢j

= =~ E =; ~ o o o T i m e ~ o o

• Z ~ Z , Z

L o c k e do p e n



The app l ica t ion o f pro tec t io n to rura l d is t r ib ut ion systems 26 3

Fuse t ( ) t a l clearance t ime

2 inst. + 2 delayed trips

2 inst. + 1 delay edtrip

i inst. trips

inst. trip

I - C ()-ordinati(} n v \ \

( . ' u r r e n t




The modified fuse characterist ics are then superimposed upon the cumulativt ime/current curves of the recloser, as shown on Fig 15.3 .3F. The upper l imi t focorrect co-ordination is given by the intersection of the derated fuse melt incurve and the last instantaneous tr ipping curve of the recloser (shown as A), whil

the lower l imit is obtained from the intersection of the fuse total clearing t imcurve w ith th e recloser final delayed tr ipping curve (sho wn as C).

W ith faul t currents betw een A and B, co-ordinat ion wi l l be obta ined o nly i f thfault is removed before the second instananeous tr ip, and currents in excess of wil l cause the fuse to blow before any operat ion of the recloser. Point A is therefore regarded as the upp er l imit s ince above this value the full advantage of a seconinstantaneous t r ip i s not obta ined. At currents below the lower l imi t , locking opeof the recloser wil l occur before the clearing of the fuse, provided that the currenis in excess of the minimum value required to tr ip the recloser, normally twice irated current .

The range of co-ordination is obviously influenced by the slope of the fuscharacteristic curve, the steep slope o f the fast blow ing fuse restr ict ing the range comparison wi th that of the s low blowing type. The la t ter type are thereforem ploy ed w ith reclosers, and i t has been foun d tha t the size of fuse is dictated bco-ordinat ion requirements ra ther than load currents , and fuse ra tings som ewh

higher than those in use on sys tems w i tho ut rec losers are general ly em ploy ed.The dual mode fuse referred to in Chapter 5 is designed to afford improved co

ordination with reclosers by having fast blowing characterist ics at low currenand slow blowin g characteristics at high cu rrents .

(c ) Development o f a l terna t ive opera t ion cycles :Operat ional exper ience ofreclosers and fuses in co m binat ion in it ia l ly indicated that the num ber o f supplinterrupt ions on the protected l ine were reduced by some 90% of thei r or ig inanumber, and that the operat ion of a fuse was a def ini te indicat ion of a pers is tenfault . A fter tw o or three years, howev er, a sharp increase in the num be r of fusoperat ions on transient faults occurred inferring that fuse deteriorat ion in servichad, in fact , taken place.

The effect of this deteriorat ion can be offset , and the ini t ial performance of rec loser /fuse com binat ion m ainta ined, b y the replacement of all fuse e lements aintervals of appro xim ately two years. Al ternat ively, the fuse deter iora t ion factor o

0-75 m ay be reduced, b ut th is is a t tended by a reduct ion in the range of co-ordination and m ay no t , in any case, be the com plete so lution. In view of the smanum ber of pers is tent faults occurr ing on recloser protected sys tems, how eveconsiderat ion was given to the el imination of fuses entirely. In addit ion to reducininstal lat ion and operat ing costs and avoiding unnecessary interruptions due ttransient faults , this course permitted an improvement in the overal l system performance since delayed recloser tr ipping operat ions could be el iminated and discrimination with back-up relays thereby faci l i tated. I t was appreciated that alpersistent faults would result in the locking open of the recloser causing a greatenum ber of consum ers to be affected b y interrupt io n of supp ly bu t i t was fel t tha



The appl ica t ion o f pro tect ion to rura l d is t r ibu t ion sys tems 26 5

Reclosers having al l instantaneous tr ipping operat ions, as indicated in Fig. 13.3.were therefore in t roduced.

Reclosers wi th delayed t r ips have been used wi thout associa ted fuses , but theimpose unnecessary strain on the system due to the long clearance t imes for pe

s istent faults. In ad di t ion, the problem of d iscr iminat ion wi th back-up re lay prtect io n sti ll remains.

Operating experience of reclosers on systems from which fuses have beeeliminated indicates that the incidence o f supply in terrup tion s is reduced to consis tent ly low level . The main cr i ter ion to be employed when comparing thmethod of appl icat ion wi th that employing fuses , however, i s that of the produof the number of consumers and t ime for which suppl ies are in terrupted, takeover a period of t ime rather than for an isolated incident .

(d) Fau lt indicators:On systems protected by reclosers w i th the a ll ins tantaneoutripping sequen ce, dam age d ue to persistent faults is great ly restr icted due to thvery sho rt du rat io n for which the fault cu rrent is al lowed to p ersist . At t imes threnders fault location difficult, and adequate line sectionalising facilit ies arnecessary to assist the location of faults . Where fuse mounts already exist , themay be converted to sect ionalising points by the f i t t ing of solid l inks in place of th

fuses.The instal lat ion of fault indicators at s trategic posi t ions on an overhead-l in

circuit aids the location of faults b y indicating the passage o f fault curre nt . Onsuch device provides a f ixed p hase fault set t ing of 100, 200 or 4 00 A and minimum earth fault set t ing of 5% of the phase fault set t ing. Indication is displayewh ilst the setting cu rren t is exc eede d or the line is dead a fter fault clearance, reset ing taking place automat ical ly on res tora tion of supp ly.

The device is activated by three detector coils located inside special insulatom ou nted on the l ine crossarm. The m agnet ic f lux surroun ding each l ine con du ctosupported by the insulators induces a voltage proport ional to the l ine current ithe associated coil and these voltages are applied via flexible leads to solid-statphase and earth-fault measuring circuits contained in a w eat he rpr oo f housinmounted on the pole below the crossarm.

At the set current value, a signal is applied to a trigger circuit which dischargea capac itor in to the ind icato r op eratin g coil. W hilst the line is energised the devic

receives a pulse every few seconds from a second capacitor to retain i t in the resestate bu t the o perat ing circuit is arranged to override the reset t ing circuit.

The device derives the energy necessary for operat ion of the measuring anindicat ing funct ions f rom the l ine i t sel f by means o f capaci tors form ed b y conducting glazes on the upper shed and internal coil housing of the insulator. Sincthere is no stored energy available prior to energising the line, the device takeapproximately two seconds to operate when c los ing a l ine on to a faul t and th it ime may be increased if the source impedance is high and the voltage reduced. the l ine protect ion clears the fault within that t ime, no indicat ion is made.

For close up faults when the line voltage may fall to zero sufficient energy i



2 6 6 T he a p p l i c a t i o n o f p r o t e c t i o n to r u r a l d i s t r i b u t i o n s y ste m s

Closed ,

F a u l ti n c e p t i o n

O p e n

¢¢ ¢¢

o qZ -

¢¢

• - 112

o qz , -

L o c k e d

o p e n

Time . - - - - - - - -e .

F i g . 1 5 . 3 . 3 G Trave l / t imediagram fo r all-instantaneous recloser

Tr i p r o d

Succ es s ive u up o s i t i o n s o f ~ . . . . . . dm o v i n g p i s t o n

. . . . . ' dR e s e t p o s i t i o no f m o v i n g p i s to n ~ 1 ~

N o n - r e t u r nva lve s

S e r i e s c o i l

M o v i n g p i s t o n s t o p

C a p t i v e p i s t o n

I r o n c o r e ~ O i l i n t a k e



The appl ica t ion o f pro tec t io n to rura l d is t r ib ut ion systems 26 7

The indica tor hous ing and the de tec tor co i l s may be ins ta l led or removed by thuse of l ive l ine te chn ique s.

A noth er type of l ine faul t indica tor m ay be used as a por table device for a t tachment to poles at s t rategic points on a l ine during faul t f inding operat ions and

subsequent removal for use on later faul ts on other circui ts . Alternat ively, a versiosui table for permanent instal la t ion is avai lable .

The device is f ixed to the pole ne ar to the base and is responsive to ch anges ithe magnet ic f ie ld produced at this level by currents f lowing in the l ine conductorsth is f ie ld be ing zero under normal ba lanced load condi t ions s ince the vec tor sum othe phase currents is zero . U nder the unbalanced cond i t ions obta ine d dur ing faul thowever, a resul tant f ie ld is produced and an earth faul t current of 20 A i

suff ic ient to cause opera t ion of the indica tor. The phase to phase faul t cur renrequi red to opera te the device i s dependent on the conductor conf igura t ion , thm inim um value for l ines of hor izonta l con s t ruc t io n to BS 1320 be ing 900 A ansom ew ha t le ss fo r l ines o f t r iangu la r to rm at ion , a l though opera t ion may no t occuin the case of faul t s whic h cause currents to f low in the oute r c ond uctors only.

The opera t ing sequence of the ins t rument i s in i t ia ted when a sys tem dis turbanccauses the rate of change of the m agn et ic f ie ld to be suff icien t to induce a vol tagof the set value in the detector coi l . I r respect ive of i ts s tate , the device immediatel

resets to the s ta r t of i ts seque nce and for 50 ms op era t ion is inhib i ted. T his delais in t roduced to avoid unwanted opera t ion on magnet i s ing inrush t rans ients but ia t the end of this period the f ie ld is s t i l l adequate to produce the required vol tagin the de tec tor co i l , the device wi l l indica te a f te r a fur ther 20 ms. Fol lowing th isfurther operat ion, i .e . reset t ing, is prevented for one second so that the device iun affe cte d b y the change in fie ld caused b y the clearanc e of the fau l t by a circuibreaker. T he de te c to r c i rcuit is then re ins ta ted and any fur ther sys tem dis turbanc

wil l reset the device and restar t the ope rat ing seque nce. I f , how eve r, no such s ignais rece ived , the device rese ts au tomat ica l ly af te r 8 h . The t imes quoted above arprese t dur ing manufac ture but may be var ied to su i t par t icu lar requi rements .

O perat in g pow er is derived frown internal bat t er ie s , a ten year li fe being claimefor those used in the permanently instal led version.

(e) Sectionalisers.A method of i so la t ing faul ty sec t ions of a rec loser pro tec tednetwork wi thout employing fuses i s the use of au tomat ic l ine sec t ional i sers . Thesdevices are pole mounted o i l d isconnectors which are a r ranged to open automat ica lly dur ing a predeterm ined 'dead t im e ' of the associa ted rec loser, thus i so la t ing thfaul t before loc kou t o f the rec loser occurs . This charac ter i s t ic ensures co-ordina t ionwith the recloser at a l l values of faul t current up to the ful l ra t ing of the device.

The sec t ional iser funct ions by coun t ing the passage of pulses of fault cur rent lethrough by the rec loser, these pulses f lowing through the opera t ing so lenoid caus ing

a capt ive pis ton in an oi l f i l led cyl inder to be pul led down against a spring. A nonreturn valve in the crown of the capt ive pis ton al lows oi l f rom the underside to pas



268 The applicat ion o f pro tect ion to rural distr ibution systems

which is forced upward by the spring when the recloser operates to cut off the faucurrent in the solenoid. The oi l column in turn l i f ts a moving piston. After a presenumber of pulses, the off column formed is suff icient to l i f t the moving pistoagainst the tr ip bar of the sect ionaliser, the contacts then being opened by th

act ion of a spring which has been charged during manual closing of the device. Thbasic arrangement of the sect ionaliser is shown in Fig. 15.3.3H.

( f) Gr adin g o f rec losers with o ther form s o f pro tec tion:In a typical recloserprotected sys tem, as depic ted in Fig . 15.3 .3D , the opera t ing character ist ic of threcloser mu st be betwee n those of the fuse and the protective re lay on the sourcsubstat ion circuit break er. A ny m aldiscrimina tion, in addit ion to causing unnec essa

loss of supplies, m ay also de lay the restorat ion of these supplies by presenting false indicat ion of the locat ion of a fault to the operat ing staff .A m ethod for determining the co-ordination betwe en recloser and fuse has bee

set out in Sec tion 15.3 .3 (b), and i t is essential to ensure that the size of fuinstal led permits a range of co-ordination adequate to cover fault currents l ikely tbe encountered on the protected spur.

The protect ive relay associated with the substat ion circuit breaker wil l usuallbe of inverse defini te minimum time type, and when a persistent fault occur

be yo nd the recloser this relay ma y be energised for two very brief periods followeby two some what longer t ime in tervals.

Since l i t t le reset t ing of the relay disc can take place during the very brief deat imes , cumulat ive forward movement of the re lay wi l l occur. In order thadiscrimina tion b etw een the circuit brea ker and the recloser is obta ined , it therefore necessary to take th is in to account wh en choosing the t ime se t t ing of thre lay. An opera t ing t ime of approximate ly twice that of the t ime delay t r ippin

characteris t ic of the recloser, as in Fig. 15.3.4L, is usually sat isfactory.Compliance wi th th is requirement may a t t imes be di ff icul t agains t the background of the overa l l scheme of protect ion fur ther back towards the source o

S o u r c es u b s t a t i o n

R e c l o s e r 1 R e c l o s e r 2

1 2 ] 1 2 1

I L o c k s o p e n L o c k s o p e na f t e r 4 o p e r a t i o n s a f te r 3 o p e r a t io n s

N . B . I f o n l y o n e r e c l o s e r is f i t t e d w i t h a ne a r t h f a u l t r e l ay t h i s s h o u l db e e m p l o y e d as r e c l o s e r N o . 2 .



The app l ica t ion of pro tec t io n to rura l d is t r ibu t ion sys tems 26 9

supply. Where , however, fu l l d iscr iminat ion cannot be obta ined , the ins ta l la t ioof rec losers wi l l s t i l l show a cons iderable improvement in the qual i ty of supplsince some 90% of the faul ts occurr ing wil l be cleared in the nonpersis tent s tage bthe ins tanta neou s t r ips of the rec loser which perm i t d iscr im inat ion . The d isad

vantage of th is a r rangement l ies in the fac t tha t when maldiscr iminat ion occurs , thopera t ing s ta ff may be presented wi th a mis leading indica t ion of the loca t ion of thfaul t and valuable t ime lost in restor ing supplies .

W ith a l l - ins tantaneous t r ipping rec losers the d i ff icu l ty of obta in ingdiscr iminat ion be tween the rec loser and c i rcui t breaker i s grea t ly reduced. Whereclosers of this ty pe are opera ted in ser ies, how ever, the differences in theoperat ing t imes of uni ts of various rat ings are so small as to render t ime gradin

unre l iab le . R ecourse m ust th erefore be ma de to grading by the num ber oopera t ions to lock out , as in F ig. 15 .3 .3 J . The rec loser mo st remo te f rom the supplsource is set to lock open af ter two or three unsuccessful reclose operat ions, whilsthe one nearer the source is arranged to com plet e a full sequen ce of four t ripbefore locking open. Th us , i f the cu rrent to a fault bey ond the mo re rem oterec loser is of such va lue as to cause s im ul taneou s op era t ion of both rec losers, thmore remote rec loser wi l l lock open to i so la te the faul t whi ls t supply to the hea l thsec t ion i s mainta ined through the rec loser nearer the source .

W hen a recloser with an earth -faul t t r ipping featu re is em plo ye d in ser ies wi th rec loser not so f i t ted , as may occur due to p iecemeal grow th of a ne tw ork , it mu sbe bo rne in mind tha t a ll ear th - fauh t r ipping opera t ions are ins tantaneo us . Threc loser wi th the ear th- faul t fea ture must therefore be employed as the moreremote uni t of the pa i r ; o therwise fa lse indica t ion of faul t pos i t ion may be g ivens ince the magni tude of the faul t cur rent may be h igh enough to opera te the ear thfaul t fea ture but insuff ic ient to cause overc urrent t r ipping of the rec loser not s

f i t ted .

15 .3 .4 Subs ta t ion c i rcui t breakers

The loca t ion on a rura l sys tem of the pole-m ounte d rec los ing devices previouslym ent io ned ma y be governed by cons idera t ions of normal cu rrent ra t ing andbreaking capaci ty, and these may demand tha t the rec loser be ins ta l led somedis tance f rom the pr im ary subs ta t ion . In such cases, the sec t ion of line be tw een thsubs ta t ion and the device i s wi thout the benef i t of h igh speed automat ic rec los ingand faul ts on tha t sec t ion , w heth er pers is ten t or n onpe rs is ten t , w i ll a ffec t thecon t inu i ty of supply to the rema inder of the l ine . The advantage of providingautomat ic rec los ing of the subs ta t ion c i rcui t breaker to minimise in ter rupt ions osupply on th is sec t ion wi l l therefore be apparent .

Control relays for circui t breaker reclosing schemes have been avai lable tbr anul nbe r o f years . The s implest o f these provides a single closing impulse a f ter

p rede te rmined t ime de lay, wh i ls t ano the r to rm pe rmi t s a preselec ted nu m ber orec losures wi th se lec ted t ime in terva ls be tw een them . This la t te r re lay, know n a



270 The appl ica t ion o f pro tect ion to rura l d is t r ibut ion sys tems

which are spaced a number of tapped holes. Into selected holes are screwed pinwhich project through the disc, and as this is rotated the project ions in turn actuaa contact to energise the closing circuit , the t ime between reclosures ( the deat ime) being determ ined by the spacing of the p ins. I f the c i rcui t breaker remain

closed after a reclosure, the relay is reset by the energising of the m oto r reversfield th ro ug h an auxil iary switch on th e c ircuit break er. Th e f inal pin has a somewh at longer projec t ion than the others and if the reclosure ini t iated b y it unsuccessful, con t inued rota t ion of the d isc causes it to op era te a locko ut contacwhich disconnects the supply to the foward f ie ld of the dr iv ing motor, prevent infurther reclosure. The electr ical circuits of this type of relay are indicated in Fig1 5 . 3 . 4 A .

The t ime in terval between t r ipping and reclosure genera lly em ployed w i th thabove re lays is comparat ively long and ma y vary f rom 10 to 120 s . The num ber oreclosures pe rmit ted is depe nden t on the nature of the sys tem protectedand may be two or three where i t consists almost entirely of overhead l ines, ansom ewhat less where the p ropor t ion of underground cable is appreciable . Ope rat inexperience indicates that with the use of these relays, in conjunction with inverst ime protec t ion, successful rec losure occurs in appro xim ate ly three out of foufaul ts , the number depending on the speed of opera t ion of the protect ion.

The improved qual i ty of rura l suppl ies obta ined by the use of the pole-mountedautomatic circuit recloser indicated the advantages of high speed fault clearance anthe desirabil i ty of providing a similar operat ing sequence on substat ion circuibreakers . This course removes the normal current and breaking capaci ty l imi ta t ionof the recloser and exten ds the reclosing facil it ies to the whole of the protec teline.

k

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T h e a p p l ic a t i o n o f p r o t e c t i o n t o r u r a l d i s t r i b u t i o n s y s te m s 271

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F i g . 1 5 . 3 . 4 B B a s i c e c lo s i n g s c h e m e f o r s u b s t a t i o n c i r c u i t b r e a k e r

To provide this feature, the circuit breaker is f i t ted w ith a com bina tion of highspeed and inverse-time pro tection , each having app rox im ately the same rang e of set-t ings. O n the occurrence o f a fault , the high s peed protection operates to tr ip the

circuit b reaker, w hich is reclosed by an aux ill iary t iming relay after a brie f t imeinterval. The tripping circuits of the high speed relays are then held open , either byan aux iliary sw itch on the discharged spring closing mecl+anism or by an aux iliaryrelay , so that if the fault persists after reclosure, F urther trippin g o f the circuitbreaker is accomplished by i .d. m .t , relays. The control schem e is arranged to lockope n the b reak er on the operatio n of the inv erse tinm relays, bu t if the reclosureis successful, the relays reset to prov ide a full ope rating cycle on subsequent faults.The basic circuit arrangem ent of a simple spring closing schem e is shown in Fig .

15.3 .4 B and the operat ing sequence diagram in Fig. 15.3 .4 C.

i " a u l t H i g h s p e e d R e t l , , , , e , I I ) M I i r ipi n c e p t i ( ) n t r i p

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272 The appl ica tion o f prote ct ion to rura l d is t r ibut ion systems

This sequence is known as repetitive single-shot reclosing and was adopted tlimit the number of breaking operations at the higher source substation fault leveto a figure within the capability of standard circuit breakers. Subsequent investigations and service experience have show n tha t b reakers curr ently available in thcountry are capable of carrying out many more breaking operations beformaintenan ce b ecom es necessary than those required by cert ificat ion tests , part icularly at the lower duties.

In the early 1960s, a num ber of Area Boards introduced individual autoreclosing schemes for use with ground mounted circuit breakers but in 1966 report recommending standard reclosing schemes was issued by the ElectricityCouncil. The schemes, all incorporating the single shot sequence with optional shor

or long dead times were for:

( i )

(ii)

( i i i )

(iv)

Single-shot reclosing for circuit breakers having hand charged spring mechanisms.Repetitive single shot reclosing for breakers with motor wound spring closingmechanisms. This scheme utilises an auxiliary switch on the spring chargingmechanismto use the rewinding t ime as the reclaim t ime of the sequence.Repetitive single-shot reclosing for circuit breakers with solenoid closingmechanisms. In this case the dead and reclaim times are determined by atiming relay.Repeti t ive single shot reclosing for breakers with solenoid or motor woundspring mechanisms and incorporating precumulative lockout and cumulativelocko ut alarm features.

In all these schemes reclosing is initiated only by the operation of the high speedprotection thus ensuring that it takes place only after a fault and not followinmanual tr ipping of the breaker. The repeti t ive schemes incorporate cumulativoperation counting relays which l imit the number of tr ipping operations on faultto an extent determined by the m aintenance requirements of the breaker. When thpreset number of operations has been achieved, the inverse t ime protection onlis effective and further reclosing is inhibited.

The schematic diagram for scheme (iv) is shown in Fig. 15.3.4D.

A subsequent report concerning protection against lightning discussed the useof automatic circuit reclosing in this context and recommended the adoption olong dead times and short reclaim times, the figures quoted being 10-15s and 5srespectively. These times enable the system to recover fully from a fault andminimise the possibility of unnecessary lockouts on repetitive faults. This reporalso suggested that, with improved knowledge of circuit breaker capabilities, multishot reclosing schemes might be employed with advantage. Modifications of theoriginal standard schemes have therefore been made to permit the optional use oan addit ional counting relay to extend the operating sequence up to a maximum othree reclosures (i e four tripping operations) on any one fault



T h e a p p l i c a t i o no r o t e c t io n t o r u r a l d i s t r i b u t io n s y s t e m s 2 7 3

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I I I I a a rm r e l a y I C u m u l a t i v e . p e r. P r e - l o c k o u tC o n tr o l s w it c h ~ r e l y a,I . . . . la y / ~ _

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M x Jt or s u p p l y M o t o r s u p p l y( m a y h e d . c . ) ( m a y b e d . c . )

M e c h a n i c al c o u n t e r t o r e c o r d t o t a l n u m b e r o f o p e n i n g o p e r a t io n s .A u x i l i a r y s w i t c h d o n c l o s i n g s p r in g m e c h an i. ~m is ( ) p e n w h e n s p r i n g is c h a r g e d . A u x i l i a r y s w i t c h e o n c l o s i n g s p r in g m e c h a

c l o s e d w h e n s p r i n g i s c h a r g e d ." r im i n g r e l a y t o h a v e a n o r m a l l y o p e n t i m e d e l a y c o n t a c t A c l o s i n g a f t e r a p r e s e t t i m e d e l a y o f 0 . 5 - 1 .0 s o r 5 - 2 0 s l ', )r a p e r

n o t l e s s t h a n 0 . 6s a n d n o t g r e a t e r t h a n 2 s a n d a n o r m a l l y o p e n t i m e d e l a y c ~ m t a c t B c l o s in g a f t e r a p r e s e t t im e d ~ : l ay o f 3 0 -o r 3 . ~ - 6 0 s . T h e r e m a i n i n g c o n t a c t s c) p e ra te i n s t a n t a n e o u s l y.

A u x i l i a r y r e l a y m u s t o p e r a t e in n o t m o r e t h a n 8 m s . T h e n o r m a l l y ( ~ p e n c ( m t a c t s m u .~ t h a v e a l O O m s d e l a y o n d r o p c ~ ff w h er e l a y i s s h o r t e d .

N o r m a l l y < )p e n c o n t a c t o n t h e c u m u l a t i v e o p e r a t i () n - c ~ u n t i n g r e l a y m u s t n o tc l o s e in l e ss th an I S ins .R e c l o s i n g r e l a y in t e r n a l - b r i d g i n g c o n t a c t is c h t s e d ~ h e n t h e r e l a y a ss e m b l y is w i t h d r a w n fr o m) t h e c a se .

F i g . 1 5 .3 . 4 D C i r c u i t d i a g r a m o f r e p e t i t i v e s i n g l e . s h o t r e c l o s i n g s c h e m e f o r c i r c u i t b r e a k e r

w i t h m o t o r - w o u n d s p r i n g o r s o l e n o i d - c l o s i n g m e c h a n i s m a n d h a v i n g p r e l o c k o u t



2 7 4 T h e a p p l i c a t io n o f p r o t e c t i o n t o r u r a l d i s t r i b u t io n s y s te m s

facil i t ies is no w available in solid-state form . T hese relays afford high speed pro-tect ion with character is t ics shaped to prevent operat ion by t ransformer magnet is inginrush currents together with a choice of various inverse t ime overcurrent and earthfault characteristics by means of interchangeable plug-in modules. Sensit ive earth

fault options are also available.Flexible control of the auto reclosing sequence and protect ion operat ion

select ion is provided by changeover l inks m oun ted on th e face of the relay. U p to3 or 4 reclosures per fault inciden t m ay b e obta ined and the instan tan eousprotect ion can be inhibi ted for any t r ip operat ion of a mult iple-reclose sequence.Both dead and reclaim t imes are adjustable but whils t the former may differ foreach reclosure, the preset reclaim time is applicable following all reclosures. An

operations counter and circuit breaker inspection and lockout alarms are provided.In addition to greater flexibil i ty in application, the solid-state protection andreclosing control re lays have the meri t of occupying l i t t le control panel space com-pared to their electromagnetic counterparts.

The suitabili ty of a circuit breaker for autoreclosing duties depends upon itstotal break t im e, the m echanical s tabi l ity of i ts operat ing mechan ism and, in thecase ofoil circuit breakers, the effect of any residual gas pressure in the tank whensuccessive breaking operat ions occur within a very short t im e. In add i t ion, taking an

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The appl icat ion of protect ion to rura l d is t r ibut ion sys tem s 275

overal l v iew, the breaker ' s abi l i ty to perform a number of operat ions before requir ing maintenance m ust be taken in to account .

The vacuum interru pter is ideally suited to this du ty . This device possesses higspeed of fault clearance, the current being interrupted at the f irst current zero aft

t r ip in i t ia tion unless opening of the contacts takes place wi thin 3 ms of that p oiwhen clearance is effected at the next zero point . Such a characterist ic permits h igh degree o f co-ordinat ion betw een the vacuum breaker and any associa ted fuseThis is i l lustrated in Fig. 15.3.4E which shows the co-ordination between both oand vacuum circuit breakers and fuses. The fuse characterist ics have been plotted 75% o f the publ ished data to a llow for deter iora t ion of the fuse l ink and the in tesection of the fuse and circuit breaker characterist ics determines the l imit incurrent beyond which co-ordinat ion wi l l not be obta ined.

I t is not possible to em plo y sectionalisers in series w ith su bstat ion circubreakers having repetitive single shot reclosing facilit ies. Since the sectionaliser hno breaking capaci ty and must open dur ing a dead t ime in terval in the operat insequence, it would isolate the line section before the single reclosure irrespective whether the fault was of a transient or permanent nature. Reclosers having ahigh-speed tr ipping sequences m ay be used in the role o f sectionalisers in cojun ct ion wi th substa t ion b reakers , however. W hilst the f irst t r ip o f bo th breaker an

recloser may occur simultaneously, the subsequent high-speed tr ips of the reclosewill ensure discrimin ation w ith the inverse t ime prot ect io n of the circuit breakand final clearance by the recloser.

The applicat ion of mult ishot reclosing schemes to substat ion circuit breakerperm its the use of associated sectionalisers at a lower cost th an tha t o f reclosers.

In terrup t ions of su pply on l ines protected by th is form of reclosing are reduceto a level s imilar to tha t obta ined by the use of po le-m oun ted reclosers and circubreaker m aintenance requiremen ts have been fo und to be l ight .

The provis ion of any form of automat ic reclos ing on a c i rcui t breaker requiresome form of pow er c los ing m echanism. I f fu l l advan tage is to be der ived f rom thfeature , th is m echa nism m ust be of the solenoid , m oto r wou nd spring or o ther selcharging type.

15 .4 Sensitive earth-fault p rotectio n

I t has been m ent ione d earl ier that ear th-fault currents on rura l netw orks may be overy low m agnitud e owing to long line lengths, the use of neu tral earthing resistorand diff icult earthing co ndit ion s. Su ch circum stances arise w hen an overheaconductor breaks and fal ls on ground of high resist ivi ty or across a hedge ohays tack .

The 1947 edi t ion o f the Overhead Line R egulat ions required that such

conductor should be rendered dead but compliance wi th th is requirement was isome cases not possible since the fault current under such a condit ion might be les



276 The appl ica tion o f protec t ion to rura l d is t r ibut ion sys tems

requ irem ent is no t inc luded in the current (19 70 ) edi t ion of the R egula t ions but v iew of the impl ica t ions o f a b roken conduc tor r emain ing a l ive , some means au tom at ic d isconn ect ion is c learly desi rable .

To avoid a dang erous con di t ion ar ising on bro ken con du ctor faul ts , a sensi tiv

form of ear th-faul t protec t ion , responsive to pr imary ear th-faul t currents of thorder of 5-10 A, i s required . Such values represent the min im um set t ing wh ich cabe sa t i s fac to r i ly employed , s ince normal sys tem unba lance cur ren t s , fo r exampdue to the uneq ual l ine to e ar th capaci tance cu rrents of varying lengths of singphase spur l ines, ma y ap proac h the lower value. Even so th is sens i tiv ity m ay binsuff ic ient to detec t broken conductor faul ts in which the conductor on the s idof the break rem ote f rom the source fa lls to the gro und . The c on du ctor in thins tance remains energised v ia the h igh-vol tage windings of any t ransformers conec ted to the line beyo nd the po in t o f f au l t and the fau l t cu r ren t ma y be l imi ted a very low value by the impedance of these windings . I t s magni tude wi l l bde te rmined by the number o f t r ans formers connec ted and the load connec ted tthem, but in the ext reme case of t ransformers on open c i rcui t wi l l cons is t of thmag ni t is ing currents o nly .

W hilst possessing a low faul t se t t ing , the com po nen ts m ust be suff ic ient ly robuto wi ths tand the e ffec ts o f muc h heav ie r f au lt cu r ren ts which ma y occur f rom t im

to t ime .W ith a re lay w hich opera tes a t such low pr ima ry currents , the effec t of thmagnet is ing current drawn by current t ransformers in id le shunt has an appreciablbear ing on the value of pr imary current required to cause opera t ion of the re lay.

For this reason, therefore, early instal lat ions ut i l ised a moving-coil relay elemenenergised by a core balance current t ransformer mounted on the cable terminat ionThe ra t io of such a curren t transfo rm er m ay be indepen den t o f the c i rcuit ra tinand faul t level and can be ar ranged to provide the op t im um set t ing , genera l ly of thorder of 5 A.

When applying such a scheme to exis t ing swi tchgear ins ta l la t ions , howeverconsiderable expense and inconvenience m ay be involved as it m ay be necessarto b reak down the cab le end box in o rder to mount the core ba lance t r ans formeand to f it the insula ted g lands essentia l for such a schem e, i f the current t ransform eis m oun ted ex te rna l ly.

The deve lopm ent o f r e lays having e x t rem ely low burdens , o f the o rder o

0431 VA or less , has enabled sensi t ive earth-fault protect ion to be provided bconnect ion of the re lay in the res idual c i rcui t of the current t ransformers employefor the convent ional ear th-faul t protec t ion . Such re lays of the polar ised e lementype enable se t tings as low as 2% or 3 % of the pr im ary cu rrent ra t ing to be ob ta inewh ilst the corresp ond ing stat ic version p rovides a set t ing range of 1% to 16% othe ra ted current .

H igh-speed c learance of the low fault currents in que s t ion is unnece ssary anindeed m ay be und es i rable , since the h igh sens it iv ity of the pro tec t ion m ight thegive r ise to ope ra t ion on m om en tary sys tem abno rm al i t ies . I t is usual , therefo reto ap ply a t ime- lag to the t r ipping o pera t ion and values of app rox im ate ly 1



The appl ica tion of protec t ion to rura l d is t r ibut ion sys tem s 277

tect ion and reclosers having delayed tr ipping and earth-fault features is shown Fig. 15.4A.

The operat ion of sensi t ive earth-fault protect ion applied to an overhead l ine f requent ly an indicat ion of the exis tence of a dangerous condi t ion on the c i rcui

e .g . a broken conductor ly ing on the ground. For th is reason, some ear l ier autoreclosing schemes are arranged to inhibit the reclosing feature after trippininit iated by sensi t ive earth-fault protect ion. Experience has shown however that thcond i t ion to wh ich th is form of protect ion is des igned to respond ar ises f requen tdur ing normal operat ion of an overhead sys tem and some under takings now emploautomatic reclosing in such circumstances. A reasonable degree of successful re

1 0 0

R a n g e of ea r th f a u l tc u r r e n t s c l e a r e d b ys e n s i t iv e E / I " re l a y s

10

F a u l t c u r r e n t s c l e a r e d b y

r e c lo s e r w i t h e a r t h f a u l tt r i p p i n g f e a t u re

,., 1__~ Fau lt cur re nt sLv cleared by

I recloser wi thips

Delayed f .overcur ren t t r ip

0.1

~ _ _ m a x im u m ./ / / / / / / / / / / / . i / 7

Earth faul t t r ip ~ m i n i m u m

.01 /Pick up currentof sensi tiveear th faul t re lay

10

Cur r en t - Amps

I00 1000



278 The appl ica t ion o f pro tect ion to rura l d is t r ibut ion sys tems

closing has been achieved and the more recent schemes include an option foselect ion of au toreclosing fol lowing the op erat io n o f sensi tive earth-faupro tec t ion .

Sectionalising devices on overhead l ines protected by sensi t ive earth-faul

protect ion should be of the type which breaks a l l three phases s imul taneouslyotherwise the unbalance caused by the sequent ia l opening of individual phases maexceed the set t ime delay and cause the relay to operate. In some cases, switcheare provided in the tr ipping circuit to render the protect ion inoperat ive whilscarrying ou t swi tching operat ions .

15 5 Are-suppression coils

Pr ior to the in t roduct ion of the unear thed crossarm form of overhead l ine const ruct ion, faul ts on rura l sys tems were predominant ly to ear th . To minimisinterruptions due to these earth faults some systems were designed to operatwi th the neutra l point unear thed, but i t was found that under faul t condi t ionthe capacitance currents between the healthy phases and earth were sufficiento permit arcing at the fault posi t ion. The capacitance currents were invariablof low magni tude and consequent ly were di ff icul t to detect by the normaprotect ion. Fur thermore the arc ing could be prolonged and dangerous overvol tagemight occur. Such sys tems are not now permit ted by s ta tute .

To overcome this , the arc-suppression or Petersen coil was introduced. Adescribed in Chapter 1, this takes the form of a reactor connected between thsystem neutra l point and ear th , the coi l impedance being such that under ear tfault con dit ions the capacitance current to earth o f the sound phases is neutral iseby the inductive curren t of the coil . The system l ine-to-earth capacitance wil l, o

course, vary with the amount of l ine in service at any one t ime, and tappings mustherefore be provided on the coil to ca ter for d i fferent operat ing con di t ions . W ita l imited number of tappings, however, i t is vir tual ly impossible to neutral isexact ly the capaci tance current for every sys tem combinat ion, and in pract ice thcurren t at the fault is the difference betw een the system capacitance and coicurrents .

The difference current is normally very small and can be tolerated by thesystem. If the coil is continuously rated, an earth fault may be al lowed to persisfor a period sufficient to enable switching operat ions to be carried out to isolatthe faulty sect ion with the minimum loss of supplies. This faci l i ty is invaluablwhere no standby supplies exist , but suffers from some disadvantages. Under earthfault condi t ions the vol tages to ear th of the sound phases of the whole sys tem arraised to the full l ine-to-line voltage and this increase may cause insulationbreakdown on these phases a t points on other l ines of the sys tem, as indicated iFig. 15.5.A. The resultant condit ion is termed a 'cross-country ' fault and

constitutes a phase-to-phase fault against which the arc-suppression coil is ineffect ive. For this reason, the transfer of exist ing networks to arc-suppression coi



T h e a p p l i c a t io n o f p r o t e c t io n to rura l d is t r ib ut ion systems 279

S e c o n d a r yw i n d i n g o fsupp ly t r ans fo rmer

C o n t i n u o u s l yr a t e d a r c

s u p p r e s s i o nco i l

R Y B

Il llil

1 ,-,

- Feeder 1

_ ~ S u b s e q u e n tf lashover-:- on phase B

%

' ,, , C ross coun t ry fau l t

~ Orig ina l fau l ton phase R

l. 'eeder 2

Fig . 15 .5A C r o s s ~ o u n t r y f a u l ts o n s y s t em p r o t e c t e d b y a n a rc -s u p p r es s io n coil

A u t o m a t i cs w i t c h

Ear th ingresis tor

l LS h o r t t i m e r a t e da r c s u p p r e s s i o n c o i l

Fig . 15 .5B A u t o m a t i c s h o r t ~ : i r c u i t in g s c h e m e f o r s h o r t t i m e r a t e d a r c -s u p p r es s io ncoil

insulat ion weak spots occurr ing during a faul t is thereby reduced and their locat ionfaci l i ta ted.

Where alternative supplies are available or the persistence of the voltage increasecannot be tolerated, a short t ime rated coi l f i t ted with automatic short-circui t ingequip m ent , as in F ig . 15 .5 .B , m ay b e emp loyed . W i th th is a r rangement , i f the fau ltcond i t ion persists for more than a few second s, the sy stem neutral point isautom atica l ly con necte d to ear th , e i ther direct or through a res is tor in paral le l w iththe coi l , thus permit t ing the passage of suff ic ient ear th-faul t current to be detected

and the l ine isolated by the normal feeder protect ion. In some instal la t ions, theshort c i rcui t ap pl ied to the coi l is autom atical ly remov ed af ter a t ime-lag suff ic ient

to perm it th e isolat ion of the faul ty l ine. T his ensures that the system is w ithou tthe benef i t o f the a rc-suppress ion co i l for the minimum of t ime but may lead to the



280 The appl ica tion o f protec t ion to rura l d is t r ibut ion systems

isolated faulty feeder is large. To o vercom e this , coils w ith autom atic tap changinmay be em ploy ed, bu t th is natural ly adds to the cost of the coil .

As an arc-suppression coil is effect ive for a fault anywhere on the completsystem, i t is desirable to provide indicat ion of the faulty feeder when the coil

brou ght in to op erat ion. W ith autom at ic shor t c i rcuit ing, the t r ipping of the feedcircuit breaker is a clear indication. Where a continuously rated coil is usedindication of a faul ty feeder may be obta ined by the use of a wa t tmetr ic type re lasufficiently sensitive to detect the small difference currents referred to earlier ithis section. To energise such relays, however, i t is necessary to provide a voltagtransformer on each feeder circuit breaker and the arrangement is therefore cost ly

The a djus tm ent or tun ing of a coil in relat ion to the pro tected s ystem is carrieout by applying an earth fault to the system and varying the coil tapping to givm axim um voltage across the coil wi th the norm al amo unt of sys tem connected. T hreactance is then reduced slightly to ensure that a reasonable balance will exiswh en a section of the netw ork is removed for any reason. Some degree of mis tuninis permissible without seriously affect ing system operat ion, and for 11 kV systemthis tolerance is sufficient to al low sections of networks to be transferred froother substa t ions w hen necessary to m ainta in suppl ies . For sys tems where f requevariat ions in length of l ine connected occur, automatic on-load tuning equipment

available but is normally not considered economic.When viewed in the l ight of the amount of l ine which i t may protect , an arc

suppression coil is a comparatively cheap i tem of equipment. This advantage mabe somewhat offse t by the cost of s t rengthening insula t ion weak spots and thpossible transfer of single phase spur lines or line transposition in order to achieva phase balance of capacitance currents . I ts operat ion is , however, independent ofault current levels and the co-ordination of exist ing protect ion is not affected bits use. Stat is t ics indicate that a reduction of some 40% in the number of supplinterrupt ions com pared wi th fuse protected sys tems is achieved on sys temprotected by arc-suppression coils , al though a greater reduction might be expecteon older l ines with a greater number of points having earthed metalwork.

15 .6 Performa nce/cost com parison of protective equipm ent for rural system s

I t wil l be appreciated from the preceding sections of the chapter that in thappl ication of protec t ion to rura l sys tems, the pr imary funct ion o f the c learance ofaults is all ied to the rapid resto rat ion of supplies. In com paring the perform ance ovarious forms of protec t ion available, therefore , some cognizance must be taken othis factor. U nfo rtun atel y, in such an assessm ent, an infal lible cri terion is diff icuto f ind and the influence of part icular local condit ions must be weighed.

The qua l i ty of rural supplies may be judged in terms o f

( a )(b)

Interrupt ion s per 100 km o f l ine per annu m .The p roduc t o f the num ber o f in terrupt ions and the km of line affected pe



The appl icat ion o f protect ion to rura l d is t r ibut ion sys tem s 281

( c )(d)

Tota l consumer h ours los t per annu m .Consum er hours los t per consumer pe r ann um .

None of these takes in to account a l l the fac tors which cause i r r i ta t ion or inconvenience to the consumer, such as t ime of day or a spate of in ter rupt ions of supplto a pa r t icu la r consu me r, bu t tha t o f in te r rup t ion pe r 100 km per annu m i s usua llmost readi ly avai lable and is therefore most commonly used.

The cos t o f each fo rm of p ro tec t ion mus t en te r in to any compar i son which imade be tween them, and s ince the amount o f ne twork p ro tec ted by each fo rmvaries, the cost per ki lom etre o f line prote cted m ay serve as a useful basis. This cosshould take in to account opera t ing cos ts such as maintenance charges and cos t ofuse replacem ent , in add i t ion to capi ta l cos t , and m ay wel l be expressed in termof to ta l annu al charges.

Opera t ing exper ience wi th the types of protec t ion covered by th is Chapteindica te tha t the performance f igures shown in Table 15.6A may be expected .

Table 15 6A Performance figures for various types of pro tec tion

Type o f p ro t ec t ion

H.V. fusesArc-suppression coilsPole-mounted rec loserSu bsta t ion reclosing circui t Periodic reclosingbrea ker Single-shot high-speed reclosing

In te r rup t ions pe r

10 0 km p e r annum

3116

273

The signif icance of these f igures should be viewed in the l ight of the fol lowinobservat ions.

(a) Fuses: Individually these ca ll for on ly a small capita l ou t lay , but for adequ atepro tec t ion of a ne tw ork a large nu m be r m ay be requ ired . The inc idence of a faurequires the replac em ent o f the fuse e lem ent and th is may involve h igh labour an

t ransp or t cos ts. Deter iora t ion w hich occurs in service may cause unnecessary supplfailures.

(b) Arc-su ppression coils:A single coi l wil l protect the whole of the system towh ich i t is conn ected and i t is therefore low in terms o f capi ta l cos t per mi le of l inIn add i t ion , very l it t le m aintenan ce is required . I t i s, how ever, e ffec t ive on ly agains ing le phase - to -ea r th fau l t s , which amount to some 50% of the to ta l number o

faul ts on overhead l ines of unear thed const ruct ion , and, in addi t ion , requires thathe l ine insula tion b e in sound con di t ion in order to w i ths tand t rans ient ove




of remote alarm system is desirable when the coil is instal led at an unattendesubsta t ion.

( c ) Po le -mo un ted r ec loser s:Each main l ine of the network requires a separate

recloser for prote ct io n and , as the capital cost o f each instal lation is some 4 0% the cost o f an arc-suppression co il , the tota l cost m ay be higher than th at o f a coprotect ion sys tem. The high degree of protect ion is provided only on that por t ioof the l ine on the s ide of the recloser rem ote f rom the source of supply, and ra tecurrent and breaking capaci ty l imi ta tions m ay res t ric t the exten t o f th is por t ioN ationa lly, the average length of line pro tected per recloser instal led is o f the o rdof 12 ki lometres . The cost of per iodical maintenance must be taken in to accoun

but , due to the reduced number of supply in terrupt ions wi th the use of rec losersystem operat ing costs are greatly reduced.

(d ) Su bs ta t ion r ec los ing c i r cu i t b reaker s :The cost o f providing autom at icreclosing features o n sub stat ion circuit break ers is that of addit ional relay equim ent on ly and is general ly bu t a small percentage of the cost of the circuit breakitself . There is meri t , too, in the fact that such features may also be readily applieto exist ing switchgear. High-speed tr ipping may, with considerable advantage, alsbe provided at low cost .

The use of reclosing circuit breakers affords the benefi ts of automatirestorat io n to the w hole of the associated feeder in contras t to restr ict ions imposeby th e l imited break ing capacity of the p ole-m oun ted recloser. The average length l ine protected by substat ion reclosing circuit breakers is nat ionally some 20 km pcircuit break er.

Rapid fault clearance combined with automatic reclosing provides the lowesincidence of supply in terrupt ions and, because of i t s performance, low cost another advantages, the provision of this feature on substat ion circuit breakers haconsiderable advantage.

Mult iple reclosure sequences offer some improvement in system performanccompared with single-shot schemes and also permit the use of sect ionalisers to l imthe extent of supplies interrupted. The cost of the sect ionaliser at present availablis how ever some two-thi rds of that of a rec loser and largely on th is accoun t thehave not been w idely used in th is coun try.

Fault passage indicators, whilst greatly facilitating fault location, are generalltoo expensive to permit their permanent instal lat ion in the profusion required tderive their ful l benefi t whilst the portable form involves delay and expense iset t ing out and collect ion for each fault location.

The el imination of h.v. fuses from networks ut i l is ing automatic reclosing offereconom ic and o perat ional advantages .

15.7 Prim ary netw ork s in rural areas



The appl ica t ion of prote ct ion to rura l d is t r ibut ion sys tem s 28 3

pr imary networks in rura l areas . Ring main schemes are commonly employed tafford al ternat ive supplies and use is also made of feeder t ransformers and teefeeders. Overhead l ines are general ly supported on wood poles and switchgear kept to a minimum, single circuit breaker substat ions being common pract ice eve

thou gh involving some loss of ope rat iona l f lexibi l ity.Pilot channels for protect ion are seldom available. Sometimes combined with a

ear th ing conductor, they may be carr ied on the overhead l ine suppor ts but succircuits are expensive and l iable to faults caused by cl imatic condit ions and otheexternal influences. In addit ion, i t is necessary to guard against the effects oinduc t ion f rom the p r imary conduc tors .

On some 3 3 kV sys tems, par t icular ly those having a predom inance of steel towlines, arc-suppression coils are sometimes used for protect ion against earth faultW ith the increas ing use of wo od pole unear thed ty pe const ruct ion, however, theuse is rarely extended to new systems.

On simple r ing main systems, current and t ime graded direct ional overcurrenand ear th-faul t protect ion has been employed for many years . Such protect ionhowever, has long operat ing t imes for some types of faults and is l imited in applict ion to r ing mains having not more than f ive substat ions. These l imitat ions togethewith a general lack of pi lot channels for unit schemes, have provided the incentiv

for the developm ent of the cheaper forms of d is tance protect ion incorporat inswitched measuring elements, as described in Chapter 9.

The protect ion of feeder t ransformers has been discussed in Chapter 12 but orural systems overall schemes utilising pilots are rarely used for the reasons stateabove. Ins tead s impler forms of protect ion are usual ly employed. One such schemfor a single circuit is show n in Fig. 15.7A . A t the sou rce end of the line the circubrea ker is equipp ed with instanta neou s ea rth-fault and inverse defini te m inim umtime overcurrent relays, the lat ter incorporat ing high-set instantaneous elementThe rapid clearance of l ine faults by the use of relays w itho ut t ime lags minimisedamage at the fault point and con sequ ently increases the possibi l ity of successfreclosure of the circuit breaker.

• I . . . . . . "1I , I

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H . V. L . V. H . V.

N.P.D. r e l a y N.P.D. r e l a y

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(a) Vo l t a g e t r a n s f o r m e r e n e r g i s e d

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I I L v

H.V. L.V.

~ , •

T I _ _ ~ nser hushing

~ D. relay

,,g,

i l

T~ " T " b u s h i n g s

~ . P. D . r e l a y

Z ~ - - .°

Fig . 15 .7B

( b ) C o n d e n s e r h u s h i n g e n e r g i s e d

N e u t r a l p o i n t d i s p la c e m e n t p r o t e c t i o n

Faults on the h.v. winding of the t ransformer are detected by the protect ion atthe source end w hils t res t ricted earth-faul t pro tect ion is app l ied to the second arywinding. In addi t ion, overcurrent and ear th-faul t protect ion is f i t ted on thesecondary side to guard against busbar faults or the failure of circuit breakers toclear faul ts on the outgoing feeders . Protect ion against inter turn and other internalfaul ts which cause gas evolut ion is provided by a Buchholz gas and oi l re lay.

As an al ternat ive the t ransformer may be protected by an overal l scheme, butsuch an arrangement requires current t ransformers on the h.v. s ide, usual ly accom -modated in the terminal bushing turrets .

Intertripping o f th e source circuit breaker is necessary on the occurrence of at ransformer faul t , and this is accom plished by a fault throw ing switch conn ectedbe tw een one phase and ear th or, in the case of arc-suppression coi l system s,be tw een tw o phases . Th e fau l t th rowing swi tch is c losed on the opera t ion o f thetransformer protect ion by an electr ical ly re leased m anua l ly charged spr ing m ech-anism and wil l ensure the t r ipping of the remote h.v. c i rcui t breaker.

Where duplicate feeder transformers afford supplies to a substation, similarschemes of protect ion to those used for s ingle uni ts may be appl ied. An ear th faul t



T h e a p p l ic a t i o n o f p r o t e c t i o n t o r u r a l d i s t r i b u t io n s y s t e m s 2 8 5

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; - - - - I . . . . . ' - - ~I I, : . . . . . ~I

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Fig. 15.7C S c h e m e o f p r o t e c t i o n f o r d u p l ic a t e f e e d e r t r a n s f o r m e r s

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F i g . 1 5 . 7 D

m

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S in g l e e l e m e n t d i r e c t io n a l o v e r c u r r e n t r e l a y f o r p r o t e c t i o n o f d u p l ic a t e f e e d e rt r a n s f o r m e r

tr ip but the fault will st i l l be back-fed through the transformer. Since the neutralpoint of the t ransformer h.v. winding is not usual ly ear thed, the vol tage

to ear th o f th e tw o heal th y l ines wil l r ise to the ful l l ine- to- l ine vol tage. Thiscondit ion may be detected, and the t ransformer I .e . c i rcui t breaker t r ipped,



286 The applicat ion o f pro tec t ion to rural distr ibution systems

Protectiverelay contact

Timing or o.c.b. Auto /non-autorelay aux. switch switch

] Au x. relay ~ ~ ~ Clis: [(hand reset)

contactor ~ t gr s w i t c hcoil

Trip

F i g . 1 5 . 7 E Simple non repe titive single-shot reclosing scheme

of protect ion uses a relay connected to respond to a residual voltage, the relabeing energised by a vol tage t ransformer or condenser bushings , the la t ter methobeing somewhat cheaper. The arrangements for d i ffer ing t ransformer connect ionare shown in Fig. 15.7B. A t ime-lag relay having a set t ing of approximately 5s normal ly employed in conjunct ion wi th the neutra l d isplacement detect ing re lay prevent operat ion when faul ts occur on other sect ions of the network. A typicscheme for the protect ion of duplicate feeder transformers is shown in Fig. 15.7C.

A noth er form of p rotect ion wh ich has been successfully emp loyed experm ental ly to br ing abo ut t r ipping under the cond i t ion descr ibed above uses sensitive reverse power relay to detect the reverse magnetising current of thtransfo rme r. W ith m od ern transfo rme rs ut i lis ing cold rol led steel cores, how evethe loss and ma gnetising curr ents are very small and special relays and cu rren

transformers m ay be necessary. C om pensat ion for phase angle errors on the currentransformers may be required and is provided as an integral part of the relay, thfeature being cut-out dur ing heavy through faul t currents . The appl icat ion of thscheme is general ly l imited to unteed paral lel feeder transformers since tees intrduce the poss ibi li ty o f incorrect o perat ion .

Protect ion against the back-feeding of phase-to-phase faults occurring on thl ines of dupl icate feeder t ransformers may be afforded by di rect ional overcurrenprotect ion instal led on the lower voltage side of the transformer. For this purpos

the 3 0 ° relay con nec tion is u nsatisfac tory and the 90 ° (or 90 ° minu s 45 ° ) conection should be used. Where the transformers are delta/star connected, a s inglelement relay, connected as shown in Fig. 15.7D, may be employed for thipurpo se since a phase/phase fault on the d elta side will pro duc e a 2:1:1 currendistr ibution on the secondary side. (See Chapter 12).

The principles of auto m atic circuit reclosing discussed in Section 15.3.1 applequal ly to rura l 33 kV and 66 kV systems. Feeder c i rcui t breakers on such sys tem

are therefore general ly f i t ted with reclosing faci l i t ies , usually of the nonrepeti t ivsingle shot type, that is the type in which only one reclosing operat ion is providel k f h i i b k i b i i F hi



T h e a p p l i c a t i o n o f p r o t e c t i o n t o r u r a l d i s t r i b u t i o n s y st e m s 2 8 7

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1 5 . 7 E .

T h e u s e o f a u t o m a t i c r e c lo s i n g f e a t u r es p e r m i t s t h e u s e o f i n s t a n t a n e o u s p r o-

t e c t i o n i n a d d i t i o n t o t h e m o r e u s ua l i .d a n . t . f o r m s , a s d e s c r i b e d i n S e c t i o n 1 5 . 3 . 4

a n d s u c h s c h e m e s a re b e i n g i n c r e a si n g l y u s e d o n r ur al p r i m a r y s y s t e m s . A s a r e su l t ,f a ul t d a m a g e is m i n i m i s e d a n d t h e n u m b e r o f p r o l on g e d i n t e r ru p t i o n s o f s u p p l y

due t o l i ne f au l t s i s r educed .

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c i rc u i t b r e a k e r s o n t h e l o w e r v ol t a g e s i d e o f s o m e s y s t e m t r a n s f o r m e r s . T h i s s h o u l d

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The app l ica t ion o f p ro tec t ion to ru ra l d i s t r ibu t ion sys tems 289

only be carr ied out i f the primary circui t is heal thy, consequently t i le reclosinfeature is ini t ia ted by a vol tage relay energised from a vol tage t ransformer on thsecondary side of t i le main t ransformer. A typical scheme is shown in Fig. 15.7F.

Extensive use is made of automatic reclosing in a single circuit breaker sub

stat io n, the schem atic diagram o f w hich is show n in Fig. 15.7G. In considerinthe fun ct ion ing o f this schem e, let it be assumed that l ine A is nor m ally the incoming supply, whils t l ine B is t i le outgoing supply to an adjacent substat ionA fault o n line A is cleared b y tile trip ping o f the local and rem ote h.v. circuibreakers together wi th the in ter tr ipping of t rans form er A lower voltage c i rcubreak er to prevent a back feed to the faul t . The l ine is sub seq uen tly re .energiseby the au tom at ic reclos ing of the rem ote b reaker and th is is then fo l lowed by th

reclosing of the local h.v. circuit breaker if t i le voltage relay controlling the reclosing feature of this breaker remains energised for a period exceeding i ts t ime-lasetting . This t ime-lag is set to a value in excess of the op eratin g time of thpro tect ion at the rem ote end of the l ine so that on persis tent faul t no reclosing othe local breaker occurs . The auto m at ic rec los ing of the t rans form er lower voltagcircui t breaker is governed by restorat ion of supply to the t ransformer ament ioned in the previous paragraph.

O per at ion of the schem e in the event o f a faul t on l ine B is s imilar to th a

described abo ve, excep t that the local b,.v. c ircuit b reaker recloses auto m atical laf ter a pre de term ined t ime lag w ith ou t vol tage restorat ion con trol . I t thus acts tc lear persis tent faul ts on t i le outgoing feeder in a manner s imilar to that in whicthe circuit breaker at t i le remote end of l ine A clears such faults on that l ine.

On transformer faul ts the t ransformer protect ion is arranged to t r ip the local h.and app rop riate t rans form er l .v. c ircui t-breakers and to close the faul t- throw insw itch to en sure the t r ipping o f the rem ote h.v. c ircui t brea ker. After a t ime-la

suff icient to ensure operat ion of this la t ter circui t breaker, the faul ty t ransformer i so la ted by the opening of the pow er opera ted d isco nne ctor, fo llowing wh ich thlocal and remote h.v. c ircui t breakers are reclosed automatical ly.

15.8 Bibl iography

Regula t ionsOverhead l ine Regula t ions , HMSO

Articles'Line pro tect i on by Petersen coils" by H W illott Taylor and P F Str i tzl(].1l:7 .82,page 3 87 , A pril 19 3 8)'Automatic circui t reclosers ' by Peirson. Pollard and Care(].11£l:',102 Pt .A, (6) ,1955)

'Auto-rec los ing sw i tchgear in d is t r ibut ion prac t ice ' b y S H Money and J Harr(Proc. lEE,115, (2) , 19 68)



29 0 The applicat ion o f pro tect ion to rural distr ibu tion systems

'H igh speed reclosing on 11 kV rural netw orks' by S H Money(E lec. Times,18June 1959)'High speed tripping and reclosing on rural networks' by S H Money(E lec. Times,15 & 22 Jun e 1961)

Reports'Arc suppression co ils and auto-reclosing switchgear', The Elec tricity Counci(DRP Report No. 1,1962)'Repo rt on standardisation of auto-reclosing facilities on 11 kV ground m ountemetalclad distribution switchgear', The Electricity Council, (ACE Report No.11966)

'Lightning protection of distribution networks', The Electricity Council, (RepoRef. EC R/R 566, 1972)'Repo rt on auto-reclosing schemes for 11 kV distribution netw orks The ElectriciCouncil (ACE R eport N o. 54 , 1977)



Chap ter 16

The appl ica t ion of p ro tec t ionto u rban and m et ropo l it an sys tem s

by K.A.J.Coates

16.1 In t roduct ion

In the early days of electr ici ty supply, many small generat ing stat ions supplied theiown local areas and there was l i t t le intcrconnect ion. As the load grew, larges ta t ions were bui l t and t ransmiss ion networks were super imposed which in terconnected the larger s tat ions and a ugm ented supplies to areas wh ere the local gene rat iowas inadequ ate . A com plex sys tem of gene ra t ion , t ransmiss ion and d is t r ibut ioevolved having a variety of different vol tage levels for generat ion and transmissional tho ug h distr ib ut io n was usua l ly at 6-6 kV or 11 kV . M uch of this s t il l exists buwi th the s i t ing of new s ta t ions away f rom large concent ra t ions of popula t ion anthe in t rod uc t ion of 275 kV and 4 00 kV transmiss ion a new pat tern is emerging

Typical ly, energy wil l be received at a 400 kV supergrid point where i t wil l bet ransformed down and d is t r ibuted by sys tems opera t ing a t 132 kV and 33 kV tpr imary subs ta t ions which in turn wi l l t ransform to 11 kV. At the h igher loaddensi t ies , for e xam ple , in c i ty cen t res there are econom ic advantages in having onlone system level be tw een 40 0 kV and 11 kV and if the 33 kV level is dispensewi th , as may be the case , the whole sys tem needs only two s tages of t ransformat ionamely 400 /132 kV and 132/11 kV.

At the higher system levels , the uni ts , e .g. t ransformers, busbars , cable circui tsare large and repair and replacement t imes are lengthy. Fai lures at this level tend taffect large numbers of consumers tor long periods. To safeguard against suchextensive outages, plant and circui ts are duplicated and usual ly operate in paral lelPro tec t ion is requi red to iden t i fy c orrec t ly and in i tia te the d iscon nect ion o f anfaul ty uni t a t the same t ime remaining stable against faul ts at lower system levelfor which o ther protec t ion exis ts . The provis ion of adeq uate , of ten com plexpr ote ct io n at high s ystem levels is expensive bu t i ts cost is usual ly a small percentag

of the capi tal value of the main plant .At the lower sys tem levels the uni ts becom e progressively more nu m erou s



292 The applicat ion o f pro tect ion to urban and m etrop oli tan systems

service remains to be protected. To correspond wi th these condi t ions thepro tect io n bec om es simpler and cheaper bu t m ust remain rel iable so that the scaof servicing involved is con tained w ithin reasonable l imits .

Econo mic considerat ions dem and a t tent io n to the faul t suscept ib i l ity of thvarious classes of equipment and if faults are rare then only minimal protect iomay be jus t i f ied and some r isks accepted. Protect ion a t some other point i s therelied upo n to give disco nne ction.

Pro tec t ion may be necessary to comply wi th an exp l ic i t o r impl ied s ta tu torrequ irement ; the p rov is ion o f p ro tec tion on a consu m er ' s se rv ice is a requ i remenunder the Elect r ic i ty Supply Regulat ions (superseded) .

Al thoug h the pr imary funct ion of p rotect ion is usual ly to minimise loss o

supply due to faults i t may also be valuable in reducing damage to capital planby l imi t ing fault cu rrent d urat ion .Thus the ex ten t and type of p ro tec t ion a fforded a t any po in t may depend upo

a number of factors. The availabil i ty of comprehensive fault s tat is t ics enables fauprobabil i t ies to be calculated which serve as a guide to the value of protect ion aa part icular point on a system. Because protect ion affects the way in which sys tem operates unde r fault cond i t ions it is rarely that p rotect ion arrangements cabe assessed in isolat ion fro m b road er system design considerat ion s, however.

16 .2 Character is t ics o f urban and m etro po li tan areas

Towns and ci t ies usually have a concentrat ion of commercial premises at thcentre , surrounded by predo m inant ly residentia l areas wh ich in turn merge wi tsurrounding semi-rural areas. Industr ial premises may be present , scat tered in smapockets or concentrated in a part icular area.

Typical ly, the dis t r ibut ion sys tem comprises a 415/240 V network suppl ief rom 1 lkV /41 5/2 40 V subs ta tions wi th a capac ity rang ing from 300 kV A to 100kV A. 1 lk V feeders each supplying perhaps a doze n such substa t ions , emanate f ropr imary substa t ions which in turn may be suppl ied a t 33 kV, 66 kVor 132 kV. In these pr im ary substa t ions there are usual ly two o r more t ransformerranging in capaci ty f rom 5 M VA to 60 M VA. This type o f d is t r ibut ion sys tem comm on ly supplies the wh ole range o f load densit ies from , say, 100 MW /km 2 in th

centre o f the largest ci t ies to the 5-12 MW /km 2 typical of residential areas do w n tthe m uch lower densit ies of semirural areas. At the highest load densit ies sub stat iom ay b e only 30 m ap ar t com pared wi th 2 00m apar t in resident ia l areas. In the la ttareas vol tage regula t ion in the 415/240 V network may affect the spacing osubsta t ions , a considerat ion which does not apply in the high densi ty areas wherspacing is determined ent i re ly by the load and substa t ion t ransform er capaci ty.

In the central areas substat ions are of necessi ty often si tuated in the premises othe larger consum ers . As an a l ternative to 41 5/2 40 V s upply , some large consum em ay be afforded supply a t 1 l kV but th is ar rangemen t is more com m on where thload is indust ria l. The 4 15 /24 0V and l lk V dis t r ibut ion is carr ied o ut a lmo



The application o f protection to urban and m etropoli tan system s 293

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294 The appl icat ion of protec t ion to urban and m etropol i tan s /s tems

1 6 . 3 D i s tr i b u t i o n s y s t e m p r o t e c t i o n - r ad ia l I .v. s y s t e m s

The e lem ents o f a d is t r ibut ion sys tem are show n in Fig . 16 .3A, and wi ll be used td iscuss the pro tec t ion aspects.

1 6 . 3 . 1 S e r v i c e s

The f i rs t pa r t o f the d i s tr ibu t ion sys tem to be p ro tec ted i s the se rv ice cab le , and th ii s ach ieved by a se rv ice fuse a t the t e rmina t ion o f the d i s t r ibu t ion company 'se rv ice cab le on the consumer ' s p roper ty. For spec ia l r easons th i s may take thfo rm of a c i rcu i t b reaker, bu t in the major i ty o f cases the bes t and cheapesprotec t ion i s a h igh breaking capaci ty (h .b .c . ) fuse . Usual ly, the s ize of the fus

shou ld be re la ted to the cur ren t -ca r ry in g cap ac i ty o f the se rv ice cab le and no t to thload app l ied fo r o r taken by the consum er.

1 6 . 3 . 2 L . V . c a b l e s

The cables which make up the l .v. d is t r ibut ion network require protec t ion againfaul ts and i t i s common pract ice to f i t car t r idge fuses in the phase conductors oeach o utgoing cable a t the l .v. busb ar in the substa t ion . Fuses m ay also be insertein the un derground ne tw ork a t cab le junc t ion s where l ink boxes ex i s t.

In choosing the s ize of fuse the fo l lowing three points need to be taken in tcons ide ra t ion , bea r ing in mind tha t they a re no t a lways compat ib le and a compromise is of ten necessary.

(a) The fuse provides pro tec t ion for overcu rrent as a result of faul ts and noover load. O ver loads can o ccur on d is t r ibut ion cables thro ug h increas ing dem andbut these are unl ikely to be of such a magni tude tha t they cannot be carr ied safefor severa l hours wi thout damaging the cable . Over loads can a lso occur througal tera t ion due to load t ransfers , and in these cases the increase in load m ay be m ucgreater. N evertheless the re is s t il l an interval o f t im e befo re the cable m ay bdamaged. The s ize of fuse must be chosen so tha t i t does not b low a t t imes of peaload, but a t the same t ime safeguards the cable agains t overcurrent due to breakdown of insula t ion . The s ize of fuse i s therefore chosen more in re la t ion to th

shor t - term ra t ing o f the cable than the s ize of the load. Thu s a cable considereappropr ia te fo r a 300A load cou ld be fused wi th a 600A or 800A fuse . A fu r thefactor wh ich needs to be considered is the degree of d iscr iminat ion wh ich should bobta ined between the l .v. fuse on th is cable and the protec t ion on the t ransformein the substa t ion .

( b) Fau l ts o n t he con sum er ' s p r emi s e s wh i ch a re n o t c l e a re d by t he c o nsu m er ' so wn fu se s o r wh ich occu r be tw een t he d i s tr ibu t io n c om p a ny ' s cu t ou t ( f u se ) and t hconsumer ' s f u se s , mus t be c l e a r ed by t he d i s t r i bu t i on co m pany ' s f u se , andthere fore the fuse a t the sub s ta t ion or in the ne two rk m us t be such as to d i sc r imin a t



The appl icat ion o f protect ion to urban and m etropol i tan sy s tems 295

I t shou ld be no ted he re tha t the rem ark m ade p rev ious ly, tha t is tha t the s i ze ofuse in the d i s tr ibu t ion co m pa ny ' s cu tou t shou ld be re lated to the cur ren t -ca r ry incapac i ty o f the se rv ice cab le , r equ i res no modi f i ca t ion by the po in t s made aboveThat i s to say, where the d is t r ibut ion cable and service cable are of the same s ize

the fuse on the d is t r ibut ion cable should be the larger. This fac t wi l l he lpd i sc r imina t ion .

(c) The m in im um fusing cur ren t o f the fuse m us t be le ss than the m in im umear th-faul t curren t . This i s of ten d i ff icul t to achieve w hen the den si ty of the load low, or in d is t r ibut ion sys tems where the s ize of d is t r ibutor i s chosen to sui t thcurre nt a t every p oint ( tha t i s a t ree or tapered sys tem ), and therefore w here thconductors become smal ler and smal ler as the d is tances f rom the source becomlonger.

In o rder to ensure tha t an ear th faul t a t the en d o f a long length of smal l sec t iocable is cleared, a fuse c ould be used in a l ink bo x whe re the cables are join ed , thfuse ra t ing being chosen to d iscr iminate wi th a la rger fuse fur ther back in thnetw ork . Fuses can a lso be used in th is way in an a t t em pt to im prove thcon t inu i ty o f supp ly by t ry ing to l imi t the num ber o f consum ers which wil l bwi thout supply in the event of a cable faul t .

1 6 . 3 . 3 S u b s t a t i o n t r a n s fo r m e r s

Cons ider now the p ro tec t ion o f the t r ans formers in the subs ta t ion . Due to thelarge number, the re la t ive unimpor tance of each uni t to the sys tem as a wholethei r good record of re l iabi l i ty, and the h igh percentage cos t of g iv ing com pletp ro tec t ion , economics p lay a la rge pa r t in de te rm in ing the type o f p ro tec t ion tbe used.

In the s imple case of a s ingle t ran sfo rm er, plain h.v. fuse pr ote ct io n is thcheapes t , but careful considera t ion must be g iven to the choice of fuses . In urbaareas it is no t un usual to g ive a supply to a single co nsum er in the ord er o f 25 0 kVat 41 5/2 40 V. Whi le i t might be fe lt des irable to supp ly such loads d i rec t f romsepara te subs ta t ions , th is i s not a lways economical or prac t icable , and they wihave to be suppl ied f rom the 4 15 /24 0 V n etw ork s . Fo r such a load, the servicfuse sh ould be ra ted a t 400 A, w hich necess i ta tes an 800 A fuse on the o utgoindis t r ibutor a t the subs ta t ion in order to mainta in a ra t io of 2 .

On an 1 lkV sys tem, the ra t io o f t r ans form at ion w ould be som eth ing under 2and therefore to m ainta in a ra t io of 2 w i th an 1.v. fuse ra t ing of 800 A, the h .vfuse ra t ing should not be less than 64 A. The discr iminat ion which would be produced by such an ar rangement i s i l lus t ra ted in Fig . 16 .3 .3A which shows typicacurves for the fuses ment ioned. The curves used, however, must be those for thac tua l fuses employed . I t mus t be remembered tha t the reac tance and s ize o f tht ransfo rm er se ts an upp er l imi t to the p rospect ive 1 .v. faul t curren t , and th a t an

imp edan ce in the faul t i tse l f wi l l fur the r reduce th is curre nt . There is no p oint it ry ing to achieve d iscr iminat ion a t currents grea ter than those l ike ly to be found



296 The applicat ion o f prote ct ion to urban and m etrop oli tan systems

-~ |

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M a x i m u m s h o r t c i r c u itc u r r e n t f o r5 0 0 k VA t r a n s fo r m e r

S 0 I 0 0 5 0 0

C u r r e n t i n p r i m a r y a m p s a t I I k V

Fig. 16 .3 .3A Ty pical fuse characteristic fo r disc rimin ation

No m ent ion has yet been made of re la t ionship b etween the size of the h .v. fusand the s i ze of the t ransform er, but in consider ing th is aspect i t must bremembered that s imple fuse protect ion may give l i t t le or no protect ion for eartfaults at the middle of the h.v. winding and that , as the rat io of the size of fuse tfull load current increases, less of the winding is covered for earth faults.

Fur thermore , fuse protect ion is not in tended to guard agains t overcurrents duto overloading of the transformer, but primari ly against overcurrents due to fault

These facts , coup led w ith a l imited choice o f h.v. fuses, ma kes for a certain lack ore lat ionship between the full load current of a t ransformer and the ra t ing of thh.v. fuse.

In the UK, the rat ings of h.v. and l .v. fuses appropriate to the various sizes odistr ibution transformers in use have been standardised and an Electr ici ty SuppIndustry Standard No. 12-8 deals comprehensively with this subject .

Where eco nom ic considerations jus t i fy a greater degree of prote ct ion, th en thsingle transformer can be protected by a circuit breaker with overcurrent anearth-fault pro tect io n. If relays are used instead o f direct-acting trip coils , the n thprotec t ion can be set more accurate ly, but th is in t roduces the problem of providinthe n ecessary source of supply for t r ipping the circuit breake r. If on ly two-po lovercurrent relays are instal led, the set t ing must be chosen to ensure that the relawill op erate for a phase-to-phase fault on the sec ond ary side of a delta stat ransformer which p roduces a 2 :1:1 dis t r ibut ion o f current in the p r imary c i rcuiWhatever the type o f pro tect ion chosen for a substa t ion t ransform er i t is necessa

to bear in mind that i t is required to operate and clear the fault before operat ioof the protect ion on the outgoing feeder a t the pr imary substa t ion f rom which i t



The app l ica t ion o f p ro tec t ion to u rban and me t ropof i tan sys tems 297

When two t ransformers feed a common busbar, s teps may need to be taken tensure discr iminat ion when the heal thy t ransformer feeds the faul ty t ransformthrough the l .v. s ide. In such a case neither fuses nor overcurrent and earth-faurelays wil l provide this discrimination and furthermore circuit breakers must bprovided on bo th the h .v. and l.v. sides of the t ransformers . The protect ion of twsuch transformers could take f ive forms:

(a)

(b)

(c)

(a)

(e)

Suitable unit protect ion, for example Merz-Price, which wil l provide discr iminat ion for phase and ear th faults on b oth s ides of the t ransformers .Overcurrent and earth fault on the primary side, restr icted earth fault othe secondary side, and intertr ipping, which wil l provide discrimination fo

ear th faul ts on b ot h s ides of the t ransformer.H.V. and l .v. overcurrent and Buchholz, which wil l provide discrimination fofaults inside the tran sform ers.Overcurrent and ear th faul t on the pr imary s ide , d i rect ional protect ion othe seco ndary side, and intertr ippin g, wh ich wil l provide discrimin ation fophase and ear th faul ts on b oth s ides of the t ransformers .Frame leakage, Buchholz, h.v. overcurrent and intertr ipping which wilprovide discrimination for internal faults and earth faults on b o th sides othe t ransformers .

In practice, in radial l .v. systems, t ransformers are rarely operated in paral lel on co m m on l .v. busb ar because their failure rate is no t high eno ugh to jus t ify the coof the ad di t ional swi tchgear and p rotect io n equ ipm ent involved.

The quest ion of whether to ap ply uni t busbar protec t ion to e i ther the l.v. or h .vbusbars cann ot be ser iously considered. The cost is out of all propo r t ion to the r is

of failures and the small advantage to be gained by installing it .

1 6 . 3 . 4 H . V. c a b l e s

For the protect io n of the h .v. cable feeding the sub sta t ions , the same considerationapply as in the case of the protec t ion of the su bsta t ion t ransforme r. I f there is on lone h.v. cable and the possibi l i ty of back-feed need n ot be conside red, then plaiove rcurrent and earth-fault pro tect io n at the m ain sub stat ion is sufficient . If thh.v. cable is run ei ther direct ly or indirect ly in parallel w ith oth er h.v. cables, somform o f uni t o r d i rectional protec t ion is necessary.

Again, the relatively heavy cost of providing pro tect io n in such circumstancehas to be considered, and therefore a layout is often chosen which avoids h.v. cablerunnin g in paral lel , yet gives facil it ies for al ternative means o f supply. Su ch a la y ouses r ing mains, as i l lustrated in Fig. 16.3.4A and the r ing is normally open at convenient po int . Under such c i rcumstances the prote ct ion a t the main substa t io

would again be overcurrent and earth fault . The set t ing of the earth fault relashould be as low as possible consistent with sufficient discrimination against th



298 The appl ica t ion o f prote ct ion to urban and m etropof i tan sys tems

set t ing sh ould be as high as possible consistent w ith the size o f cable and its shorterm rat ing, as already described in con nectio n w ith fuses for 1.v. cables, anprovide discr iminat icn w i th re lays closer to the source of supp ly.

I f grading wi th overcurrent and ear th-faul t protect ion a t the pr imary substa t iois possible, there is advantage in installing similar protection at a point on the rinmidw ay be tween the p r imary subs ta t ion and the normal ly open po in t . For thosconsum ers nearer the pr imary substa t ion th is g ives a probabi l i ty of a 50% reduct io

t°¢1 k V

E

ff---qNote" ] EF I] signifies Earth fault indicator

1 1

Normallyopen



The appl icat ion o f protect ion to urban and m etropol i tan sys tems 299

in outage t ime for h.v. faul ts .W hen a faul t occurs on an h .v. cable in a radia lly opera ted r ing ma in sys tem

substa t ions are shut d ow n unt i l the faul t i s loca ted and supply is res tored byresect ioning.

The process o f fau l t loca t ion is fac i l ita ted i f ear th- faul t ind ica tors a re ins ta llea t appropr ia t e po in t s on the ne tw ork as shown in F ig. 16 .3 .4A. The ma jo r i ty ofaul ts a re e ar th faul t s , or com m enc e as ear th faul t s before changing to phase faul tand as such can be de tec ted by a co re -ba lance cu r ren t t r ans fo rmer f i t t ed over thh .v. cab le . The cu r ren t t r ans fo rm er opera tes an a t t r ac ted a rma tu re ty pe re lay hav ina manual ly rese t f lag indica tor. Inspect ion of the indica tors immedia te ly af te r f au lt enab les the ex te n t o f the pa th o f ea rth - fau l t cu r ren t to be de te rm ined , l eadinto quicker ident i f ica t ion of the faul ty cable sec t ion . The re lay has a br ie f t ime de lato prevent response to t rans ie nt unbalance and i s sensi tive to ear th faul t cur rents o50A or m ore in the m ain con duc tors . I t is usual ly necessary to ensure tha t the f ludue to ear th- faul t cur rent in the phase conductor i s not nul l i f ied by the re turn othe same current , in reverse d i rec t ion , in the cable shea th . The e lec t r ica l cont inui to f the cable shea th is the re fo re b rok en , usua l ly a t the po in t a t wh ich the cu r rent ransformer i s f i t ted . Successfu l use of th is par t icu lar indica tor i s dependent upoal l indica tors be ing rese t soon af te r the event in readiness for any fu ture faul t . A

al te rna t ive type i s ava i lab le wi th an automat ic vol tage opera ted rese t fea ture . Thvoltage source is the l .v. supply in the substat ion in which the device is instal ledand rese t t ing there fore takes p lace a t the ins tant w hen l .v. suppl ies a re res tored .

If i t is essent ial to m ain tain a f i rm su pply on a given bus bar u nde r faul t condi t ions on one of the feeding c i rcui t s , two or more h .v. cables must be run inpara l le l to these busbars , each su i tab ly equipped wi th pro tec t ion to ensure d iscr im inat ion and a uto m at ic i so la tion o f the faul ty c able . In genera l , the d is tanceare shor t in urban ne tworks , and therefore a uni t pro tec t ion scheme wi th p i lowires can be used.

An h .v. ne twork which affords f i rm suppl ies to four subs ta t ions in th is way i sshow n in Fig . 16 .3 .4B. Each cable c i rcui t is equippe d w i th uni t p ro te c t io n us ing p i lo t wire sys tem, thus there are a to ta l of seven pro tec ted zones . A faul t in anprotec ted zone causes t r ipping of the c i rcui t breakers a t each end; the affec tedcable is los t but supply is m ainta in ed by the o th er cables. I t is necessary to ensurtha t the d i ff e ren t cu r ren t d i s t r ibu t ion a f t e r a f au lt a t any po in t on the ne tw ork

does not give r ise to overloads.The m eth od affords effec t ive pro te c t io n agains t cable faul t s but there are o the

e ffec t s which need to be t aken in to cons ide ra t ion . At each subs ta t ion the p ro tec tedzone ex tends on ly to the po in t on the ma in c i r cu i t conduc to r s a t wh ich the cu r rent ransfo rm ers a re ins ta lled , so tha t the busbar, swi tchgear and m ost of it s in ternaconnect ions are outs ide the zone . There i s a poss ib i l i ty of p i lo ts be ing openci rcui ted , a t r ip c i rcui t fa i lure or fa i lure of a c i rcui t breaker to t r ip for mechanicareasons . A cable faul t coupled wi th fa i lure of the uni t pro tec t ion , or a swi tchgeafaul t , a re s imi lar in tha t they can only be c leared by c i rcui t breakers fur ther back inthe sys te m result ing in the loss of a l l suppl ies to the netw ork under co ns idera t ion



30 0 The application of pro tection to urba n and m etropo litan systems

direct ional protect ion, busbar protect ion, p i lo t -moni tor ing e tc . , the cost of sucaddit ional equipment is diff icult to just ify and the method usually adopted is tinstall overc urren t and earth fault i .d.m .t, relays at selected poin ts to sectionalisthe ne tw ork and limit th e e xte nt o f ou tage. In Fig. 16.3.4B, the i .d.na.t , relay

, , , = , , , , , ,, ,,

: - ( 0 3 _ . . .

D @ E3

lng r t , ) ; l ~

Un it~protected h v r ing ma in systemg, 16.3.4B

{instal led in the r ing separates the netw ork into two groups each o f two feeders an

only one group is shut dow n for the type o f fault und er considerat ion.The shut dow n of the group involves operat ion of a to ta l of three re lays , two oh f d h i b i d h i l il h f l i l d b




pr im ary substa t ion wh ich wi ll be t r ipped i f i ts re lay opera t ing t im e is exceed ed. is instruct ive to consider h ow faults at differen t p osi t ions affect the operat insequence of the three netw ork re lays and the to ta l c learance t im e. I f a faul t occuin a feeder cable-box a t the pr imary substa t ion v i r tua l ly the whole of the faucurrent wi l l f low di rec t ly through i t s own c i rcui t breaker which t r ips a t the re laset t ing. The faul t curre nt pa th is then via the othe r three circui ts , feeding thfaul t f ro m the rem ote end . The faul t is now seen by a to ta l of four re lays , the thron the feeders plus the r ing sect ionalis ing relay. If the feeder impedances arroughly similar and the r ing sect ionalis ing relay has a lower t ime set t ing i t wil l bthe f irst to t r ip thu s c ut t ing off fault current in two feeders. The remaining feedis the only c i rcui t now feeding the faul t and i ts re lay opera tes to c om plete the fau

clear ing seq uence . The to ta l faul t c learance t im e is therefore a com bina t ion o f fu lsequent ia l and s imul taneous re lay opera t ing t imes . By compar ison, a faul t a t thremote end of the network, say on the busbars a t a subs ta t ion , produces s igni f icacur rent in al l relays s im ultan eo usly . Th e r ing sect ionalis ing relay wil l be the f i rs t opera te thereb y c lear ing the faul t f rom tw o feeders and f inal c learance comes wh ethe two remaining feeder re lays have bo th o pera ted . Under these c ircum stances thtotal clearance t ime wil l be different from the previous case and almost certainshor ter.

With para l le led feeder ne tworks , therefore , the pos i t ion of the faul t has a pronou nced effec t u pon the re lay opera t ing sequen ce and it is necessary to f ind thcon di t ion wh ich gives the m ax im um tota l c learance t im e. This is done by se lec tina faul t pos i t ion , performing network analys is to ascer ta in faul t current d is t r ibut iocalculat ing relay op erat ing t imes to f ind the f irs t relay to o pera te and how far thothers have moved tow ard the i r opera t ing points . The netwo rk con di t ion af tthe f i rs t relay has operated is then analysed to ascertain the revised current distr ib

t ion and re lay ca lcula t ions performed again to ascer ta in the next re lay to operaand so on unti l the faul t is cleared. The whole process is then repeated for faul ts adi fferent pos i t ions u nt i l the m ax im um tota l c learance t ime has been establ ishedThe opera t ing t im e o f the tr ans former p ro tec t ion a t the p r imary subs ta tion m uthen d i sc r imina te w i th the m axim um to ta l c lea rance t im e .

The ca lcula t ions are not so onerous as would appear because the fac torcon tr ibut ing tow ards the longest t ime becom e app arent af ter only a few fauposi t ions have been t r ied . O vercurrent and ear th faul t re lays need to be considereas two separa te groups . Phase faul ts appear throughout the sys tem as overcurrenand as a resul t al l overcurrent relays are involved in the t ime grading considerat ionThe maximum t ime de lay i s f ixed by p ro tec t ion cons ide ra t ions on the p r imarsubs ta tion t r ans former and the m in imu m by the necessi ty to d i sc r imina te wi tp ro tec t ion on any d i s t ribu t ion t r ans former. The p re fe r red m in im um d i sc r imina t iointerval of 0-4s between stages usual ly severely l imits the number of s tages that cabe em ployed be tw een the upper l imi t a t the p r imary subs ta t ion and the lowe

l im i t a t the d is t r ibut ion substa t ion . Because ear th-faul t current does no t app ear asuch thro ug ho ut the ent i re sys tem there are usual ly fewer stages of ear th-faul




The m etho d of affording fi rm suppl ies using unit p rotect ion by pi lo ts w i th bacup overcu rrent and earth-fault p rote ct ion has the meri t o f relat ive simplici ty anis eco nom ic if the pi lots are laid w ith the main cables. A nu m ber of oth er arrangme nts have been used. Uni t prote ct ion by pi lo ts may be modif ied to include th

switchgear busbars in the protected zone so that each successive protected zonoverlaps the next to this extent . As a result back-up protect ion is no longer requireto cope with b usb ar faults bu t oth er r isks, such as pi lot circuit failure, rema in. Timgraded direct ional schemes as described in Chapter 2 el iminate the pi lot diff icultare responsive to busb ar faults and oth er failures bu t require a source o f polarisinvoltage usually derived from voltage transformers which add to switchgear costDistance protect ion is inapplicable to high density urban distr ibution because

difficulty in achieving sensitivity to the short distances involved and again a voltasource is necessary.

16.3 .5 Pr imary subs ta t ions

Prim ary s ub statio ns are the po ints in the syste m wh ere su pp ly is received at e.la.t ransformed to h.v, and fed via busbars and circuit breakers to the h.v. distr ibutionetw ork. T he h .v. d is t r ibut ion voltage com m on ly in use in the U ni ted K ingdom 1 lkV ; a t e .h .v, the voltages are typical ly 33kV , 66kV or 132kV , the t rend beintowards 132kV. There are usually two or more transformers ranging in size fromabout 5 MVA to 60 MVA, the very large t ransformers having double secondarwindings to l imit the fault level on the 11 kV side and the norm al curren t rat ing the low er voltage switchgear to an econ om ic value. In add it ion to the variety ovoltages and transformer capacit ies in use there are variat ions in the method oneutral earthing, the arrangement of busbars and the faci l i t ies for voltage contro

There may be local e .h.v, switchgear controll ing the transformers and possibly othecircuits or the transform ers ma y be co ntrolled b y rem ote e Ja.v. switchgear in wh iccase each transformer and i ts associated e.h.v, cable is known as a transformefeeder. The ru n ni ng arrangem ent is usually w ith pairs of t ransfo rme rs in paral lto afford a f irm supply to the 1 l kV b usbar but schemes of separate t ransform erunning arrangements exist under which loss of supply to a sect ion of 1 lk V busbis res tored b y coupl ing autom at ical ly to a heal thy sect ion.

The requirements of the protect ive gear are primari ly:

(a)

(b)

(c)

to clear a faulty 1 lk V feeder from the system before higher level pro tect iooperates,to c lear a faul ty t ransformer or t ransformer feeder before the protect ion oany paral lel healthy circuit operates and before any higher level protect iooperates,to l imit the effect of busbar faults on other parts of the system.

In add it ion to these primary requ iremen ts there is a secon dary requ ireme nt to lim



The appl icat ion o f protect ion to urban and m etropol itan systems 303

Requirements (a) and (b) are c lear cut , but (c) poses problems as to the extent othe provis ion to be made agains t busbar faul ts . At one extreme i t may be arguetha t bus bar faults are rare and th at provided higher level outage is pro tected againthen only a s ingle busbar wi th a nonauto sect ion swi tch between t ransforme

posi t ions , is jus t i f ied . On the othe r hand the mag ni tude of an outage, wi th poss iblengthy res tora t ion t ime m ay be considered as jus t i fy ing a dupl icate b usbar ar rangm ent wi th uni t protected sect ions . As one might expect , an arrangement somwhere betwee n these tw o extrem es is usual ly adop ted bu t the busbar ar rangemem ay n ot rest on secur i ty considerat ions a lone.

The var ious schemes of protect ion appropr ia te to pr imary substa t ions are deaw ith in detai l in Cha pters 12 and 13.

16.4 Dis t r ibut ion sys tem prote ct ion - in terconne cted l.v. sys tems

Va rious in tercon nec ted 1.v. system s are in use in large cit ies in this cou ntr y anabroad and such systems involve protect ion considerat ions over and above thosof a radial system .

Interconnected l .v. systems are part icularly suited to ci ty distr ibution becausthe l .v. cables required to supply the load natural ly form a practical ly continuou

grid over large areas. Thus the substat ions which supply such a network may easibe operated in paral lel through the l .v. network via solid l inks or cartr idge fusesThere are savings in both capital and running costs. The savings take place becausto a greater degree than on a radia l network, an in terconnected network couple

I18

m

6

1

- " r - - - - 1

I

I 1

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L6

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304 The appl ica t ion o f pro tect ion to urban and m etropo l i tan sys tems

together the loads vi r tual ly all of wh ich have some diversi ty one wi th anoth er. Ththe maximum demand supplied by the transformers is less and fewer are required otheir capacity is less. The percentage loading o f transfo rm ers tend s to equalisbecause they are effect ively in paral lel , giving the condit ion for minimum coppe

loss and since the total transformer capacity is reduced the total iron loss is lesLosses in the l .v. network are also reduced because cables are fed from both endand b ecause the nu m ber of parallel paths is increased.

The other important feature of interconnected l .v. systems is the general ladopted pract ice of coupl ing together the l .v. networks suppl ied f rom substa t ionon different h.v. feeders and arranging that the l .v. network is immune to the losof a single h.v. feeder, the load being taken up on the remaining feeders in th

same group. This is a valuable advantage because the greatest single cause of losof sup ply to consu me rs is h.v. feeder faults .

16.4.1 Th e l.v.n e t w o r k

There are variat ions in the m eth od of dealing w ith the 1.v. net w ork . Fig. 16.4.1shows a ful ly fused arrangement, cartr idge fuses being f i t ted in the phasconductors at al l l ink boxes. If the fault current distr ibution fol lowed the idea

pat tern shown the fuses would discriminate and only those a t each end of thfaulty distr ibu tor wo uld op erate. In practice irregulari ties in the patte rn o f impedances between junc t ions upset the dis t r ibut ion of fault current and unwan tefuse operat ions takes place. This si tuat ion is complicated by a further effect whicis that some faults clear themselves leaving fuses intact, particularly with the larges ize of fuse . The form of const ruct ion of the cable, the con duc tor mater ial and thprospective fault current all have a bearing up on the prob abil i ty o f fault selclearance. If all these factors are favourable then very nearly all faults will clea

which leads to the proposit ion that the fuses are unnecessary, or relat ively so. Othis basis som e in tercon nec ted l .v. systems have opera ted w ith large block s o f loasolidly interconnected, ul t imate disconnection for a non self-clearing fault rest inupon l .v. protect ive gear at the substat ions. A disadvantage of this degree of soliinter con nec tion is that there are operat iona l difficult ies in restorat ion after a nonself-clearing fault . B etween the tw o extrem es of ful ly fused and solidly interconnected arrangements there are hybr id schemes which pe rform w ell .

1 6 . 4 . 2 S u b s t a t i o n s

In an interconnected l .v. system, the transformers are effect ively connected inparal lel across the h.v. and l .v. networks and any faulty unit is required to berapidly isola ted f rom both networks . The usual protect ion considerat ions for thparallel operat ion of t ransformers ap ply and i t fo l lows that protec t ion is required tin torrupt quickly the backfeed to a faul ty t ransformer f rom the l .v. network

Al though un i t p ro tec t ion of the t rans former cou ld be employed , economic considerat ions favour the use of direct ional protect ion on the l .v. s ide because thi




h.v. cable faul t . L .V. protec t ion for faul ts in the forward d i rec t ion , e .g . on the l.vnetwork, i s usual ly by d i rec t -ac t ing t r ips shunted by t ime fuses , the swi tchgeabeing in the form of an a i r c ircui t breaker.

Protec t ion on the h .v. s ide of the t ransformer i s required to respond to in terna

faul ts and faul ts on the h .v. and l .v. connect ions . This requirement may be met id i fferent wa ys . I f the loss of an h .v. feeder due to a t ransfo rm er faul t is an accepter isk then local protec t ion can be omit ted , re l iance being placed upon the feedepro tec t ion a t the pr im ary su bsta t ion to g ive fault c learance . W ith such an ar rangem ent howev er there is d i ff icul ty in d iscr iminat ing betw een the current due to heavload on the feeder and tha t due to a faul t on the l .v. s ide of a t ransformer. Locaprotec t ion of the t ransformer overcomes th is d i ff icul ty and a t the same t imenables sens i t ive ear th-faul t protec t ion to be provided economical ly for the t ransformer pr imary. The swi tchgear i s usual ly an o i l c i rcui t breaker and the protec t iois by direct act ing tr ips with a t ime fuse delay for overcurrent faul ts andins tantan eou s op era t ion for ear th faul ts , the la t ter having a suff ic ient ly low se t t into pro tec t the w hole of the pr im ary winding. An in ter t r ip to the l .v. c i rcuit break eensures tha t the l .v. back-feed is bro ken wh en the h .v. c i rcui t breake r o pera tes . H .Vfusegear is a possible al ternat ive to the h.v. circui t breaker, again with an inter tr ito the l .v. circui t breaker, but this does not provide the sensi t ive earth-fault pro

tec t ion .

1 6 . 4 . 3 T h e h .v. n e t w o r k

I t is usual for the h .v. ne two rk supplying an in terco nne cted l .v. ne tw ork tocomprise a group of f rom four to e ight h .v. feeders , each feeder opera ted radia l land supp ly ing typ ica l ly ab ou t t en 50 0kV A subs ta t ions . There m ay be p rov is ion focoupl ing feeders toge ther a t the i r ext rem it ies and e lsewhere to th e ex tent tha t i fsec t ion of h .v. cables i s lost du e to faul t , supp ly to a ll subs ta t ions m ay be res toreby swi tching. Al ternat ive ly, such coupl ing fac il it ies m ay b e absent in w hich cassupply to an area which has suffered the loss of an h .v. feeder can only bemainta ined through the in terconnected l .v. ne twork. A disadvantage of th is ar rangement i s tha t a f au l t a t the p r imary subs ta t ion end o f a f eeder may impose undustress o n the l .v. ne tw ork s ince the la t ter is required to su ppo r t th e who le of th

load of the feeder unt i l the faul t is located and repa i red .As an al ternat ive to radial op era t ion and at a rather higher capital cost , feeder

may be equipped wi th uni t protec t ion and opera ted in para l le l . Supply to substat ion s is the n f irm against an y single h.v. cable faul t and un der this co nd it ionload is no t r equ ired to be sup por ted th roug h the l .v. ne tw ork , o the r than tha t duto the loss of a subs ta t ion t ransformer.

The h .v. ne twork may be in ter leaved or b locked. In an ideal in ter leaved networkthe h .v. cables are routed so tha t ne ighbour ing substa t ions are suppl ied f rom di f

ferent feeders . In a b locked netw ork one feeder supplies a nu m be r of ne ighbou r insubstations wh ich is te rm ed a b lock and th is b lock adjoins b locks suppl ied b y oth e




of blocks is large and the blocks may be interleaved, giving a distr ibuted blocarrangement. Interleaving reduces the burden of current across the l .v. networwh ich occurs under feeder outage condi t ions .

1 6 . 4 . 4 P r o t e c t i o n o n a d i s tr i b u t ed b l o c k l .v . i n t e r c o n n e c t e d s y s t e m

Fig. 1 6.4 .4A shows a dis t ributed block sys tem suppl ied f rom a group of fou1 lk V feeders fed f rom one sect ion of busbar a t a pr im ary su bsta t ion. Each feedeon leaving the pr imary substa t ion supplies an l.v. in terconnec ted block throu gh fousubsta t ions and th is b lock is coupled to s imi lar b locks on other feeders each by fosets of fuses, termed fr inge fuses. The feeder then continues to a more remote are

where i t supplies another interconnected block similarly arranged. The diagrashows a to ta l ly symmetr ical format ion for s impl ic i ty of presenta t ion but a numbof var ia t ions are poss ible . For example , any feeder may supply more than twblocks and the number of t ransformers feeding a b lock may be more or less thafour ; th is in tu rn affects the num ber of f ringe fuses betwe en blocks . The extent twhich such va r ia t io ns m ay exis t i s determined by a full know ledge of how the new ork operates un der var ious faul t con di t ions bu t , in general , sym m etry and unformity ease protect ion discr iminat ion considerat ions . Al though not shown in thdiagram the config urat ion of the 1.v. netw ork is frequ ently such as to al low direfusing betwee n diagonally oppo si te b locks , e .g . A1 and C1, thereby improving thunifo rmity of loading wh en a feeder i s los t due to faul t.

Three features o f the w ay in which the netw ork is required to operate undefault con dit ion s m ay be no ted at this s tage. An 1.v. ne tw ork fault in an interconnected block, i f i t does not self-clear, results in shut-down of the block, clearancbeing ob tained by op erat io n of al l the 1.v. circuit break ers on transform ers feedin

that b lock and by o perat ion of the f ringe-fuses wh ich conne ct i t to o th er b lockAn h.v. cable phase fault is cleared by feeder prote ct io n at the primary substat ioand by reverse-power protect ion on the l .v. circuit breakers on al l t ransformersuppl ied f rom that feeder. Both the near and remote blocks on the faul ty feedeare therefore involved in this operat ion. An h.v. cable earth fault is cleared bfeeder protect ion at the primary substat ion but the cable may remain al ive back-fefrom the 1.v. ne tw ork since the reverse-power pro tect io n will no t necessariloperate under this condit ion. The cable can therefore, via the transformercon nec ted to i t , provide pa ths in paral lel w ith l .v. cables and assist in the transfer opow er across the sup po rted 1.v. block s.

Fig. 16.4 .4B shows the prote ct ion arrangements . The 1 lk V feeder protec t ion a two-pole overcurrent and single-pole earth-fault relay with a normally inverscharacteristic on all elements.

The transformer h.v. circuit breaker in each substat ion is equipped with direcacting t r ips to g ive t ime fuse delayed overcurrent and ins tantaneous ear th fau

pro tect io n. A n auxil iary switch intertrips the l .v. circuit breaker.Protect ion associated with the l .v. circuit breaker is current . t ransformer operate



The app l i ca t ion o f p ro te c t ion to u rban and m et ropo l i tan sys tems 30 7

I I k V -



308 The appl ica t ion o f prote ct ion to urban and m etropo l i tan sys tems

D11 kV

A "" B r l " C

I E [ ~ l E ] 1 E l l

_15/240 V . .

S e r v i c e _ ~

to b u i l d i n g

T T w o 5A tripsL ~ with time fuses

one IA EF trip

f t uwith time fuses

, n

,w l , i

L .v . n e t w o r k X Y

F i g . 1 6 . 4 . 4 . B Distributed block system protection

o/oTor. ~ T w o 5A tripsfuses

F trip

Z[ ~ Three 5A OC trips

with time fuses

A

F ~ u s e

relay operat ing a shunt t r ip. The shunt t r ip also responds to t r ipping current from

the in ter tr ip circui t and supp ly is take n via fuses, f rom the trans form er l .v. connect ions. Supplies to the vol tage coi ls of the reverse-power relay are afforded in



The appl ica t ion o f prote ct ion to urban and m etropo l i tan sys tems 309

The reverse-power relay has to sat isfy a number of requirements. Because otransformer reactance in the fault path the power factor of the fault current ilow and the relay is therefore designed to have i ts maximum sensit ivi ty at abou55 ° lagging. Vo ltage depression m ay b e co nsiderable at the t ime w hen the relay required to op erate and the relay, toge ther with th e l .v. circuit break er shu nt tr icoil , is designed for operat ion with the phase to neutral voltage reduced to 30V, tsui t typical w orst con dit ions . Rapid d iscon nection of an h.v. cable phase fault necessary, otherwise fringe fuses will o pera te, an d this gives a requirem ent for verfast opera t ion , typical ly 30m s. Subject to fast op erat ion and stabil i ty und etransient con dit ion s, the op erat ing c urrent is som ew hat less cri tical , bu t 200% ispractical value. The relay must be stable for faults in the forward direct ion and

three-phase design rather than three separate single-phase elements overcomes thproblem of incorrect response in one phase which occurs under certainasym metr ical faul t cond i t ions .

Th e ch aracteristics o f the fringe fuse used in the l .v. ne tw or k is de term ine dhaving regard to the fact that under h.v. fault condit ions the current experienced bfringe fuses seldom exceeds 5 00 0A . In order to provide a 20 0m s delay at thcurrent to ensure discrimination with the l .v. reverse-power protect ion ( includincircuit breaker operat ing t ime) a 400A fuse with a sl ightly modified characterist iis used.

Referring to Fig. 16.4.4B and considering the chain of pro tect io n from thpr imary substa t ion down through feeder B to a s ingle substa t ion and thence to thl .v. network at point X, i t is evident that discrimination is unlikely to presenproblems except possibly between the t ime fuses ei ther side of the substat ion tranform er. These fuses are effectively in series, how ever, so that correct discrim inatioserves only as a guide to fault condit ions and the performance that may be obtaine

by grading the t ime fuses is adequate for this purpose. Correct discrimination maalso be anticipated if considerat ion is extended to point Y on the other side of fr inge fuse and thence to point Z on the remote side of reverse-power protect ion a substat io n on feeder C. The reverse pro tect io n at Z operates, all oth er prote ct iopro tect io n on feeder B remaining stable. Correct discrimin ation at point Y however dependent upon there being only one fr inge fuse in the circuit and thgives r ise to a fu nd am ental principle w hich relates the nu m ber of fringe fusebetwe en in terconnected blocks to the to ta l t ransforme r capaci ty in the block. Threlat ionship which has been established is that the number of fuses should noexceed on e fuse per 750kW A of t ransformer cap aci ty.

I t is necessary to consider the stabil i ty of h.v. feeder overcurrent protect ionwhen an l .v. block supplied by the feeder experiences a non-self-clearing fault . Aeach substat ion supplying the faulty block the l .v. circuit breaker overcurrenprotect ion is subject to that substa t ion 's individual contr ibut ion but the feedeovercurrent protect ion is subject to the sum of these contr ibut ions . In addi t iothe feeder carries the load o f a hea lthy bloc k w hich i t also supplies. There is iconsequence a poss ibil ity of fai lure of discriminat ion betwe en the feeder pro tect io



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The appl icat ion o f protect ion to urban and m etropol i tan sys tem s 311

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31 2 The app/ication o f pro tect ion to urban and m etrop oli tan systems

other considerat ions, the need to maintain stabil i ty places an upper l imit on thtota l capaci ty of substa t ions which may be coupled together to form an in terconnected bloc k. T he w orst con dit ion is for an l.v. phase/phase fault giving a 2:1:dis t r ibut ion o f current in the h .v. phases under w hich the mo st heavi ly loaded phas

has an effect ive transformation rat io of 1:22. Fig. 16.4.4C shows a comparison othe protect ion character is t ic of the h .v. feeder protect ion adjus ted for load on thhea lthy b lock against the characterist ic of the l .v. overcu rrent pro tect io n of four l .circuit breakers in paral lel . The proximity of the curves indicates that the l imit ofeeder protect ion stabil i ty is reached with four l .v. circuit breakers f i t ted wit120015 curren t t ransfo rme rs and 7.5A t ime fuses. This prote ct io n is app ropriatto a 750kVA transformer so that the upper l imi t of t ransformer capaci ty in ain te rconnec ted b lock is four 75 0kV A t rans formers .

In addit ion to the condit ion of feeder stabil i ty for a non-self-clearing fault iei ther block direct ly connected to the feeder i t is necessary to consider the effecon stabil i ty when the fault is in a remote block under which condit ion fr inge fuseare required to operate to isolate the faulty block. The feeder carries the load of iown two blocks plus the load due to the fault current in the fr inge fuses. As in thprevious case the w orst con dit ion is the 1.v. phase]phase fault . Fig. 16.4.4D showscom parison of the protec t ion characteris tic of the h .v. feeder protect ion adjus te

for the load on its tw o blo ck s against the c haracteristic o f six fringe fuses in paralleIt is seen that discrimin ation fai ls i f the num ber of fringe fuses exceeds six.

The protect ion discrimination requirements described above are l imit ing condi t ions which apply to a theoret ica l ly regular network and they therefore afford guide to design. Beneath these l imits i rregulari t ies , which occur principally in thl .v. network, may impair current sharing and cause instabil i ty under fault or loacurrent sup por t condi t ions . In pract ice , these condi t ions are checked by a com puteprogramm e which by ne twork reduc t ion conver ts the num erous impedances makinup the 1.v. ne tw ork into equivalent impedan ces betw een su bstat ion s and fringe fuposit ions. Currents are then calculated at these posi t ions for three condit ions:

( a )

( b )

( c )

Cu rrent f low in the l .v. ne tw ork and tran sform ers due to an h.v. feeder phasephase fault , af ter the primary substat ion circuit breaker has opened.The same condit ion but after reverse-power relays have operated, todetermine the l.v. current f lows to supp or t the peak load in the blocks whic

have lost h.v. supp ly.The supp or t con di t ion again bu t for an h.v. feeder phase/ear th faul t wh en thh.v. cable remains in circuit as a l ink between transformer primaries andforms a paral lel connection across the supported blocks, giving a differendistr ibution of l .v. current .

Under condi t ion a ) , current in the fr inge fuses is checked against the 5000A limit

at which discrimination with reverse-power relays fai ls . The l .v. phase/neutravoltage is checked to establish that a minimum of 30V phase to neutral is availabl



The app l ica t ion o f p ro tec t ion to u rban and me t ropo l it an sys tems 313

c ) , the method of analysis s imulates load current by the applicat ion of a l imitedfault of appropriate magnitude. The transformer and fr inge fuse currents thuderived are su bsequen t ly super imposed upo n the norm al loads a t these points tgive a true repres entat io n of the con dit io n, enabling poin ts o f excess loading to bidentif ied. Since the h.v. system is arranged in the form of a r ing main and supplyto all sub stat ions ma y be restore d, under single outage co nd it ion s, by switching, thsafe loading is that which can be sustained unti l switching is complete, a period ofsay, two hours .

Analysis o f the netw ork in th e w ay described enables it s performance undefaul t condi t ions to be determined wi th considerable accuracy and the few sourceof instabil i ty w hich m ay exist are readily identif ied. T hey can usually be remedie

by modifying the impedance pat tern of the network, i .e . by changing f r inge fusposit ion s or switching s ubstat ion s to oth er blocks. If this is no t possible the procedure ado pted is to forfe i t som e o f the suppor t capabi l i ty of the netw ork toensure that the protect ion discr iminat ion requirements are met under faul tcondi t ions .

16.4 .5 Su pply to la rge poin t loads

Large point loads due, for example , to major off ice premises f requent ly producedem ands in excess of 1000kV A. Such loads may be supplied from substa t ionsincorporated in the norm al way in to a d is t r ibuted block l.v. in terconnected sys tembut the arrangement tends to be uneconomic in respect of the aggregate capacityof connect ions to the l .v. network required to suppor t the in ternal load of thepremises when h.v. supply is lost . Also there may be diff iculty in locating anynearb y fringe fuses so as to avoid excess loading und er the sup po rt c ond it ion. Th esedrawbacks are overcome by instal ling two or more t ransformers each equipped wi thl.v. reverse pow er pro tect io n and supplied from a different h.v. feeder. Fig. 16.4.5 A,shows such an arrangement us ing two 750kVA transformers and the protect ivegear is s imilar to that already discussed. I t should be noted that the l .v. busbarforms a direct l ink between transformers on different feeders. Since this direct l inkexists in the posi t ion normally occupied by fr inge fuses i ts presence may affect thenu m ber of fr inge fu se s permissible elsewhere betw een the same two feeders.

Stabil i ty co nsiderat ion s m ay also l imit the nu m ber of fuses tha t can be instal ledbetw een the l .v. busbars and the external n etw ork.Where a nu m ber of large point loads are grouped together the netw ork m ay take

the form show n in Fig. 16.4.5 B. Small loads interposed betw een the m ajor, direct lyfed, loads are supplied by single l .v. distr ibutors laid from point to point and fusedat each end. Ult imate clearance of an l .v. cable fault now rests on two fuses onlyand the feeder stabil i ty considerat ions which relate to a similar fault under theinterconnected block condi t ion no longer apply.

The electr ical instal lations o f large poin t loads may include p lant driven by largeinduct ion motors . Vol tage depress ion on the sys tem supplying such motors causes



3 1 4 T h e a p p l i c a t io n o f p r o t e c t i o n t o u r b a n a n d m e t r o p o l i t a n s y ste m s

P r i m a r ys u b s t a t i o n

N o r m a l l y

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®A _ A

Serv i cest o b u i l d i n g

H . V. n e t w o r k

H . V. n e t w o r k E x t e r na ln e t w o r k



The appl icat ion o f protect ion to urban and m etropol i tan sys tem s 315

1 1 kV f eed e r s

L . V. n e t w o r k ~ -14 . r- - -" l -

Fig . 16 .4 .5B Supply to a group o f f our poi nt loadsN o t e : h.v. and I .v. swi tchgear omi t ted for c la r i ty

with t ime over several cycles. This current ma y cause un wa nted opera t ion o f locareverse-power pro tect ion if the vol tage depression is due to a faul t on feeders othethan those affording the su pply. Th e effec t i s dep end ent upo n both the size of thindu ction m ot or load and the character is t ics of the reverse pow er relay and iunl ike ly to cause problems i f the motor load i s less than about 400 h .p . Thecharac ter i s t ic of the re lay may be compared wi th the ca lcula ted charac ter i s t ic othe genera ted current and i t is poss ib le to de term ine the e xten t to wh ich the rela

needs to be de layed to prevent the effec t f rom taking p lace . Al ternat ive lyaddi t ional equipment may be ins ta l led to b lock re lay opera t ion whi le motor curren



31 6 The appl ica t ion o f prote ct ion to urban and m etropof i tan sys tems

in respect o f a faul t on one of the feeders affording supply, und er which condi t iofast clearance is desirable for motor s tabi l i ty reasons.

1 6 . 4 . 6 S u p p l y t o h .v. c o n s u m e r s

If supply is afforded to a consumer at high voltage from a single h.v. feeder and thh.v. feeder suffers a phase faul t , th e feeder is shut do w n and the co nsu m er lossupply; reverse-power re lays in a l l subs ta t ions connected to the feeder opera te anisola te any back -feed f rom the l .v. ne tw ork . I f the faul t is an ear th faul t , how evethe reverse-power re lays do no t op era te and the co nsum er ' s load is back-fed f rothe 1 .v. ne tw ork . The load the re fo re appears as an add i t iona l co m m i tm ent on th

l .v. network and is supplied from a heal thy source via fr inge fuses. Addit ional fr ingfuses to ca ter for th is load c ann ot b e provided because the to ta l nu m ber of f ringfuses is l im i ted by the nu m ber of t ransform ers feeding the l .v. ne tw ork , apreviously discussed.

The di ff icul ty may be overcome, a t the same t ime affording a h igher degree osecur i ty of supp ly to the con sum er, by taking sup ply f ro m two h .v. feeders anusing di rec t ional pro tec t io n . Fig . 16 .4 .6A shows such an ar rang em ent . The i.d .mrelay is se t to d iscr iminate w i th s imi lar re lays a t the pr im ary substa t ion and d etecfaul t current in e i ther d i rec t ion corresponding to a faul t on e i ther feeder. Threverse-power relay which has a s imilar characteris t ic to the relay used with thin tercon nected l .v. sys tems detec ts the d i rec t ion o f faul t current and i ts changeovecon tact d i rec ts t r ipping ac t ion to the c i rcuit breaker facing a faul ty h .v. cable . Thuthe supply to the consu m er is f irm against an h .v. cable faul t and the bu rde n o n thin terconnected l .v. ne twork is removed under feeder ear th-faul t condi t ions . I t mabe noted tha t a faul t on the h .v. busbars supplying the consumer resul ts in th

open ing o f one local c i rcuit break er on ly, faul t current is sus ta ined on the o thelocal c i rcui t breaker but c leared by opera t ion of the feeder swi tch a t the pr imarsubs ta t ion end .

An a l ternat ive method of in tegra t ing the suppl ies to h .v. consumers wi th the h .vnetw ork supplying an in tercon nected l .v. sys tem is to sup ply the h .v. consu m er oconsumers f rom a uni t protec ted h .v. ne twork and to provide a spur feed to subs ta t ions supplying the in terconnected l .v. sys tem from a c i rcui t breaker f i t ted wi ti .d .m . t , p ro tec t ion .

16 .5 P r iva t e gene r a t i on

In sys tems which supply urban and metropol i tan areas there may be consumerwh o ope ra te pr ivate genera t ing p lant . The machines are usually of the synchro noutype wi th capaci t ies in the range 300 to 3000kW, the smal ler s izes opera t ing a41 5 /2 40 V and the l arge r usua lly at l lkV . Th ey m ay opera te con t inuous ly i

para l le l wi th the publ ic supply or as s tandby equipment wi th changeover ofac i l i t ies for inf requent para l le l opera t ion . Since the consumer may be afforde



The app l ica t ion o f p ro tec t ion to u rban and me t ropo l i t an sys tems 317

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S u p p l y t o a n h v c o n s u m e r in a s y s te m w h i c h a l s o s u p p li es an i n t e r c o n n e c t e d



318 The appl icat ion o f protect ion to urban and m etropol itan systems

in terface betw een the publ ic supply and the genera t ion m ay take a var iety odifferent forms, too numerous for considerat ion here in detai l . There are howevepro tect io n problem s peculiar to the interface wh ich can be considered in generterms and which are separate from the specif ic generator protect ion considerat ion

dealt with in Chaper 12.I t is im po rtan t tha t the private generat ion should not jeopardise the publi

supply and equally that faults on the public supply should not interfere with thprivate generat ion. The presence of generat ion increases the fault level on thsystem and if generation is at high voltage this increase particularly affects thwhole of the netw ork on the pr ima ry substa tion c oncerned. Pract ica lly the wholof the genera tor ' s faul t current contr ibut ion appears a t the pr im ary substa t iobusbars with the result tha t this is usually the point of highest fault level on th e h.system. The effect needs to be determ ined to ensure that the fault rat ing of switcgear throu gho ut the sys tem is not exceeded.

The neutra l point o f the con sum er ' s sys tem is required to be ear thed . I f thconsumer 's generat ion is at high voltage, the system earthing of the public supply available during paral lel operat ion but the consumer must provide his own earth fper iods when independent opera t ion is taking place . This may be by means oswitched star point earthing but a preferable arrang em ent is the provision of a

ear th ing t ransforme r connected via a nonauto m at ic c i rcui t breaker to thconsum er ' s busbar and k ept perm anen t ly in c i rcuit . The t ransform er is required thave normal protect ion against internal faults under which condit ion isolat ion by tr ipping the switches control l ing sources of supply to the busbar to which i t conn ected. System e ar th faul ts m ay be detected by ear th-fault protect ion in thconnect ion between the t ransformer s tar point and ear th . Normal overcurrent anearth-fault p rotec t ion at the supply terminals is provided b y i.d.m .t , relays to safguard the public su pply against faults on the con sum er 's instal lat ion. This will alsrespond to phase and possibly, earth faults , in the reverse direct ion, i .e . on the incoming publ ic supply b ut m ay be suppleme nted w i th reverse-power protec t ionpolarising voltage being obtained from the voltage transformer provided fosynchronising purposes. The same source provides reference voltages for neutravoltage displacem ent p rote ct io n whic h is the principal protec t ion for earth faults othe incoming public supply. The object ive in respect of faults on the incominsupply is to clear such faults without affect ing the stabil i ty of supply to the con

sumer ' s load f rom his genera t ion but problems may ar ise due to the protect iobeing sensi t ive to voltage depression caused by unrelated faults on the public supplsystem.

Where gene rat ion is at low voltage the gene rator s tar point is conn ected direct lwi th the c onsum er ' s ear th e lec t rode sys tem and then to the ear th ing fac il ity associted wi th the incoming supply. The la t ter normal ly has a d i rec t connect ion wi th ths tar point of the secondary winding o f the incoming supply t ransformer. With th iarangement , res t r ic ted ear th-faul t protect ion may be appl ied to the genera tor i t sebu t for faults outside th e restr icted zone and a t the interface w ith the publisupply overcurrent protect ion by i d m t relay is usual Where there is more tha



The appl icat ion o f protect ion to u rban and m etropol itan system s 319

one generator three-wire machines may be used together with a stat ic balanceconnected to a busbar common to a l l machines , thereby providing a s ingle s tapoint which is earthed and effect ive for the complete system.

16 .6 Fu tu re t r ends

In the appl ication of protect ion techniques there is , in co m m on with other aspecof electr ici ty distr ibution, a need to practice due economy in affording a rel iablsupply to consumers .

Using data based upon comprehensive fault s tat is t ics available in the UK it iincreas ingly co m m on practice to apply m etho ds of cost benef i t analysis and sys te

rel iabil i ty evaluation to system design and protect ion in order to meet thiobject ive , and the t rend can be expected to cont inue. C oupled w i th these processexists the faci l i ty of carrying out analysis of networks for al l types of faults usincomputer programmes which may be readi ly updated to sui t changing networcondi t ions . The precise requirements of protect ion schemes may therefore bdetermined and thei r performance predic ted.

So far as equipment i s concerned the present t rend is towards the developmen

of solid state devices which duplicate the performance of their electromechanicacounterparts; a radical change is therefore taking place in equipment design, adist inct from i ts ap plicat ion. The solid state devices are more com plicated and havmore elements than their electromechanical equivalents but a part icular feature that their operat ing currents can be very much smaller. If advantage is to be takeof th is fea ture then a red uct ion in the s ize of ins t rum ent t ransform ers is a probablt rend fo llowed poss ib ly by the in t roduc t ion of new method s of de tec t ing the manitudes and phase angles of currents and voltages in the main conductors.

A logical extension of the applicat ion of solid-state techniques in complesystem s could lead to sub sti tut io n of individual sol id-state relays by integratesystems which receive al l necessary data and perform the appropriate protect ivfunct ions an d, in addi t ion such funct ions as indicat ion, a larm and meter ing.

16.7 Bibliography

StandardsESI Standard 12-8: T he appl icat ion of fuse-l inks to 1 lk V and 6-6k V /415 V dis t rbu t ion ne tworksESI Standard 37-2: MV dis t r ibut ion fuseboardsESI Standard 41-5: Indo or m etalc lad swi tchgear; ra t ings up to 250 M VA at 6-6kVand 350 M VA at 1 lk VESI Standard 41 -12: Non-extensible r ing main eq uipm ents incorp orat ing aautom at ic fuse-switch and swi tches; ra tings 250 MV A and 1 lk V and 150 MV A a6 .6kVESI Standard 48-2: Fa ul t passage indicators for 6-6kV and 1 lk V und ergroun d an



32 0 The application o f prote ction to urban and m etropofitan systems

Engineering Rec om m enda tion G 26 " The installation and o perational aspects private generating plant (The Electricity Council)

A r t i c l e

'Application of test results to the calculation o f short-circuit levels in large industrisystems with co ncen trated ind uction m otor loads' by C ooper, Maclean and WilliamProc. IEE 1969, 116, (11)



Th e app lica t ion o f p ro tec tion to

t r ansmis s ion sys t emsb y J . C . W h i t t a k e r

Chapter i7

17.1 Genera l p r inc ip les of appl ica t ion of pro tec t ion to t ransmiss ion sys tems

17.1.1 I n t r o d u c t i o n

This cha pter descr ibes the appl ica t ion to t ransmiss ion sys tems of the varioupro tec t ion sys tems descr ibed in prev ious chapters , and g ives the reasons for

choos ing a par t icu lar p ro tec t ion sys tem for a g iven appl ica t ion . T he cha pter is baseon the prac t ice fo l lowed in the U ni ted K ingdom . This prac t ice , wh ich is con s tan t lychanging as new requi rem ents a r ise and as new pro tec t ion sys tems becom e ava i lab li s in f luenced not on ly by technica l and economic cons idera t ions but a l so byhis tor ica l fac tors ; i t i s a lso based on the p ro tec t io n requi rem ents for so l id ly ear thedt ransmiss ion sys tems, such as ex is t in the Uni ted Kingdom. The de ta i led pro tec t ionrequi rem ents for o th er types o f power sy s tem , for exam ple for a t ransmiss ionsys tem ear thed through arc suppress ion co i l s , would be apprec iab ly d i ffe ren t f rom

those descr ibed in th i s chapter, a l though m any of the bas ic requi rem ents wo uld bs imi la r. Compar ison of the prac t ice in the Uni ted Kingdom wi th prac t ice in o thecountr ies br ings to l ight many interest ing s imilar i t ies and differences, but i t ibeyond the scope of th i s chapter to a t tempt to expla in the reasons for the d i f -fe rences , o r to descr ibe in de ta i l the m any d i ffe ren t techniques used overseas .

17 .1 .2 Sys temdes ign cons ide ra t ions

I t is essent ia l that there should be close l ia ison between the system design engineeand the pro tec t ion engineer a t a very ear ly s tage in the des ign of a power sys temW ith h igh fau l t l eve ls , for exam ple 35 000 M VA at 40 0k V , one of the m ain l imi tat ions in system design is the speed at which faul ts , in par t icular three-phase faul tscan be cleared from the system. This faul t -c learance t ime governs the s tabi l i ty l imiof the pow er sys tem w hich in turn governs it s des ign . For exam ple , if there a re teedfeeder c i rcui ts , faul t -c learance t imes may be appreciably increased, and the saving ithe cos t o f c i rcu i t b reakers as a resu l t o f tee-connec t ing c i rcu i t s may have to be



32 2 The app lication of pro tectio n to transmission systems

S t a t i o n S t a t i o nA B

S t a t i o n C

Fig. 17.1 .2A T r a n s f o r m e r t e e c o n n e c t e d v ia a c i r c u i t b r e a k e r t o a fe e d e r

offse t b y the sys tem des ign engineer aga ins t the cos t o f re inforc ing the pow ersys tem to ensure tha t i t can wi ths tand the longer fau l t -c learance t imes .

No rm al ly, the sys tem des ign engineer assum es tha t the op era t ion of a ll p ro-tec t ion on the sys tem wi l l be fu l ly d i sc r imina t ive ; bu t i t is essen t ia l tha t he checkwi th the pro tec t ion engineer tha t d i sc r imina t ion i s feas ib le wi th the par t icu la rc i rcu i t conf igura t ion he proposes . The impor tance of any pro tec t ion d i ff icu l t ies canthen be assessed toge ther wi th the cos t o f overcoming these d i ff icu l t ies . A s impleexam ple o f th is i s a 120 M VA t ransform er tee-con nec ted v ia a c i rcu i t b reak ermidway a long a 257 kV l ine (F ig . 17 .1 .2A) . As wi l l be apparen t l a te r in th i scha pter, i t can be very d i ff icu l t and cos t ly to provide a fu l ly d i sc r imina t ive h ighspeed fau l t c learance for such a c i rcu i t . The par t icu la r fau l t s w hich a re d i ff icu l t toc lear d i sc r imina t ive ly a re those occu r r ing be tw een the t ransfo rm er h .v. c i rcu ibreak er and the t ransform er h .v. bush ings , as these fau l ts ma y be wi th in the f ir szone reach of any acce le ra ted d i s tance pro tec t ion ins ta l led a t s ta t ions A and B.

1 7 . 1 .3 F a c t o r s w h i c h i n f lu e n c e t h e c h o i c e o f p r o t e c t i o n

The fo l low ing a re some of the fac tors involved in choos ing a su i tab le pro tec t ionscheme for a g iven appl ica t ion .

17 .1 .3 .1 P lan t to be pro tec ted : The f irs t factor to b e con sidered in cho osing asu i tab l e p ro t ec t ion scheme i s t he n a tu re o f t he p l an t t o be p ro t ec t ed , fo r exam plgene ra to r, t r ans fo rme r, f eede r e t c .

The de t a il ed cha rac te r is t ic s o f t he p l an t t o be p ro t ec t ed m us t t hen be examinedto de te rmine i f any spec ia l cons idera t ions apply in choos ing a su i tab le pro tec t ionscheme. As an example , a feeder may cons is t o f a l ength of overhead l ine connec ted in se ries w i th an und ergroun d cab le . Not a ll p ro tec t ion schem es su i tab le fopro tec t ing overhead l ines a re necessar i ly su i tab le for a combina t ion of overhead

l ine and cab le , fo r exa m ple the re la t ive ly low imp edance of the cab le m ay be such



The application o f protection to transmission sys tem s 323

tha t the e ffect ive length of the feeder i s too shor t for the sa t i s fac tory em plo ym enof d i st ance p ro tec t ion .

1Z 1.3 .2 Probability o f various types o f fault:The var ious types of fau l t which

may occur on the p lan t to be p ro tec ted shou ld nex t be cons ide red , t oge the r wi tthe proba bi l i ty of such fau lt s occurr ing .

For exam ple , on a t ransform er c i rcu i t the m ajor i ty of fau l ts wi th in the t ransform er tank are ear th fau l t s , the r isk of a phase- to-phase fau lt c lear of ear th wi th ithe t ank be ing low. Breakdown o f the t r ans fo rmer co re in su la t ion , and in t e r tu rnfaul ts a lso occasional ly occur.

Ear th- faul t p ro te c t ion is therefore essentia l, and B uchholz pro tec t ion is requi reto de tec t core fau l ts and in te r turn fau l ts wh ich are c lear of ear th .

Fas t and fu l ly d iscr iminat ive phase-fault p ro tec t ion , for exam ple b iasedharm onic res tra ined overal l d i ffe rent ia l pro tec t ion , can o f ten only be jus t if ied tprovide phase- faul t p ro tec t ion for the connect ions to a t ransformer. These connect ions, unless they are phase segregated, for example consis t ing of s ingle-corcables ra ther than open copperwork , may present an apprec iable phase fau l t r i sk .

Another example i s on a genera tor not only can ear th fau l t s and phase fau l toccur, bu t the genera tor can be severe ly damaged by negat ive phase sequencecurrents . These cur rents a r i se f rom e i ther uncleared unbalanced fau l t s on th

sys tem or f rom s ingle or two-phase open c i rcu i t s . The la t te r may resu l t f rom c i rcu i t b reaker fa i l ing to make or break on a l l i t s th ree phases , or f rom brokejum pers on a line . These r i sks, and the i r resu l tan t e ffec ts on the gene ra tor, have tbe assessed and i t is usual ly found to be necessary to provide negat ive phassequence pro tec t ion for the genera tor.

1 Z1 .3.3 Loa d and fau lt currents:T h e m a g n i t u d e o f t h e m i n i m u m a n d m a x i m u mfaul t cur rents a t appropr ia te poin ts on the sys tem must be es tab l i shed in order tde termine the sens i t iv i ty and s tab i l i ty requi rements for the pro tec t ion . The maxim um expected poss ib le cont inu ous load cur rent m ust a l so be kn ow n as th is wide te rmine the r equ i red con t inuous the rma l r a t ing o f the p ro tec t ion . I t m ay a lsaffec t the m inim um permissib le phase- faul t se t t ing , and the se t t ings of any ovecurre nt , ra te of change of cur ren t , o r d is tance s ta r t ing equipm ent .

The charg ing cur rents of ove rhead l ines and cables m ust a lso be taken in toaccount , as they may inf luence the minimum permiss ib le pro tec t ion se t t ings .

17.1.3.4 Vo ltage and current ratings o f protected plant:Not on ly shou ld thenorm al co n t inuous vo lt age and cu r ren t r a tings o f the p ro tec ted p lan t be t aken in taccoun t in choos ing the p ro tec t ion r equ i red , bu t any spec ia l r equ i remen t s such ashor t t ime over load ra t ings , m ust be co ns idered .

As an example , i t may be permiss ib le to over load a power t ransformer for a fewminu tes wi th sa fe ty, s ay to 50% overload , t hus necess it a ting a m in imu m overcurren t p ro tec t ion se t ting g rea te r than 150% of the r a ted cu r ren t o f the t r ans fo rmer.

Another example of a shor t - t ime over load ra t ing requi red for a few minute



324 The application o f protect ion to t ransmission system s

occu rs in the case of cer tain overhead-l ine circui ts w here i t is desirable to ut i l ise thshor t - t ime thermal capabi l i ty of the overhead l ine to enable quick s ta r t ing gasturb ine or pumped-s torage p lant to re l ieve the over load condi t ion .

For some uni t sys tems of pro tec t ion , par t icu lar ly those u t i l i s ing Pos t Off ice

pi lo ts , i t i s h ighly undes i rab le tha t cont inuous compar ison of the cur rents a t eacend of the pro tec te d c i rcu i t be perm i t ted under expec ted load cond i t ionsotherwise a m om en tary in te r rup t ion of the s ignall ing channel m ay resu l t in inadver ten t t r ipping and s ta r t ing ( fau l t .de tec tor ) re lays a re therefore inc luded to cont rot ransmiss ion . Fo r pow er l ine car rie r sys tem s the con t inuo us transmiss ion of car r ies ignals m ay a lso cause u naccep table in te r ference to be rad ia ted in f requ enc y bandreserved for norm al ly quiescent car r ie r. Fu r the rm ore , some p ow er line carr ie r t ranmi t te rs may have no cont inuous thermal ra t ing but only a shor t - t ime ra t ing of a few

minu tes .In addi t ion to the shor t - t ime over load ra t ing , the pro tec t ion sys tem i .e . bo th th

re lays and assoc ia ted cur rent t ransformers , must be des igned to wi ths tand , botthe rma l ly and dynam ica l ly, t he m ax im um fau lt cu r ren t w h ich can f low in thepr ima ry p lant i t is pro tec t ing . T his shor t t im e faul t cur ren t ra t ing is norm al ly for 3 or 1s r a ted d ura t ion . The cho ice o f t ime depen ds on the exp ec ted m ax im um fau lclearance t imes. W here on ly l s ra ted eq uip m en t is avai lable , i t m ay be necessarto invest iga te the therm al t ime con s tant of the pro tec t io n bear ing in m ind tha t i t

very unl ike ly tha t the magni tude of the fau l t cur rent can in prac t ice remain a t i tm ax im um value fo r the to t a l du ra t ion o f the f au lt .

1 Z1 .3.5 Necessi ty or otherwise fo r high speed operat ion:T h e m a x i m u m p e r m i s .s ib le dura t ion of fau l t cur ren t i s governed by tw o fac tors :

( a )( b )

l imi ta t ion of damage to p lan teffec t of long fau l t -c learance t imes on the sys tem which i s feeding the fau lcur rent .

The effec ts of long fau l t -c learance t im es m ust be assessed , not only wi th regard tthe damage a t the poin t of fau l t , for example an o i l f i re may be unnecessar i ls ta r ted in a fau l ted t ransformer, but the p lan t feeding the fau l t may a lso bedam aged by the fau l t cur re nt , for exam ple a cable throu gh wh ich the fau l t cur renpasses ma y be dam aged.

Fast faul t -clearance t imes espec ial ly for three-phas e faul ts m ay be essent ial tm ain ta in sys tem s tab i l i ty. On the Br i ti sh Supergrid sys tem s tab i l i ty s tudies indica ttha t a t som e 400 k V s ta t ions sys tem s tab i l ity wil l be end angered i f a c lose-up threephase fau l t is a l lowed to persist for m ore than 80m s. Fast c learance t ime s are alsrequi red on p lant feeding la rge induct ion motor loads (e .g . a t s tee lworks or o iref iner ies) . The reason for this is that a prolonged dip in vol tage may resul t in thinduct ion motors s lowing down to such an extent tha t , when the fau l t i s c leareand n orm al vol tage is res tored , the m otor s draw such a h igh cur ren t a t tem pt ing tacce lera te tha t i t p rodu ces a cor respo ndingly la rge vol tage dro p be tw een the suppl



The application o f protec tion to transmission sys tem s 325

and the m oto r te rm ina ls . This reduct ion in vol tage m ay resu lt in the m otors be inunable to produce enough torque to rega in the i r normal opera t ing speed .

On the o the r han d , unnecessar ily fas t p ro tec t ion should no t be spec i fied as , w i tcer ta in except ions , the fas te r schemes a re more expens ive , more complex and , i

many cases , more l iab le to malopera te . For example , d i s tance pro tec t ion which haan opera t ing t ime of under 10ms usua l ly requi res more fa i thfu l vo l tage inputs thaare norm al ly obta inable f rom m ost des igns of capac i tor vo l tage t ransformers .

In quo t ing pro te c t ion opera t ing t imes for assessing the i r e ffec t on sys tem performance i t has to be borne in mind tha t the t imes quoted for a g iven pro tec t ionsys tem can usua l ly only be app rox im ate , as so m any fac tors in f luence the ac tuaopera t ing t im e obta ined on an ind iv idua l fau lt .

Some of the pr incipal factors involved are:

( a )

b )c )

(a)

(e)

(,0

(h)

the m agni tude of fau lt c ur ren t (usua lly expressed as a m ul t ip le of the se t tingof the p ro tec t ion )the type of faul t , e .g . phase- to-ear th , phase- to-phase or 3-phasethe ra t io o f the im pedance of the source of the fau l t cur ren t to the im pedanc ebetween the re lay ing poin t and the poin t of fau l tt he m agn i tude o f t he d . c . com pon en t p re sen t i n the f au lt cu r ren t , and theprefau l t magnet i sa t ion s ta te of the cur ren t t ransformers

the magn i tudes and phase ang le s o f ha rmon ic componen t s and nons inuso ida lwavefo rmsthe m agni tude and d i rec t ion of the load cur ren t f lowing im m edia te ly pr ior tothe faul tthe resis tance of the faul t i tsel fthe charac te r is t ics o f any s igna ll ing channels used be tw een the loca l andrem ote c i rcu it b reakers . T he charac te r i st ics of the chann el m ay be appre-c iab ly modi f ied by t rans ien t e lec t r ica l in te r fe rence in the channel a t the t ime

of the faul t , including interference from adjacent s ignal l ing channels .

W i th an increas ing a m ou nt o f genera t ion be ing conn ec ted in to the t ransmiss ionsys tem a t h igher vo l tages , i . e . the genera tor / t ransformers be ing swi tched a t 400 and27 5k V ins tead of 132 or 33k V , the ra t io of reac tance to res i stance of the sourceimpedance for fau l t s on the t ransmiss ion sys tem is s teadi ly increas ing .Thisresultsin a h igher probabi l i ty of a la rge d .c . component in the fau l t cur ren t when thefaul t occurs a t o the r tha n a t the m axim um poin t -on-wave of the vol tage . Hence

i tem (d) in the l is t i s beco m ing of increas ing im por tan ce becau se , i f the m ain orin te rpos ing cur ren t t ransformers sa tura te , very long opera t ing t imes may occur.

17.1.3.6 Imp ortanc e of security of supply:To ob ta in fu l ly d i sc r imina t ive p ro -t ec t ion fo r a l l t ypes o f f au l t may no t a lways be economica l ly ju s t i f i ed , depend ingon the impor t ance o f t he load be ing supp l i ed . Fo r example , i n t he a r r angemen tsho w n in Fig. 17.1.3A , the l .v. busb ars are suppl ied solely from the tw o t rans-formers and the o nly form of phase fau l t p ro tec t ion is inverse def in i te m inim um

time ( i .d .m. t . ) overc ur ren t pro tec t ion . I f a phase- to-phase fau l t c lear of ear th occurs



3 2 6 T h e a p p l i c a t io n o f p r o t e c t io n t o t r an s m is s io n y s t e m s

- , , , ,,, - H .V . bu sb ar

If i ~ 0 ( ,

T l

i e to phase fau lt

L . V. b u s b a r

L o a d

Fig. 17.1.35o, Nondiscr iminat ive t r ipping for t ransformer I .v. fau l t

i 1 " H .V. bu sba r

T I T 2

e to phase faul t

~ o ~

1 T- i ' .... , , _ , L . V . b u s b a r

L o a d

Fig . 17 .1 .3B Discr iminat ive t r ipping for t ransformer I .v. fau l t




on the 1 .v. conn ec t ions of t ran sform er 2 , the fau l t cur ren t i s suppl ied equal ly bybo th t ransform ers and henc e , neglec t ing load cu r ren ts , the l .v. overcu r ren t re lays ont ransform er 1 and t ran sform er 2 wi l l opera te in the same nom inal t ime, t r ipp ing thl .v. c i rcu it b reake rs o f bo th t ransfo rm er 1 and t ransform er 2 . A ny load cur ren t wi l

tend to cause t ransform er 1 re lay to op era te before t ransform er 2 . In add i t ion , thh .v. overcu r ren t pro te c t ion wi l l ope ra te on t ransform er 2 thus c lear ing the fau l tA ny load con nec ted to the l .v. busb ar would thus lose i ts supply.

If , on the other hand, in addi t ion to or in place of the l .v. inverse def ini temin imum t ime ove rcu r ren t p ro t ec t ion , d i r ec t iona l l . v. ove rcu r ren t p ro t ec t ion o rt ransform er overa l l d i ffe ren t ia l p ro tec t io n were provided , e i ther of these formof pro tec t ion would opera te before the i .d . rn . t , overcur ren t pro tec t ion and only thfau l t ed t r ans fo rmer wou ld be t r i pped , t he reby ma in ta in ing the supp ly to the load

(Fig. 17.1.3B).

17.1.3 .7 C ompa tibility with existing protection:When choos ing a pro tec t ionscheme and i ts se t tings due regard m ust be pa id to the charac te ri s tics of an y ex is tinpro te c t ion on the pow er sys tem. I f the se t t ing of a d i s tance pro te c t ion on the t ransmiss ion sys tem has to be such tha t i t can ' see through ' the s tep-down t ransformerand thus respond to fau lt s on the d is t r ibu t ion sys tem , i ts t ime se t ting m ust be h igenough to d i sc r imina te wi th the p ro tec t ion on the d i s t r ibu t ion sys tem.

17.1.3.8 Availability of signalling channels:In t ransmiss ion ne tworks wheresubs ta t ions a re separa ted by only a few mi les , as in London, the most sa t i s fac torand econom ic p ro tec t ion fo r tw o~ nd ed f eede rs is o f t en to e m ploy p r iva te p i lowire protect ion ut i l is ing pairs of wires in a network of pr ivate pi lot cables . I t iimportant to arrange that suff ic ient spare pairs are provided in the pi lot cablew hen the y are or ig ina l ly laid to ensure th a t an a dequ ate nu m ber o f pa ir s is ava ilab

to meet a l l expec ted fu ture needs .Where pr iva te p i lo t s a re not ava i lab le , i t i s usua l ly economic in the UK to h i rep i lo t s from the te l ecom m unica t ion com pan ies fo r t he p ro tec t ion o f two -endedfeede r s . However, s ince 1978 , t he t e l ecommunica t ion compan ies have in gene rawi thd raw n the f ac i li t y fo r t he h i r e o f m e ta l li c c i rcu i ts hav ing m ax im um dec la redres i s tance and capac i t ance va lues . Hen ce m any o f t he d i f fe ren t ia l and ph asec o m p a r i s o n p r o t e c t i o n s y s t e m s d e s i g n e d t o c o m p a r e f u n d a m e n t a l f r e q u e n c yquan t i t i e s ove r t e l ecommunica t ion company me ta l l i c p i lo t s a r e no t su i t ab le fo

m os t app l i ca tions , and p ro tec t ion des igned fo r u se wi th vo ice f r equency s igna l l insys t ems i s u sed in these cases . Power- l ine -ca r r i e r channe l s can o f t en beecon om ica l ly ju s t if i ed , bu t the f r equency bands ava i l ab le to the Supp ly Ind us t ryfor th i s form of s igna l l ing , a re rap id ly used up and d i ff icu l ty may of ten beexper ienced in a l loca t ing su i tab le f requencies w i thou t the r isk of caus ing in te rfe rence in ad jacent channels , a l though be t te r u t i l i sa t ion of the ava i lab le f requencband s is ach ieved by insta ll ing car rie r equ ipm ents w hich requi re na r row er s igna ll inband wid ths than h i the r to . In a llocat ing f r equenc ie s , no t o n ly m us t a l lowance b

m ade for an adeq uate separa t ion be tw een s igna ll ing f requencies on l ines a t the sam



32 8 The app lication o f pro tection to transmission systems

vol tage , but cognisance must be taken of any mutual coupl ing of car r ie r s ignalbetween l ines of different vol tages, for example where carr ier s ignal l ing is used o275k V and 400k V lines wh ich fo l low the same rou te .

M icrowave s ignall ing channels m ay be jus t i f ied econo m ica l ly and techn ica l ly

bu t the requi rem ent for microwave l inks tha t repea ter s ta t ions must be wi th in linof s ight of each other severely restr ic ts their appl icat ion. Even greater restr ic t ionin the Uni ted Kingdom are the requi rements to obta in a l icense to opera te a microwave l ink and to obta in p lanning permiss ion for the e rec t ion of microwave aer ia land towers . The use of f ibre-opt ic s ignal l ing channels , which incidental ly arimmune to e lec t r ica l and magnet ic in te r ference problems, a re l ike ly to be used ifuture as pro tec t ion s ignal ling chan nels , par t icular ly i f the f ibre-opt ic cable can bsa t i s fac tor i ly inco rpora ted as par t of the ear thwire of an overhead- line c i rcu it .

1 Z1 3 9 Cost: Where several protect ive schemes meet the specif ied requirements ,the cheapes t schem e w hich meets the spec i f ica t ion wou ld norm al ly be chosen . Icost ing the schemes, care has to be taken to include al l the relevant costs , foexam ple capi tal ised cos ts of p i lo ts , m ain tenance cos t , e tc . (See Sec t ion 17 .5).

The cos t of a par ticu lar scheme m ay be influenced b y the need for o the rfac i l i t ies such as cont ro l , meter ing or te lecommunica t ion requi rements . The ins tala t ion of pr iva te p i lo t cables for example , may not be economic for pro tec t ion

purposes only, but may be jus t i f ied i f a l so requi red for in te r t r ipping , cont ro lmeter ing and te lephones .

1 7 . 2 M a i nand back-up protect ion and locat ion of current t ransformers

17 .2 .1 Ma inand back-up protect ion

Main pro tec t ion is def ined as tha t pro te c t ion wh ich is norm al ly expected to in i t ia tequickly and d iscr iminat ive ly, the t r ipping of appropr ia te c i rcu i t b reakers to c lear fau lt f rom the sys tem . B ack-up pro tec t io n is the pro tec t ion which is in tended topera te in the event o f a fa i lure or inabi li ty of any main pro tec t io n to c lear thfault .

The nu m ber and types o f m a in and back-up p ro tec t ion sys t em s to be ins t a ll ed ona g iven f eede r o r i tem o f p l an t depen ds on t echn ica l and econom ic con s ide ra t ionsAs an example , on the UK e lec t r i c i ty sys t em i t has been found , based on cos t -bene f i t ana lyses , t ha t t he p rov i s ion o f some measure o f r edundancy fo r the ma inpro tec t ion on the 400 and 27 5kV sys tem s can be econ om ica l ly ju s t if i ed ; bu t no t

excep t in excep t iona l c i r cums tances , fo r the 132kV sys tem. These ana lyses t akacc oun t of the es t imate d c os t to the na t ion of a loss of supply, i.e . no t only the losto the supp ly au thor i ty o f i t s s a l e s r evenue bu t a l so the cos t t o consumers . Fopro tec t ing the 400 and 275kV sys tems the gene ra l p r inc ip le adop ted i s t ha t noc red ib le de fec t i n a s ing le componen t o r secondary c i r cu i t o f the p ro tec t ion ot r ipp ing sche m es sh ou ld r e su l t in the nonc lea ranc e o f a p r im ary f au l t from thsys tem.

Back-up p ro tec t ion fo r the 400 ,275 and 132kV sys tems i s u sua l ly o f the non

uni t type , e .g . def in i te minimum inverse t ime overcurrent or ear th- faul t p ro tec t ion



The application o f prote ction to transmission sys tem s 329

or somet imes in spec i f ic appl ica t ions d is tance pro tec t ion . In order to reduce apprec iab ly the r isk of the back-up pro te c t ion be ing ca lled on to ope ra te a cons iderable measure of redundancy i s provided in the main pro tec t ion a r rangementsAlso most 400kV and 275kV c i rcu i t b reakers a re equipped wi th c i rcu i t -breaker fa

p ro tec t ion .

17.2.1.1 Main protection: In order to ensure fast , d iscr iminat ive and rel iable mainpro tec t ion where more than one ma in p ro tec t ion sys t em is p rov ided they shou lbe as dissimilar as possible.

For example , in the case of feeder pro tec t ion :

a )

b )

( c )

(d)

(e)

The f i r s t and second main feeder pro tec t ion , where prac t icable , should u t i l i s

en t i re ly d i ffe ren t pr inc ip les , p referab ly a d i ffe ren tia l sys tem and a d is tancesys tem.The f i r s t and second main pro tec t ion should opera te in to independentt r ipp ing sys tems. (F ig . 17 .2 .1A ) .I f two d is tance pro tec t ions a re used , where poss ib le one should be of theblocked type and the o ther should e i ther acce le ra te or in te r t r ip .Th e f i rs t and second m ain pro tec t ion should preferab ly em ploy d i ffe ren ttypes of s ignal l ing channel , e .g . power- l ine-carr ier and Post Off ice pi lot , bu

where this is not possible and pi lot c i rcui ts must be used, the two channelsshould be separa te ly routed .Two d i ffe ren t manufac turers should provide the f i r s t and second mainpro tec t ion .

N o . I d . c .s u p p l y

N o i d . c .s u p p l y

N o . 2 d . c .s u p p l y

_ I- - - I

mI

l i l

: :L :a o nIIII

I

alz-

II

ch

l N o . 2 d . c .S e c o n d m a i n s u p p l yp r o t e c t i o n - - ,,,

III

,,

II

I

F i g . 1 7 . 2 . 1 A Provis ion of two d iscre te t r ipp ing sys tems

IIIIIIIIII

IIIII N o . ] @

Tr i p Ico i l s I

I N o . 2r y ' y ~ e



33 0 The application o f pro tec tion to transmission systems

For main p lan t and i t s assoc ia ted connec t ions fu l l dupl ica t ion of the pro tec t iom ay not be technica l ly or ec ono m ica l ly jus ti f iab le in which case , e .g . for t ransform ers or generator~,, th=~ pro tec t ion of each i tem of plan t can be g roup ed into twgroups of du pl ica te or supp lem enta l re lays , each group be ing fed f rom sep ara te d .

suppl ies in to tw o segrega ted t r ipp ing sys tems.

17.2.1.2 Ba ck upprotection: Inverse def in ite min im um t ime overcur ren t pro tec t ionis usua l ly provided as back-up pro tec t ion for feeders , t ransform ers an d genera torsbut i t suffe rs f rom the d isadvantage o f having re lat ive ly long opera t ing t imes , whicm ay be unacce ptab le f rom sys tem s tab i l i ty or large indu ct ion m oto r load con

siderat ions. I .D.M.T. re lays of e lectromechanical design are cheap, s imple andrel iable but re lays of s ta t ic design may have more f lexible character is t ics , e .g . choice of var ious inverse t ime character is t ics .

Faul t c learance may a l so be nondiscr imina t ive wi th back-up overcur ren t protec t ion . For example , F ig . 17 .2 .1B represents a smal l par t o f an in te rconnec tednetwork . In such a ne twork , as the fau l t cur ren t may f low in any d i rec t ion depending on the fau l t loca t ion , i t i s imposs ib le to grade sa t i s fac tor i ly the back-upoverc ur ren t pro te c t ion se t tings on feeders, and i t is usua l , in the absence o f spec if i

opera t iona l requi rem ents , to g ive the re lays the same nom inal se t tings .

x ,x /

x /

1 fI II II i

I S t a ti o n S t a t io n S t a t i o n III A B C I| , = _ _

I L A ~ I L A ~ ~ - - - . ~ = ~ - - - L o a d

FA lt.'A

11."A 1 FC

J

I L A = L o a d c u r r e n t f r o m s t a t i o n A

I FA = F a u l t c u r r e n t f r o m s t a t i o n A

I F C = F a u l t c u r r e n t f r o m s t a t i o n C

F i g . 1 7 . 2 . 1 B N ondiscr iminat ive c learance o f fau l t by Ld .m. t . overcurrent pro tec t ion



The app lication o f pro tec tion to transmission systems 331

Assume in t he exam ple chosen f au lt cu r r en t can on ly be f ed f rom s t a ti ons A anC and an ear th fau l t occu rs on feeder BC c lose to end C. I f the ma in pro tec t io n oth is feeder fa i l s to opera te , the back-up overcur ren t re lays a t s ta t ions A, B, and Cmeasure the fo l lowing cur ren ts on the fau l ted phase :

A t s ta t ion A , feeder AB overcur ren t re lay m easures fau l t cur ren t ( IRA) + loadcur re n t fed f rom end A(ILA ) .At s ta t ion B, feeder BA o vercur ren t re lay me asures fau l t cur ren t ( IFA ) + loadcur ren t fed f rom end A(ILA ) .A t s ta t ion B, feeder BC overcu r ren t re lay m easures fau l t cur ren t fed f romend A(IFA) .At s ta t ion C, feeder CB overcur ren t re lay measures fau l t cur ren t fed f rom

end C( IFc ) .

I t is app aren t tha t the fau l t wi l l be c leared a t s ta t ion C by the re lay on feeder CBI t wi l l a lso be c leared a t s ta t ion A or B, o r b o t h , by the re lays on feeder AB. Botof these re lays m easure the same fau l t and load c ur ren t and hence wi l l have thsame nomina l opera t ing t imes . There wi l l therefore be a to ta l loss of supply as ta t ion B.

The cur ren t se t t ing on a back-up overcur ren t re lay i s governed by

( a )(b)

t he m in im um fau l t cu r r en t fo r wh ich the r e l ay is r equ ir ed to ope ra t ethe m ax im um load cu r r en t t he c ir cu it conce rned is r equ i red to ca r ry unde remergency cond i t i ons .

I t m ay h appen tha t t he s e t ti ng r equ i red by con d i t i on (b ) is g rea te r t han the s e t t inrequi red b y c ond i t ion (a ) , in w hich case the r isks involved m ay have to be ac cep ted

In the U ni ted K ingdom the usua l po l icy, in order to avoid the r i sk of cascadet r ipp ing in the t ransm iss ion s ys tem , is tha t the overcu r ren t re lays a re se t above thm a x i m u m l oa d c u r r e n ts w h i c h a r e to b e e x p e c t e d u n d e r e m e rg e n c y c o n d it io n s .

One m etho d of a l lev ia t ing th i s l imi ta t io n of overcu r ren t p ro tec t io n , a t leas t afa r as ear th fau l ts a re con cern ed , is to conn ec t res idua l ly one e lem ent of a th reee lement overcur ren t re lay (F ig . 17 .2 .1C) . This res idua l ly connec ted e lement canthen be g iven a lower se t t ing than the phase connec ted e lements as i t does no trespond to ba lanced load cur ren ts . The choice of a su i tab le se t t ing for the res idua l lyconnec ted e lement i s , however, d i ff icu l t as the se t t ing has to be graded not on lyw i th any o the r r e s idua l ly conne c ted ove rcu r ren t r e lay e l emen t s on the sys t em, bua lso w i th any d i s tance p ro t ec t ion on ad j acen t c i r cu i ts . Fu r the rm ore , t he m in im umfau lt cu r r en t t o be m easu red by the r e lay m ay va ry be tw een wide l imi t s ove r ape r iod o f 24h owing to changes in t he sys t em ze ro -sequence imped ances . Acom m on cause o f t hese va r ia t ions i s t he swi tch ing o f gene ra to r t r ans fo rmers i n andou t of se rvice a t genera t ing s ta t ions .

D i s t ance p ro t ec t ion w i th i ts 2nd zone and 3 rd zone r eaches i nhe ren t ly p rov ides

a back -up p ro t ec t ion , bu t t he e ffec t ive r eaches o f t he p ro t ec t ion a re m uch r educed



3 3 2 T h e a p p l ic a t i o n o f p r o t e c t io n t o t ra n s m is s io n y s t e m s

J

B, ~ r y y ~

( ) v e r c u r r e n trelay

Ni • | i i i i

N o r m a l 3 p h a se c o n n e c t i o n o f 3 e l e m e n t o v e r c u r r e n t r e la y

Y , ,, 1

B

v e r c u r r e n t

r e l a y

R e s id u a l c o n n e c t i o n o f o n e e l e m e n t o f 3 e l e m e n t o v e r c u r r e n t r ela y

F i g . 1 7 , 2 . 1 C O v e r c u r r e n t e l e m e n t c o n n e c t i o n s

by any fau l t cu r ren t in feeds be tween the measur ing po in t and the po in t o f f au l t .F ig . 17 .2 .1 D i l lus t ra tes th is effec t . A fault in th is typica l ex am ple is assum ed tohave occur red near to s t a t ion F on a feeder be tween s t a t ions E and F and the feederc i rcui t breaker a t E has fa i led to t r ip . The apparent d is tance measured by thed i s tance p ro te c t ion on the va r ious feeders i s ind ica ted by the f igures enc losed bybox es . I t wi ll be no te d tha t on feeder D-E a t s t a t ion D the fau l t i s apparen t ly 376krnins tead o f 91 + 12 = 103 k rn away, and a t s t a t ion A the fau l t i s apparen t ly 1195 kin



The application o f prote ction to transmission sy stem s 333

F i g . 1 7 . 2 . 1 D

1-18 51

5 5

i iI , 9 5 t -

58

1" 1 3 7 6 ]

91

Fault

Di | °

Ap pare n t f au l t d i s tance as measu red by r emo te d i s t ance p ro t e c t i on

away ins tead of 149k m . In prac t ice , mo re adverse s i tua t ions can eas ily occur and igenera l d is tance p ro tec t ion is unl ike ly to provide any apprec iable measure of backup p ro tec t ion fo r the sys tem w hen i t has to m easure be yon d the busba r s o f a ma jopow er s ta t ion or any o ther sub s ta t ion w i th a m ajor infeed . Occas ionally, d i s tancprote c t ion has been ins ta l led as sys tem back-up pro tec t ion in i ts own f ight , in p lacof inverse-defin ite m inim um t ime ov ercurrent pro tec t ion but i t s predic ted peformance has not been sa t i s fac tory owing main ly to the reach shor ten ing effec t .

A noth er form of back-up pro tec t ion appl icable to res is tance ear thed sys tem s is tan db y ear th- fault p ro te c t ion . This pro tec t ion sys tem cons ist s of a cur rent t ransformer, the pr imary of which i s connected in ser ies wi th the neut ra l ear th ingres is tor, and a re lay which has a long opera t ing t im e, usua l ly up to 30s . The s tan dbear th- faul t p ro tec t ion opera tes in the event of any uncleared ear th fau l t s on thsys tem and, where opera t ional ly advantageous , i t s t r ipping can be made in twos tages . Fo r exam ple , the f ir s t s tage t r ips the 1 .v. c i rcu it b reaker of the t ransform erand i f the faul t current persis ts , the second s tage, af ter a fur ther t ime, t r ips the h.v

circui t breaker.



334 The application o f pro tec tion to transmission systems

The m ax im um ope ra t ing t im e o f t he s t and by ea r th - fau l t p ro t ec t ion shou ld be sebe low the ra ted t ime of the neut ra l ea r th ing res i s tor.

I t should be no ted tha t the s ta nd by ear th- fau l t p ro tec t io n should t r ip a ll thefau l t cur ren t in feeds to the a ffec ted p ar t o f the sys tem , o therw ise par t o f thesys t em may be l e f t s t i l l ene rg i sed bu t w i thou t i t s neu t r a l po in t ea r thed . The l a t t econ di t ion is l iab le to lead to ' a rc ing gro und ' c ond i t ions w hich m ay cause very severdamage to p l an t .

1 7 . 2 . 2 Effec t o f l oca t ion o f cu r r en t t r ans fo rmers in de t e rmin ing p ro t ec t ion to beprovided

I t i s essen t ia l tha t there should be no p lan t o r connec t ions in the t ransmiss ion

sys tem which a re le f t en t i re ly unpro tec ted , and i t i s usua l to ensure tha t the var iouzones o f p ro t ec t ion in to w h ich the sys t em is d iv ided ove rl ap each o the r. To ob ta ithe r equ i red ove r l app ing w i tho u t t h e reby in t roduc ing some l ack o f d i sc r im ina t ioi s imposs ib le . In de te rmin ing a pro tec t ion scheme one of the mos t d i ff icu l t t askis to reduce th i s l ack of d i sc r imina t ion to a m in im um an d to assess the e x ten t tow h ich a scheme shou ld be com pl i ca t ed and m on ey spen t t o ca t e r fo r f au lt s occu rr ing on pe rhaps a f ew inches o f connec t ions be tween an open d i sconnec to r, o rc i rcu i t b reaker, and a cur ren t t ransformer hous ing . In th i s respec t the loca t ion o

the c u r r en t t r ans fo rm ers i n t he t ransmis s ion sys t em is o f pa ram oun t impo r t anceespec ia lly w i th regard to c i rcu it b reakers .

Cons ide r ing f i r s t c i r cu i t b r eake r s w i th cu r r en t t r ans fo rmer accommoda t ion inthe bush ings each s ide o f the c i rcu i t b reaker, e .g . dead tank b ulk o i l o r SF6 c i rcu ibreak ers , i t is easy to a r range th a t the zones of p ro tec t io n over lap w i th in thec i rcu i t -breaker ta nk (F igs. 17 2 . .2A and 17 .2 .2B) .

The r i sk of a fau l t occur r ing w i th in the tan k i s very rem ote , and i f such a fau lshou ld occu r, f rom sa fe ty cons ide ra t ions i t w ou ld no rma l ly be des ir abl e to m ak

bo th zones A an d B dead . Th ere i s , therefo re , l i t tl e d i ff icu l ty or d i sadvantage w i tth i s des ign of c i rcu i t b reaker in a r ranging su i tab le over lapping of the pro tec t ionz o n e s .

With some forms of c i rcu i t b reaker, e .g . l ive tank a i r-b las t and SF6 c i rcu i tb reake r s , fo r econo m ic and t echn ica l r easons cu r r en t t r ans fo rm ers a re o f t en loca t eon o ne s ide on ly of the c i rcu it b reaker (F igs. 17 .2 .2C and 17 .2 .2D ) . A rrangem entto ob ta in d i scr imina t ive tr i pp ing may becom e co m plex , and fu l l cons ide ra tion m usbe g iven to the r i sk of fau l t s occur r ing be tween the c i rcu i t b reaker contac ts and th

Z o n e Aii

f

Over lap zone

A~

, r " t1 , , , 8

Zone B _

v

Fig. 1 7 . 2 . 2 A Zones overlapping cir cu itb r e a k e r



The app lication o f pro tectio n to transmission systems

t

/ /e 0

335

C.T. a c c o m m o d a t i o n C . T. a c c o m m o d a t i o n

Fig . 17 .2 .2B Dead tank circuit breaker

Zone A

Zone be tween c . t .hous ing and circuitbreaker contac ts

I I

- - ' t

Zone B~ , ,

v

Fig . 17 .2 .2C Zones not overlapping circu it b r e a k e r

c . t . hous ing . The effec ts of such fau l t s both on the p lan t i t se l f and on the surrounding sys tem m ust a l so be assessed . For exam ple , in F ig . 17 .2 .2D the r isk of fau l t occurr ing a t the insu la tor suppor t ing the b las t heads on outdoor swi tchgeais not negl igible and measures must be taken for fast , and, as far as possible , dicr iminat ive clearance of suc h faul ts . I t sh ould be note d tha t such a faul t (Fig17.2 .2E) w ould s t il l be fed f ro m s ta t ion B wh en the c ircu i t b reake r a t s ta t ion A

and i t s assoc ia ted busbar se lec tors were open , provided tha t the c i rcu i t d i sconnector were c losed .

Usual ly a t busbar s ta t ions , fau l ts be twee n the c . t. hous ing and the c i rcu it -breakecon tac t s wi ll be d e tec ted by the busba r p ro tec t ion , and c lea red by the t r ipp ing oa ll the c i rcu it b reakers c onnec ted to the ap propr ia te busba r and by the t r ipping othe c i rcu it b reaker a t the rem ote end o f the c i rcu it conce rned .

Var ious a r rangements can be made for t r ipping the remote c i rcu i t b reaker, thcho ice depend ing on the type o f p ro tec t ion on the c i r cu i t and on the degree o

d i sc r imina t ion r equ i red . The a r rangement adop ted a l so depends on the ex ten t t




which increases in the complexi ty and cos t o f the pro tec t ive scheme are acceptab leTyp ica l m e thod s by wh ich the r emote c i rcu i t b reake r m ay be t ripped a re"

a )

b )

( c )

by uns tab il is ing uni t forms of pro tec t ion on the outgo ing c i rcu i ts , for exam pl

by con tac t s o n the busba r p ro tec t ion t r ip r e lays . I f a de l ay o f appro x im a te lyO.15s i s in t roduced before uns tab i l i s ing the uni t p ro tec t ion , thus enabl ingthe c i rcu it b rea ker a t s ta t ion A to ope n f i rs t , d i scr imina t ive in te r t r ipp ing canusual ly be achieved.by ope ra t ion o f t he m a in o r back-up p ro tec t ion a t st a t ion B i f t h is p ro t ec t ionis of the non-u ni t type .by me ans o f an in te r t r ipp ing s igna l wh ich is t ransm i t ted b y a p i lo t wi re ocar r ie r cur ren t channel f rom s ta t ion A to s ta t ion B. This s igna l can be in i t ia ted

by e i ther the busbar pro te c t ion t r ip re lay or a fau l t -cur ren t de tec t ing re laysuch as the cur ren t c heck re lay in c i rcu i t-breaker fai l p ro tec t ion .

The use of a cur ren t check re lay has the advantage tha t the fau l t c learance id i scr im ina t ive , bu t i t has the d isadvantage tha t the c learance is de l ibera te ly de layeand th is de lay may be u naccep tab le for fau l t-damage or s tab i l i ty reasons .

.•,~ . . n c , . . ? ~ . , n q , , ; ?

, i ,,

Fig . 17 .2 .2D Live rank air blast cir cui t breaker

Sta t ion A

R e s e r v eba r

| Maini bar / c. t . Stat i on B

, - - , - - . . . . . . . - ~

Circuit

Fig . 17 .2 .2E Fault between c.t. housing and cir cui t breaker



The application o f prote ction to transmission sy stem s 33 7

I t would appear tha t there i s a s t rong technica l case for mount ing cur rentt ransformers on both s ides of the c i rcu i t b reaker. I t must be borne in mindhowever, t ha t fo r c i r cui t b reake rs o f o the r than the dead tank ty pe , mou n t ingcurrent t ransformers on both s ides may enta i l having an undes i rab ly la rge over lazone . Fur thermore , the insu la t ion of busbars should be kept as re l iab le as poss ib land the in t rod uct ion of cur rent t ransform er hous ings m ay s lightly increase the r isof busba r fau l t s . The provis ion of roo m for c .t . hous ings on bo th s ides of the c i rcubreake r m ay a lso increase the s ize of the swi tchgear bay and he nce the s ize of thsubs ta t ion .

17 .2 .3 Tw o-s tage overcur ren t p ro tec t ion

If the m ain faul t infeed is on ly from the h.v. s ide of the t ran sfo rm er, it is somet imes advantageous , both technica l ly and economica l ly, to rep lace the t ransformel .v. overcurrent pro tec t io n cur rent t ransform ers and the assoc ia ted inverse def in i tminimum t ime re lay by a two-s tage re lay compr is ing an inverse def in i te minimumtime elem ent , energised from c. t .s o n the h.v. s ide of the t ran sfo rm er, and a d.ct imelag elem ent (Fig. 1 7.2.3A). The f i rs t s tage, the i .dJ n. t , e lem ent , is arranged tt r ip the 1.v. c i rcui t bre ake r only , and the second s tage, the d.c . time lag elem ent , thh.v. c i rcui t breaker. This two-stage relay enables faster discr iminat ive overcurren

pro tec t io n to be obta ined for the t ran sform er as the d iscr im inat ing t ime can breduced f rom the usua l minimum t ime of 0-5s for grading two i .d .m. t , re lays to f ixed t ime of , say, 0 .2s , tha t i s , the maximum ci rcu i t b reaker t r ipping t ime p lus safe ty m argin .

H . V. b u s b a r

2 n d s t a g e t r i p

/ III

. I1 s t s t a g e t r i p

2 s t a g eo v e r c u r r e n t

r e l a y

iII

L . V. b u s b a r

F i g . 1 7 . 2 . 3 A Two stage ov ercurrent protec tion



33 8 The application o f pro tec tion to transmission systems

I D M T I n s t a n t a n e o u so / c r e l a y o / c r e l a y(1 per phas e) ( I pe r phase )

rv r rx . .___ .I II I

I II

II II II I

o ~0 °_L .° ' -

L . V. e . b .t r i p r e l a y

L . _ _ . 1

Ti m e r

II

I H . V. c . b .I t r ip re lay

O I ' cL _ _ J

F i g . 1 7 . 2 . 3 BTwo-stage overcu rrent pro tec t ion d.c . c ircui ts

Typica l d .c . t r ipp ing c i rcu i ts for a two-s tage overcur ren t p ro tec t ion a re show non Fig . 17 2 .3 B. M ost i .d .m . t , re lays , having ope ra ted , t ake an apprec iab le t im e(over 100 m s) for the i r contac ts to d isengage . Hen ce , i f the t r ipp ing of the 1 .vcircui t br eak er causes " the cessat ion of faul t cu rren t , there is a dang er tha t , unlesthe t imer is imm edia te ly de-energ ised , the t iming re lay wi ll comp le te i t s operat ion , energ ise the h .v. c ircu it b reak er t r ip re lay, and the unnecessary tr ipp ing othe h.v. c i rcui t breaker would resul t . For this reason, unless the i .d .m.t , re laycontac ts have a d isengaging t im e o f less than 100 m s , an ins tan tane ous cur ren t re lais provided with contacts having a very fast disengaging t ime. The contacts areconn ec ted to open the co i l c i rcu i t o f the t im ing re lay im m edia te ly the fau lt cur renc e a s e s .

17.3 In te r t r ipp ing and pro tec t ion s igna ll ing

1 7 . 3 . 1 G e n e r a l

The var ious typ es of in te r tr ipp ing and p ro tec t io n s igna ll ing equipm ents and assoc iated communica t ion channels a re descr ibed in de ta i l in Chapter 7 .

The choice o f the m etho d to be used for in i tia t ing the t r ipp ing of the rem otecircui t breakers usual ly l ies between the fol lowing:

(a)

(b)

D.C. s igna l l ing , normal ly only used for in te r t r ipp ing , in which an uncodedd.c . signa l i s t ransm i t ted over pr iva te ly ow ned p i lo ts . The rece ive re lay ma y omay not be surge-proofed , depending on the r i sk of apprec iab le induced a .cin the pi lot wires .Vo ice - f r equency s igna l l i ng , i n wh ich a coded vo ice - f r equency s igna l i st ra n s m i t te d , n o r m a l l y o v e r a h i re d t e l e c o m m u n i c a t i o n c o m p a n y c h a n n e l , b usom et ime s o ve r p r iva t e p i lo t s o r o the r t ypes o f channe l .



The application o f protec tion to transmission sys tem s 33 9

c )

(a)

Carrier s ignal ling, in w hich a carr ier s ignal , no rm ally c od ed , bu t m ay beuncoded for cer tain s imple s ignal l ing appl icat ions, is t ransmit ted over the conduc to r s o f the p r im ary sys tem.Faul t th rowing, in which a fau l t i s de l ibera te ly in i t ia ted on the pr imary

sys tem by c los ing a fau l t th rowing swi tch . The fau l t i s de tec ted by the protec t ion a t the rem ote s ta t ion w hich in turn in it ia tes t r ipping of the a pprop r ia tecircui t breakers .

For a g iven appl ica t ion , in addi t ion to technica l cons idera t ions , the main fac torwhich govern the choice of method of t r ipping the remote c i rcu i t b reakers a re thavai labi l i ty and cost of providing the s ignal l ing equipment and channels .

Where more than one inter t r ipping or s ignal l ing channels are required forsecur i ty of opera t ion reasons they should be as d iverse as poss ib le wi th regard tbo th the design o f the eq u ipmen t s and the types o f com m unica t ion channe l usedas ment ioned in Sec t ion 17 .2 .1 .1 .

Pi lo t c a b l e N o . .,,'1I i . . . .

T r a n s f o r m e r f e e d e r 1

T r a n s f o r m e r f e e d e r 2 ~ :

P i l o t c a b l e N o . 2I f "M

Fig. 17.3 .1A Rou t ing of protect ion and in ter t r ipping channels for double c i rcui t transformerfeeder

Problems may ar i se , however, as a d i rec t resu l t o f provid ing th is d ivers i ty. Foexam ple , i f i t is assumed tha t the only com m unic a t ion channels w hich can b

econom ica l ly jus t if ied for the p ro tec t ion and in ter t r ipping for the double c ircu it ransformer feeder shown in F ig . 17 .3 .1A are two separa te ly routed pr iva te p i lo tsand i t i s a l so assumed tha t each feeder i s equipped wi th two main pro tec t ionsys tems and tha t the in te r t r ipping for each feeder is dup l ica ted , then in order tincrease the dependabi l i ty, i .e . to reduce the r isk of a fai lure to t r ip , i t is evidentha t the f i r s t main pro tec t ion for TF1 should be assoc ia ted wi th p i lo t cable No. and the second m ain pro te c t ion w i th p i lo t cable No. 2 . S imi lar ly, the f i rs t in te rt r ipping channel for T F1 should be in cable N o. 1 and the second in ter t r ipping

channel for TF1 in cable No . 2 .



34 0 The app lication o f pro tectio n to transmission systems

However, some fo rms o f ma in p ro tec t ion wi l l ma lope ra t e on load o r t h roughfaul t cur ren t in the event of the conductors in the p i lo t cab le becoming shorc i rcu i ted . S imi la r ly, w i th a s imple d .c . in te r t r ipp ing sys tem a live con du ctor in thp i lo t may, t h rough insu la t ion b reakdown, be inadve r t en t ly connec ted to ano the

con duc to r r e su l ti ng in ma lope ra t ion o f t he in t e rt r ipp ing e qu ipm en t .One of the main causes of damage to p i lo t cab les , par t icu lar ly in urban areas

is bu i ld ing or road cons t ruc t ion w ork w hen , for exam ple , shee t s tee l p i ling m aacc identa l ly resu lt in sever ing w i th con seque nt shor t -c ircu i ting of condu ctors ithe p i lo t cab le . In th i s case , in the event of the co m m unica t ion channels for TFand TF2 be ing rou ted in the m anner desc r ibed , bo th TF1 and T F2 migh t be inadver ten t ly t r ipp ed . On the o the r h and i f a ll the pro tec t ion and in te r t r ipp ing assoc iated w i th T F I were ro uted in p i lo t cab le No. 1 and tha t for TF 2 were routed i

pi lot cable No. 2 the r isk of inad verten t t r ipp ing wo uld be co nfined to a s inglpr im ary c i rcu i t .

Where the pro tec t ion or in te r t r ipp ing for more than one pr imary c i rcu i t irou ted in one p i lo t cab le the r i sk of m ore tha n one c i rcu i t be ing t r ipped in thevent of damage to tha t p i lo t can be reduced in var ious ways . For example , the usof s ta r t ing re lays for the pro te c t ion w i ll reduce the r isk of pro tec t ion m alope ra t ioand the r i sk of inadver ten t malopera t ion of the in te r t r ipp ing can be e l imina ted bus ing a voice f requen cy in te r t r ipp ing sys tem ins tead of a d .c . sys tem .

In prac t ice , an engineer ing assessment has to be made of the consequencesof the loss of supply owing to damage to a pi lot cable or equivalent s ignal l inchanne l and , w i th in the economic cons t r a in t s , t he appropr i a t e measu res t aken tprovide rel iable high-speed faul t c learance.

17.3 .2 D.C. signalling

D.C. s ignal l ing, which ut i l ises pr ivately owned pi lots between the s ta t ions concorned , has the grea t advantages of s impl ic i ty and re l iab il i ty, bu t i f surge-proore lays a re used , the opera t ing t ime may be unacceptab ly long (about 150 ms) . Thopera t ing t ime may increase apprec iab ly wi th some types of re lay i f inducedvol tages are present in the pi lot wires . Where faster operat ing speeds are required vo ice . f req uen cy s igna ll ing sys tem has to be used .

Where the pi lot c i rcui ts are very long, i t i s advisable to use a separate insulatedbat tery for energis ing the pi lots for the fol lowing reasons:

(a)

(b)

W ith som e des igns o f two-w ay two-core in te r t r ipp ing schem es , i f the in te r t r ipsend re lays a re opera ted a t bo th ends s imul taneous ly, the s ta t ion t r ipp ingbat te r ies a t the local and rem ote s ta t ions m ay be conn ec ted in para lle l v ia theinter t r ip pin g pi lots . This can lead to d iff icul t ies i f the no rm al voltages of thetwo ba t te r ies i s apprec iab ly d i ffe ren t .I t i s undes i rab le to increase apprec iab ly the capac i tance to ear th connec tedto the s ta t ion t r ipp ing ba t te ry ; excess ive capac i tance of the d .c . wi r ing can

cause relays to maloperate in the event of a s ingle ear th faul t .





34 2 The application o f pro tectio n to transmission systems

(c) A fault in the pi lot circuit , w hich m ay no t be detectable unti l a s ignal is senmight affect the securi ty of the bat te ry supplies at the stat ion from wh ich thintertripping signal is being sent; for exam ple, t r ipping o f circuit breakers athat s tat ion m ay b e delayed unti l the app ropriate fuses have cleared the fau

in the pi lots and the tr ipping supply voltage has been restored.A ny r ise in earth potential at the rem ote station due to the fault current m abe transferred through the pilot circuit and imposed on the local stationtripping battery, with resultant insulation failure (Fig. 17.3.2A). It is usualto provide 15 kV insulation to earth for batteries used to energise long pilocircuits.

17.3 .3 Pos t O ff ice s ignal ling

W hen pr ivate p i lo ts are not avai lable voice f requ ency eq uipm ents are of ten used iconjunct ion wi th h i red te lecommunicat ion company channels for protect ionsignal ling and for in ter- tr ipping. Typical opera t ing t imes for equipm ents a t presenin com m on use a re be tween 25 and 40 ms .

The main d i sadvan tages o f us ing t e lecommunica t ion company s igna l l ingchannels are:

(d)

(a)

(b)

(c)

(d)

(e)

the equipm ent i s com plex and cos tly.

the h i red pi lo ts may be subjected to unauthor ised human in ter ference . Theymay a lso be out of service when a s ignal i s required to be t ransmit ted . Inpract ice in the UK these r i sks have been low. However, the new des igns oampl i f iers and digi ta l communicat ion equipment incorporat ing sol id-s ta tedevices m ay have to be protected by ov ervol tage protect ion wi th se t tings oa few hundred volts on their external terminals . There is therefore anincreas ing r isk that the communicat ion channel may be momentar i lyinterrupted at the instant of fault by the effects of induction or r ise of earthpotent ia l caused by pr imary sys tem faul ts in the v ic in i ty of thete lecommunica t ion company equ ipment . Also , a l though mos t equ ipmentused on channels h i red by the supply companies i s independent of 240 Vmains o pera ted suppl ies and hence n ot affec ted by vol tage dips in that supplyin a few ins tances som e is not and the changeo ver ar rangem ents to a s tandbysupply may not a lways be fas t enough to ensure re l iable protect ion andintertripping.the signall ing cod e ma y be sim ulated by disturban ces in the hired circuits thucausing incorrect opera t ion. Ag ain th is is a low r isk , but the r isk m ay increasappreciably i f the te lecomm unicat ion com pany t ransmiss ion c i rcui t inc ludecarder o r mic rowave l inks , which may in t roduce unwanted f requency orphase changes in the received signal .relat ively frequent routine tests are required to prove the equipment is inproper w orking order.the propagat ion t ime of the s ignal over a long te lecommunicat ion companylink may be signif icant .



The application o f protec tion to transmission sy ste m s 34 3

1 7 . 3 . 4 Carr iers i g n a l l i n g

Carr ier s ignal l ing is widely used where the cost of the power l ine coupl ing equipm en t can be jus t i fied . In the UK , i ts use i s p rac t ica l ly un iversa l as one o f the com

m unica t ion channels for the p ro tec t ion or in te r t r ipp ing o f long feeders , un lestechnica l res t ra in ts , e .g . a length of u nde rgrou nd cable p revent i ts appl ica t ion . I t pa r t i cu la r ly use fu l a s an a l t e rna t ive to t e l ecommunica t ion company s igna l l i ngchanne l s where these channe l s may be suspec t , e .g . a l t e rna t ive rou t ing fo r twoindependen t channe l s may no t be ava i l ab le , o r where the re has been a h i s to ry oinsu la t ion p rob lems due to f lood ing .

The m ain technica l l imi ta t ion in the appl ica t ion of car rie r s igna ll ing is th a t ingeneral i t i s unsui table for c i rcui ts containing cable , owing to the loss of s igna

resu lt ing f rom the m ism atch in the charac te r i s tic imp edances of the car r ief reque ncy pa th a t the cable seal ing ends . At ten uat io n of the s igna l a lso occurwi th in a cab le, the m agn i tude o f t he a t t en tua t ion va rying apprec i ab ly wi th cables odi ffe r ing cons t ruc t ion . These l imi ta t ions can somet imes be overcome by provid inl ine t raps , o r coupl ing equipment , o r bo th , in the pr imary c i rcu i t a t the appropr ia tpoin ts b u t i t is ra re ly econom ic .

Another technica l l imi ta t ion in the appl ica t ion of car r ie r s igna l l ing i s the a ttenu at ion of the car r ie r signa l a t any tee-poin ts in the p r im ary c i rcu i t. The resu l t in

loss a t the tee-poin t can no rm al ly be reduced to an acceptab le va lue , typ ica l ly3 .5 dB , by ca re fu l ly m a tch ing the impedan ces o f the coup l ing equ ipm en t s a t eacend of the tee-connec ted c i rcu i t .

Carr ier s ignal ling has the advan tage tha t the s ignall ing cha nne l , tha t is the po w el ine i t se l f , i s no t subjec t to unauthor i sed in te r fe rence by human agencies in thesame way as hired pi lot c i rcui ts . On the other hand, carr ier s ignals are l iable to bsevere ly a t tenuated under cer ta in l ine ic ing and hoar f ros t condi t ions , and i t iadv i sab le on ce r t a in v i t a l c i r cu i t s t o back up ca r r i e r s igna l l i ng by t e l ecommuni -

ca t ion com pan y s igna l li ng . A l so , if t he s igna l has to be t ransm i t ted a long a pow el ine w h ich is fau l t ed , s a t i s f ac to ry r ecep t ion o f t he ca r r i e r signa l canno t a lw ays begua ran teed . C a r r i e r in t e r t ripp ing w ou ld the re fo re be unsa t i s f ac to ry i f t o ta l r e li anchad to be p laced on i t for the cor rec t c learance of l ine fau l t s .

Grea t care has to be taken in the des ign of a car r ie r s igna l l ing equipment toensure tha t i t wi l l no t malopera te when subjec ted to the severe in te r fe rence by thopen ing and c los ing of d i scon nec tors . This in te r fe rence is par t icu lar ly severe w hedisconn ec tors energ ise or de-energ ise con duc tors having a very smal l capac i tance

fo r example sho r t s ec tions o f busba r.

17 .3 .5 Fau l t th rowing

Fau l t th row ing , w here the fau l t l evel permi ts , is a s imple and re liab le me tho d o

t r ipp ing remote c i rcu i t b reakers . Faul t th rowing swi tches a re a t p resent ava i lab le i





The application o f prote ction to transmission sys tem s 34 5

present ins tal led on the t ransm iss ion sys tem have been e lec t r ica l ly and m echanica l ldes igned so le ly for three pole opera t io n .

17.4.2 High-speedautomat ic rec los ing

Single-shot three-phase high-speed au tom atic reclosing of c ircui t break ers wawidely appl ied in the 1950s to 27 5 kV feeder c i rcu it s . In th is con text 'h igh speedimpl ies rec losure of the c i rcu it b reaker wi th in one second of i t be ing t r ippedtypica l ly in about 0-4s . In the event of rec los ing onto a permanent fau l t in i t ia t ionof fur th er rec losures was preve nted .

In a h igh-speed auto m at ic rec los ing sequence , once t r ipping has been in i tia ted nt ime is ava ilab le for sub seque nt checks to be m ade of the vol tage condi t ions on th

sys tem. In fac t , owing to the iner t ia of the c i rcu i t b reaker ' s pneumat ic or o i l andm echan ical s ystem s, it is necessa ry to ini t ia te reclosing befo re the tr ippingopera t ion has been com ple ted .

The permissible t ime for a successful high speed automatic reclosing operat ion igovered ma in ly by"

(a)

(b)

A min imum t ime , dependen t on the t ime t aken to ex t ingu i sh the a rc a t t hepoin t of fau l t and for any ionised par t ic les a t the p oin t of fau l t to d isperse .

A m ax im um t im e , depend en t on the r a t e a t w h ich the vo l tage vec tor s a t t hestat ions at each end of the circui t swing apart . This change in phase anglebetween the vol tages i s governed by many fac tors , such as dura t ion , loca t ionand na ture of the fau l t , the des ign of the sys tem , in par t icu lar the impeda nceof the a l te rna t ive routes be tween the s ta t ions concerned , the load on thesys tem, the load on the feeder concerned immedia te ly pr ior to the fau l t , thegen erato r plan t in service at th e t ime of the faul t and the t ran sien t charac teris tics of tha t plant .

I t i s apparent tha t in order to obta in the maximum ci rcu i t dead t ime for a minim um ci rcu it reco nnec t ion t ime i t is des irab le tha t the c i rcu i t b reakers a t each end othe par t icu lar feeder should as fa r as poss ib le t r ip s imul taneous ly. Uni t forms oprotec t ion are therefore idea l for feeders which are requi red to be equipped wi th igh-speed automat ic rec los ing . Dis tance pro tec t ion can however be used providethat suff ic ient ly fast accelerat ion or blocking faci l i t ies are instal led.

I t was found dur ing the 1960s tha t on many feeders the d is turbance to theCEGB sys tem which wou ld r e su l t f rom h igh speed au tomat i c r ec losu re on to perm ane nt fau l t was unacce ptable , and for th is and o the r reasons , such as ths impl i f ica t ion o f the des ign o f c i rcu it b reakers , h igh-speed a utom at ic rec losing wareplaced by de layed auto m at ic rec los ing .

17.4 .3 Delayed auto m at ic rec los ing

Delayed autom at ic rec los ing , som et imes re ferred to as low speed auto m at ic re



34 6 The app lication o f pro tec tion to transmission systems

closing, can be def ined as reclosure which is inter locked to ensure that specif iswi tch ing and sy s tem cond i t ions a re sa ti s fied before rec losure take s p lace inupw ards o f 2s f rom t r ipp ing . The m inim um rec losure t im e is governed by the in te rlock sys tem and by the des ign of the c i rcu i t b reaker concerned; for example somtypes of o i l c i rcu i t b reaker should not be rec losed wi th in 10s of t r ipp ing in ordeto enable the arc-control device to ref i l l wi th oi l , and any gas and carbonised oi l tdisperse .

The longer t ime t aken to pe r fo rm a de l ayed au tom at i c rec los ing ope ra t ion , acom pared wi th h igh speed au tom at i c r ec los ing , pe rmi t s checks to be m ade , a ft et r ipp ing has taken p lace , o f such fac tors as whether the c i rcu i t concerned i s deador, i f i t has been re -energ ised f rom a remote poin t , whether the vol tages on eachs ide of the c i rcu it b reaker w hich i s to be rec losed a re in synch ronism or suff ic ien t l

c lose in phase and m agni tud e . In p ar t icu lar, de layed au to m at ic rec los ing has thadvantage compared to h igh-speed au tomat ic rec los ing of enabl ing fau l ted p lan t tbe i so la ted au tomat ica l ly before rec losure takes p lace . For example , in F ig . 17 .4 .3Aa f au l ted t r ans fo rmer in a bank ed pa ir can be a u tom at i ca l ly i so l a ted by ope ra t ion othe m oto r i sed d i sconnec to r M, pe rmi t t i ng the hea l thy t r ans fo rmer to be r e s to red tse rv ice wi th in a few seconds . Delayed au to m at ic rec los ing can al so be appl ied t

T r a n sA

Ai

r ans

Q l l L l a

F i g . 1 7 . 4 . 3 A A u t o m a t i c i s o la t io n o f a f a u l t y t r a n s f o r m e r i n a b a n k e d p a i r

F i g . 1 7 .4 . 3 B F a u l t s o n t r a n s f o r m e r f e e d e r



The application o f prote ction to transmission systems 34 7

Trans A _ I

1 " ° ITrans B

-M ~ F 1

F ig . 1 7 . 4 . 3 C F a u l t o n o n e t r a n s f o r m e r o f a b a n k e d p a i r i n a t r a n s f o r m e r f e e d e r

t ransform er feeder c i rcu it s as there is ade qua te t im e to de te rm ine , and to s igna l toeach en d , suff ic ien t in form at ion concern ing the fau l t to a llow the cor rec t ac t ion tobe t aken . Fo r exam ple , fo r t he p r ima ry c ir cu it shown in F ig . 17 .4 .3B i f t he f au l

occurs on ly on the l ine a t F l , th e c i rcu i t b reake rs can be au to m at ica l ly rec losedbu t in the event o f s im ul taneo us l ine and t ran sform er fau l t s F t + F2 , rec losurem us t no t t ake p l ace .

I f two t r ans fo rmers a re con nec ted a t t he end o f a t r ans fo rme r f eede r (F ig .17 .4 .3C) and a fau l t F~ occurs on one of the t ransfo rm ers , rec losure can take p lacw hen the f au l ted t r ans fo rm er has been i so la t ed .

Ty pica l sequences for de lay ed au tom at ic rec los ing of c i rcu i t b reakers a re asfol lows:

( a )

Ti m es

00.2

443

12+17+19+

A t r ans fo rme r f au l t on a banked pa ir o f tr ans fo rmers (F ig . 17 .4 .3A) .

O p e r a t i o n¢

Fau l t occu r s on t r ans fo rmer B a t F IFau l t de t ec t ed by t r ans fo rmer B p ro t ec t ion and c i r cu it b r eake r s P, Qand R t r i ppedAutomat i c open ing o f t he h .v. d i s connec to r M fo r t r ans fo rmer Bin i t ia ted

Trans fo rmer B h .v. d is connec to r M fu l ly openedCircu i t b reaker P rec losed thus re -energ is ing t ransform er AProvided vol tages across c ircu i t b reaker Q are in synch ronism or i f the

busb ar i s dea d , c i rcu i t b reaker Q i s rec losed




(b) A l ine fau l t (bu t no transfo rm er fau l t ) on a t ransform er feeder (F ig . 17 .4 .3B)

Times

00.2

1515-2

17+

Opera t ion

Line faul t a t F~ occursFau l t de tec ted by feeder pro te c t ion and c ircu i t b reake rs P and Q t r ippedCircui t breaker P reclosed thus energis ing the l ineI f the fau l t i s a permanent fau l t c i rcu i t b reaker P t r ips and no fur therreclosure takes placeProvided the l ine has remained energised for 2s , that is , the or iginalfaul t was a t ransie nt faul t , and i f the vol tages across c i rcui t bre ak er Qare in synchro nism or i f the bu sbar is dead , c i rcu i t b reak er Q is rec losed

(c) Simul taneous l ine and t ransformer fau l t s on a t ransformer feeder (F ig17.4.3B).

Times

00.2

(d)

Opera t ion

Line and t ran sform er fau l ts F I + F2 occur s imu l taneous lyFaul t s de tec ted by feeder and t ransformer pro tec t ion and c i rcu i tb reake r s P and Q t r ipped . A u tom at i c r eclos ing locked ou t by op e ra t ionof tr ans fo rmer p ro tec t ion

A fau l t on one t ran sform er o f a ban ked pa i r a t the end of a t ransform erfeeder. (Fig. 17.4.3C).

Times

00-2

44315+

17+19+

Opera t ion

Fau l t occurs on t ransform er B a t FIFaul t de tec ted by t ransform er B pro te c t ion and c i rcu i t b reakers Q andR t r ipped . C i rcu i t b reaker P tr ipped by in te r t r ipp ing s igna l f rom t rans-fo rmer B p ro tec t ion .A u tom at i c o pen ing o f t r ans fo rmer B h .v. d i sconnec to r M in i ti a tedTransformer h .v. d i sconnec tor M fu l ly opened . In te r t r ipp ing s igna lremoved, permi t t ing au tomat ic rec los ing of c i rcu i t b reaker P to proceedCircui t breaker P reclosed thus re-energis ing the l ineProvided vol tages across c i rcu i t b reaker Q are in syn chron ism or i f thebusbar is dead, c i rcui t breaker Q is reclosed

17.4 .4 Equ ipm ent des ign andprogramming

In the years pr ior to the la te 1970s most au tomat ic swi tch ing equipments cons is tedof e lec t romechanica l re lays , main ly a l l -or-noth ing re lays and t imers , mounted in

re lay cases w i th up to f i f teen re lays in each case. The re lay cases w ere in te rconn ec ted



The application o f prote ction to transmission sys tem s 349

by panel wir ing, as required, to provide the requis i te faci l i t ies a t a par t icular subs ta t ion . Som e o f the re lays were of s tandard des ign to provide , in con junct ion w i to ther re lays , de layed automat ic rec los ing fac i l i t ies for cer ta in s tandard subs ta t iolayouts , e .g . double busbar, four-switch mesh, three-switch and s ingle-switch

s ta t ions . There were , for example , groups of re lays in one or more cases whichcom pr ised s tandard des igns for 'm esh corner uni t s ' , 'm esh c i rcu it -breaker uni t s ' an' t ransformer l .v. c i rcui t -breaker uni ts ' .

With the increasingly high costs of switchgear in the 1970s, i t was found moreconom ica l to dep ar t f rom s tandard des igns of subs ta t ion , and to tee-connect obank pr imary c i rcu i t s in nons tandard conf igura t ions . This en ta i led des igning thpro te c t ion and a utom at ic swi tch ing requi rem ents very m uch on an indiv iduaci rcu i t and subs ta t ion bas is , and the s tandard c i rcu i t d iagrams for au to m at ic

swi tch ing schemes and equ ipm ent were n o longer appl iab le , except poss ib ly in thcase of double b usbar s ta t ions wi tho ut tee-connected pr im ary c i rcu it s . A typicaexample of a de layed automat ic rec los ing scheme for a double busbar s ta t ionusing electromechanical re lays is given in Fig. 17.4.4A. This s imple scheme is fothe au tomat ic rec los ing of a feeder c i rcu i t b reaker a f te r a feeder fau l t . The cor respon ding seq uence chart is given in Fig. 17.4.4B.

A fur th er d i ff icu l ty which arose f rom us ing s tandard des igns of au to m at ic rec losing equipm ents was tha t of m odify ing exis ting schemes to dea l wi th the a ddi t io

of new banked pr imary c i rcu i t s a t an ex is t ing s ta t ion . Extens ive changes in exis t ing s tandard re lays and panel wir ing were involved which were both cos t ly andt ime consuming .

Fur th e rm ore , t he use o f conven t iona l e l ec t romechan ica l r e lays to pe r fo rm large number of s imple logic and t iming funct ions required a relay panel sui tetypica l ly 4 m long for the re lays assoc ia ted wi th the au tom at ic swi tch ing of the h .vc i rcu it b reake r a t a fou r sw i tch mesh subs ta t ion . This was reduced to abo ut 2 m busing static relays.

Another problem which ar i ses f rom the use of e lec t romechanica l re lays i s thasome logic c i rcu i ts requi re m any re lay co ntac ts to be conn ected in ser ies w hich , not regular ly main ta ined , in t roduce a r i sk tha t the scheme may fa i l to opera teowing to contac t fa ilure . T he r isk of e lec t romecha nica l equ ipm ents fa i ling toopera te wh en requi red to do so has been fo und in prac t ice to be a bo ut 7% of theto ta l num ber o f r equ i red ope ra t ions .

These problems can be overcome to some extent by ins ta l l ing su i tab le des ignof sequence cont ro l le rs us ing programmable wired logic a r rangements , such as d iode m a t r ix .

However, the logic requi rements for au tomat ic swi tch ing equipments have tot ake in to accoun t no t o n ly the cond i t ions ob ta in ing a t the s t a r t o f the au tom at i cswitching sequence but a lso any changes in these condi t ions whils t the sequence iin progress . As an exam ple , i f there i s a thund ers to rm in the v ic in i ty of a subs ta t iowi th a com plex pr imary c i rcu i t conf igura t ion , a l ine m ay be s t ruck by l ightn ingresul t ing in t r ipp ing of c ircui t breakers a t that sub stat io n. I f , befo re the re is t ime t

reclose the circui t breakers , another l ine into the substat ion is s t ruck fur ther c ircui



35 0 The application o f pro tection to transmission systems

Part of voltageselection andsynchronisingscheme

Trip reJsys

L i n e ,

Neutral = ' ~ ~ S - Y Y . . ~ . , [

T RO/4m

Line disconnector~ , . . - , ~ aux i liar~ switch B- 1 L - 1 VT F/2

1 . . . - - - .

Circuit-breaker (2 sec. release)

auxi li ary switch V.T. (~ [ _ . ,O _ _ _ _ ~' - P - 3

ID - I~ ~ Buchho lz~ TRO-I _ VT F- I

O C -m

Circu it-brea kerlo w air pressurerepeat

Busbarprotect ion

~2 P--2

- o o - - - -

P - 4To indicat ion ( =circuit O.,m..O--~

V T F - 2B - 2 L K I L - 3

O O ~ I D ~ ' ~ - - ~ r - - ( D e l a y e dO~ I I °perateLK3 I I 5 - 6 ~ 0sec.)

B - L-4 D /

. . . . . . . . . . . .

C :i= 2 sec. operate---- C/4 ]- ' l ~ r , v, y ~

_ 2 s ec . r ele as e " ' " ~ ' I 'Y '~

4 ' ~ -m MC/1

C - 4

: ° ° - 3

Circuit-breakerclosing +ve

(60 sec. operate)

_ l 'C ircu i t lb rea k e r -/c lo s in g c i rcu i ts . [

Circuit-breakerclosing -ve

Fig . 17 .4 .4A Delayed autoreclosing for feeder at a busbar station



T h e a p p l i c a t io n o f p r o t e c t io n t o t r an s m i s s io n s y s t e m s 3 5

P a r t o f v o l t a g es e l e c t i o n a n ds y n c h r o n i s i n g s c h e m e

~ R

S Y A

K E Y T O U S E O F" L I N K S. . . . . .

i . F u l l s y n c h r o n i s i n g f a c i l it i e s :

1 .1 S y n c h r o . c h e c k o n l y1 .2 D e a d l in e c h a r g e o r s y n c h r o , c h e c k1 .3 D e a d b a r c h a r g e o r s y n c h r o , c h e c k1 .4 D e a d l i n e o r b a r c h a r g e

o r s y n c h r o , c h e c k

1 2 3 4 $

O O O O OI O O O O

O O O O i

2 . L i m i t e d v o l t a g e s e l e c t i o n( s y n c h r o . c h e c k n o t a v a il a b le ) :

2 . 1 N o l i n e r e f e r e n c e v. t .2 . 1 .1 A s s u m e d d e a d l in e c h a r g e

b a r al iv e . . . . . . . . . . . . . . . . . . . . . . . . I2 . 2 N o b u s b a r re f e r e n c e v . t .2 . 2 .1 A s s u m e d d e a d b a r c h a r ~

l in e a li ve . . . . . . . . . . . . . . . . . . . . . . . O2 . 2 . 2 D e a d l i ne c h a r g e b a r

a s s u m e d a li ve . . . . . . . . . . . . . . . . . . . . O

O - L i n k o u t I - L i n k i n

O O O 1

O O I O

1 O O

1 1 0 0

S

CC PDIDI R T R X -LM C -

PPYR C -S Y & S Y B -S YA -S Y N -T R O -V T F -

B u s b a r s i d e v o l ta g e m o n i t o r i n g r e l a yR e c l o s i n g r e l a yC l o s e p u l s e t i m i n g r e l a yD e a d l i n e r e c l o s e t i m i n g r e l a yP e r s i s te n t i n t e r t r i p d e t e c t i o n t i m i n g r e la yl n t e r t r i p r e c e i v e t r i p r e l a yL i n e s i d e v o l t a g e m o n i t o r i n g r e l a yM a n u a l c l o s e r e p e a t r e l a yR e c l o s e s t a r t r e l a yL i n e d e a d c h e c k s t a r t s e a l r e l a yR e c l o s e c a n c e l r e l a yS y n c h r o n i s i n g s e l e ct i o n r e l ay s m a n u a lS y n c h r o n i s i n g s e l e c t i o n re l a y a u t o - r e c l o s eS y n c h r o n i s i n g r e la yTr i p r e l a y o p e r a t e d r e l a yV. T. m o n i t o r i n g a n d s w i t c h g e a r r e p e a t r e l ay




SEQU ENCE CHA RT. I. OR DELA .yED AUTOMA. TIC RECLQSI NG OF A FE ED ER

CIRCUIT BREAKER AT A BUSBAR STATION

Feeder protectiontrip relay operated

No operation of linev.t. Buchholz

ITime delay

( I 0 sec. )

IFeeder protectiontrip relay reset

if fault persistent

trip relay reo~eratesI

C.B. tripped

Signal sent to lockoutreclosure of remote endcircuit-brea ker (usually

a prolongedintertripping signal)

Feeder circuit-breaker wasclosed, and line v.t. was alive

up to 2 sec. before fault

tFeeder protection

trip relay operated

IC.B. opened

ILine v.t. dead

INo operation of line v.t.Buchholz or busbar protn.

IBusbar v.t. alive

. . . . . .

I _

Line v.t. alive

Il-nergise

synchronising relay

ISynchronisin~ relay

operated after aminimum delay of 2 sec.

INo operation of line

v.t. Buchholz orbusbar protn, during

above sequence

( h)se C.B.

(2 sec. pulse)I

C.B. closed

IAuto-reclose O perated '

indication

ITime delay

(variable 5-30 sec.)

INo operation of line

v.t. Buchholz orbusbar protn, during

above sequence

ILine v.t. dead

busbar v.t. alive

Reclosureby hand

Inhibit fu~rtherinitiation of

auto-reclose sequence

Time ~delay(4 sec.)

IReset

auto-reclosingequipment

F i g . 1 7 . 4 . 4 B




breakers wil l be t r ippe d, necessi tat ing in some cases a change in the sequen ce o f threclosure requirem ents . I t is therefore essential that a ll the ap prop riate inp ut d ata iscanned by the auto m at ic swi tching equipm ent before each sequentia l s tep i s takenIn practice, this results in relatively simple controllers such as are sometimes used

for control l ing s imple sequential processes being unsui table for automatic switchinapplicat ions.

It also results in it being very difficult to p repare fully com preh ensive sequen cecharts or f low diagrams to show al l the logic requirements for complex automatiswi tching schemes , as so m any opt ions have to be show n. I f such a char t or d iagramis prepared it is essential that all the logic is included, i .e. the requirements fonon ope ra t ion as wel l as opera t ion of the autom at ic swi tching equipm ent .

A no ther prob lem is to ensure that the various propo sed sequence ope rat ing

t imes are correc t under unusual p r imary sys tem swi tching cond i t ions and tha tw hen necessary, the t imes are autom at ica l ly changed wi th in the equ ipm ent .

In pract ice, these diff icul t ies resul t not only in complex logic requirements fothe automat ic swi tching equipment but , equal ly impor tant , the need for comprehensive test ing faci l i t ies to enable the proposed requirements to be checked fovarious assumed faul t locat ions and system switching condit ions.

Dealing f i rs t w i th the autom at ic swi tching equipm ent i tse lf for comp lex pr imarycircui t configurat ions the interact ive logic the equipment must provide has to b

based on some form of computer technology, usual ly incorpora t ing microprocessors in the design. A microprocessor can be defined as a digital processing unico ns truc ted as one or m ore integrated circuits using large-scale integ ration (l.s .ima nufac tur ing technolog y. I t i s an ext rem ely com pact programm able e lec t ronicom po nen t f rom w hich a d ig i ta l com pute r can readi ly be cons t ruc ted b y the add i t ion o f memory, c lock , inpu t and ou tpu t componen t s . For an au tomat i cswitching equipment appl icat ion sui table interfacing devices e .g. reed relays, optoiso la tors e tc . , a re requi red be tw een the com puter and the sub s ta t ion eq uipm ent .

A great advantage o f a microprocesso r based eq uipm ent is i ts f lexibi li ty. Thehard w are, can be ord ered and instal led largely inde pen den tly of specifying thdetai led requirem ents of the au tom atic switching scheme. This great ly eases cons t ruc t ion problems. Fur thermore , most changes in scheme requi rements e i thebefore or af ter instal la t ion can be catered for by the relat ively easy process ochanging the software. The changes are usual ly accommodated by removal and replacement o f the ap propr ia te m em ory mo dules fo l lowed b y the necessary re tes ting

The microprocessor-based equipments are much smaller than their e lectro-mechanical relay predecessors , typical ly one 19 inch rack provides for a completsubs ta t ion , most of the space be ing occupied by externa l cable te rminat ions .

The equipments are also in pract ice more rel iable than electromechanical equipments , par t icular ly as by using sui table programs they can both monitor themselvecont inuo us ly and check for d iscrepancies in the input inform at ion .

The main disadvantage o f microprocessor-based equ ipm ents is that extensiveprecaut ions have to be taken to ensure that interference from the substat ionenvironment does not reach the integrated circui ts . This interference can easi ly




cause damage and malfunct ioning unless the equipment is specif ical ly designed opera te in a subs ta t ion envi ronment . One of the main sources of in ter ference caused by the open ing and c los ing of d isconnectors in the sub s ta t ion . These d isconectors can genera te h igh energy in terference over a very wide f requen cy b and ,

some ins tances up to t ens o f M Hz, and the au tom at ic swi tch ing equ ipm ent mai tse l f be cont ro l l ing the opening a nd c losing of these d isconnectors .The prepara t ion of the programme for the microprocessor-based equipment ( th

software) requires great care. One successful technique is to use in the design offica com pu ter as a design a id and to p rogram th is co m pu ter in such a m ann er tha t can sa t i s fac tor ily s imula te the inpu t logic an d a lso the equip m ent in the sub s ta t iobeing cont ro l led . In th is w ay the propo sed logic can be checked to prove i ts su itb i l ity un der var ious sys tem swi tching co ndi t ions for var ious assumed faul t loca t ion

I t shou ld be no ted tha t the co m pute r m ode l o f the subs ta tion and assoc ia tepr ima ry sys tem m ust inc lude s im ula t ion of rem ote subs ta t ions and also of thre levant secondary equipment a t those subs ta t ions , e .g . in ter t r ipping send anreceive equ ipm ents wi th the i r appropr ia te opera t ing t imes . In ad di t ion , in order fo l low and analyse the sequent ia l opera t ion of the proposed logic and of ths imula ted p la nt in the sys tem m odel , all re levant changes in inpu ts , outp uts an d thnecessary in ternal memories and t imers have to be logged, as shown schemat ica lin Fig. 17.4.4C.

Having proved the logic requi remen ts for the au tom at ic swi tching scheme, thnext s tep i s to program the automat ic swi tching equipment in the requi red manneFor electrom echa nical an d d iode logic equip m en ts this is a relat ively easy task bufor microprocessor based eq uipm ents the program has to be prepared in a su i tabform at b y a com pu ter and fed in to the equ ipm ent by sui table program m ing device .g . for coupled-charge p . r.o .m.s (programmable read-only memory) componenpulses o f electr ical energy for each bi t o f info rm ation .

A considerable am ou nt of e ffor t can be saved in program m ing m icroprocesso

based equipment i f the subs ta t ion i s sp l i t in to def ined funct ional groups of p lan

T e l e t y p e o r V . I ) . U . 1 _. _ . __i n t e r a c t i v e - - rc o n t r o l

1I S y s t e m & I

" 1 f a u l t m o d e l IL . . . . . . J

I A u t o - s w i tc h i n g I" q L e 2u i j m j n t m,.._~deljI

- L l 'Zventl o g g e r

r - - - - - - - i

" 1 E x e c u t i v e II_ . . . . . . _1

R e c o r d i n g

P r i n t o u t

F i g . 1 7 . 4 . 4 C Co mp ute r u sed fo r con t ro l l og i c design



T h e a p p l i c a t io n o f p r o t e c t io n t o t r an s m is s i o n s y s t e m s 3 5 5

R e m o t e

S u b s t a t i o n

C . B . ' s

~ =,11, =,= m

[ ~ l ~ m m m

S u b s t a t i o nN o d e s

......... C G r o u p

, ~ ,, ~.. G r o u p 2

A d j a c e n t ¢

N o d e s in

S u b s t a t i o n ~ -

/

i ' -1_I ._ i

1 ~ . ~ G r o u p 3

/ ~ G r o u p 4

~ . G r o u p 5

G r o u p s f o r A s s e m b l y o f M o d e l

L - J

/N o d e

IIIII

N o d e N o d e-- / - - - O

M o d el c o n s t r u c t e d f r o m G r o u p s

F i g . 1 7 . 4 . 4 . D Substation model

(Fig. 17.4.4D) which can then be interconnected to form a substation and standardsoftware designed to control these functional groups, in a standard manner e.g.This line has a persistent fault. Open the disconnector .

Using a library of standard software modules, programs can be built up toperform standard automatic switching functions, e.g. After transformer fault ontee connected transformer feeder, open transformer disconnector and close linecircuit breakers at mesh station . This technique has not only the advantage that theprogram can be prepared quickly but also, once a standard module of software has



35 6 The application o f pro tect ion to transmission systems

been proven for a par ticular grouping of pr imary p lant , the s tandard m odu le can bcon f ident ly incorpora ted in program s for fu ture subs ta t ions , even thou gh thau tom at ic switch ing equ ipm ent m ay be o f a d i ffe ren t type .

Assembl ies of the s tandard sof tware modules can thus be used to s impl i fy th

determ inat ion o f the logic requ i rements for a com plex auto m at ic swi tching schemw itho ut the need to prepare c om plex sequence char ts or f low diagrams. This resulin c onsiderable savings in engineering and draw ing off ice m anp ow er. I t is , o f coursessent ia l tha t the program documenta t ion incorpora tes the fac i l i ty for 'p la ilanguage ' com m ents on the tasks be ing accom pl ished a t each s tep .

17 .4 .5 Commiss ioning

A m ajor problem w i th com plex a utom at ic sw i tching schemes is to com m ission thequipment once i t is instal led in the substat ion.

D i s c o n n e c t i o n p o i n t s

Te s t p o i n t s

S u b s t a t i o n p l a n t

' . p I X ' , i l

1

- - - - - - - I ~ - - Ia I

" - - - - I

S u b s t a t i o n ~ ~ 1O O '- - -Ia u t o - s w i t c h i n g Ie q u i p m e n t ~ D I O - - - -( m i c r o - p r, , c e s s , , r i ~ ~ Ib a s e d ) - - I . I

l1

~ T e s t le a d

'__o_ L L'~ p n lp u t c r ~ ~ ~ . _ _ I / I

J S y s t e m & IJ f a u l t m o d e l IL . . . . . . . .J

1 E v e n t lo g g e r j ~ [ V . D . U .

d ;e l e t y p e , , r V I ) .i n t e r a c t i v e / - - v L . . . . . . . J c o r ngc o n t r o l l

P r i n t o u tF i g . 1 7 . 4 . 5 ACo mp uter used for on-si te testing

Next Page





The app lication o f pro tec tion to transmission systems 35 7

Operation of the primary plant i tself by the automatic switching equipment,apart from a few checking operations, is unacceptable from the p oint of view ofrequiring an unduly large num ber of operations of circuit breakers and discon-nectors to prove all the required sequences. It would also be too costly in terms of

the outage times which would be required.W ith the growth in com plexity o f the schemes it is also found that adequ atemodelling of the substation plant by relays, switches and lamps becomes toodifficult.

For complex schemes, a fairly elaborate model of the substation is requiredwhich can conveniently be provided by a transportable mini- or microcomputerprogrammed to simulate the substation concerned. In some cases it m ay bepracticable, provided it is transportable, to use the same computer for the com-

missioning tests as used for programming the automatic equipment. An exampleof this tec hniqu e is show n sche ma tically in Fig . 17.4.5 A.

17.5 Econ om ic considerations

The cost of a protection system is influenced by such factors as:

( a )

( b )

(c)(d)( e )

the com plexity of the protect ion equipment,

the voltage and current rating of the necessary ancillary equipment associa-ted with the protection, for example current and voltage transformers, linetraps, pilot cables, etc. The cost of these items may be very many times thecost of the associated protection relays,the development and engineering costs of the protection system,whe ther or not the pro tection equipm ent is being purchased in bulk, andthe special requirem ents of individual purchasers.

It is therefore no t possible to quo te general prices for protec tion system s, bu t someindication of the relative costs of prote ction relays is given in Fig . 17.5 A.

Typ e of relay Relative cost

Single-polo instantane ous differential curren t 1.0

Three-pole i.d.m .t. 2.5

Three-pole directional i.d.m.t. 5-0

Pilot wire protection operating over metallic pilots withstarting and supervision relays (p er end ) 124)

Switched high-speed distance relay for phase and earthfaults (per end)

Fig . 17 .5A Typ ical app roxima te re la t ive cos ts of pro tec t ion re lays

20.0

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In cost ing any scheme, ful l account should be taken of a l l the re levant costsinvolved; for ex am ple, the capitalised co st o f any hired pilots, the tota l installationcosts, and the probable ma intenance costs , including the cost of any necessarycircuit outages.

The cost o f the consequences of the proposed prote ct ion schemes fail ing tooperate should also be assessed, e .g . the cost of any consequent ia l interrupt ion ofsupplies to consume rs .

17.6 Typical prote ct ion appl ications in a ma jor t ransmission system

In this Section, the factors involved in selecting protection systems for typicalapplications in a m ajor transm ission sys tem are considered bu t, as i t will be seen,in some cases the choice is not easy. The schemes described in this Section arethose em ployed for protect ing high-securi ty t ransmission systems requiring fastfault clearance t imes for all fault locations. For less important transmission sys-tems and e .h.v, dis t r ibut ion systems the re quireme nts are not so onerous , andvarious econom ies and technical re laxat ions can be ma de; some of these aredescribed in Section 17.7. I t should be emphasised that the characterist ics quotedfor var ious types of protect ion refer to the protect ion systems at present avai lable .New systems are cont inual ly being developed, and these developments modify someof the factors involved in choosing a suitable scheme.

For example, the recent development of very-high-speed protect ion systemsmeasuring the direct ion of propagat ion of the s tep changes in current and vol tageproduced at the point of faul t ( ' t ravel ling-wave ' prote ct ion) have recent ly beenappl ied in some par ts of the w orld.

17 .6 .1 Feeder p ro tec t ion

In orde r to provide fast and fully discriminative pro tect ion for feeders w ith therequis i te degree of redund ancy in operat ion and divers ity of design (Sect ion 17.2.1)two separate main feeder protect ion systems are normally provided. The back-upprotect ion is usual ly def ini te minimum inverse- t ime overcurrent and ear th-fault prote ctio n. In areas of excep tionally high earth resistivity a definite-t imeearth-faul t protec t ion m ay also be provided w ith a very high sensi tivity to opera tefor high resistance earth faults.

17.6.1.1 Protect ion fo r a long overhead feeder:In this context a long overheadfeeder implies a feeder over abo ut 30 km long but under 200 km , with a negl igibleam oun t of cable in its route .

In order to obtain as much diversity in the protection as possible i t is preferableto have a current different ia l system for one main protect ion and an accelerateddis tance protect ion for the other.

In view of the distance between the substations a power-line-carrier phase-

comparison system is normally used for the different ia l system, al though up to



The appl icat ion o f pro tec t ion to t ransmission sy ste m s 35 9

40 km a voice f requency phase compar ison sys tem ut i l i s ing a te lecommunica t ioncompany channel can be used . The la t te r i s cons iderably cheaper than power- l ine-carder pro tec t ion , as no coupl ing equipment and l ine t raps a re requi red . I t i sprobable however tha t the d is tance pro tec t ion would normal ly use a

t e l ecommunica t ion company channe l fo r acce le ra t ion , and hence the des i r ab led ivers i fica t ion of com m unica t ion channels for the f ir s t and second m ain pro tec t ionis lost.

a) Carrierprotection: Practically all the power-line-carrier protection at presentin service in the U K is s ta t ic phase-comparison protec t ion. This form of protect ionreplaces older thermionic phase comparison protect ion and direct ional comparisonpro tec t ion .

The m ain advantages of phase com parison p ower l ine carr ier protect ion are:

(i) I t provides fast and discriminative pro tec tion along the wh ole length of theprotected feeder.

(i i) I t does not require a voltage inp ut, hence i t can ope rate corre ctly in the eventof a vol tage t ransformer associated with the other main protect ion on thefeeder being defective.

(i i i) I t is in general m ore sensitive tha n distance prote ctio n to high resistance earth

faults (e.g. faults caused by ionised and polluted air from grass fires).( iv) The circui t breakers a t each circuit end of the prote cted feeder are t r ipped

within a ppro xim ately 20 ms of each othe r, i r respect ive of the faul t locat ion.This feature is very im po rtan t if high-speed auto m atic reclosing is provided ; itis not important with delayed automatic reclosing.

The main disadvantages are:

( a )(b)

(c)

(a)

(e)

high costpossible difficult ies in allocating frequencies in areas where an appreciableam oun t of carr ier protect ion is installedrelat ively complex commissioning, tes t ing and maintenance procedures arerequiredunder cer ta in con duc tor icing condi tions the carr ier s ignal m ay be so at tenua-ted that the protec t ion has to be taken ou t of service to avoid the mal-operat ion of the protect ion on through faul ts

normally even short lengths of cable cannot be inser ted in the pa th o f the h.f .signal. This l im itation can be particularly difficult as i t m ay not be kn ow nuntil a relatively late stage in the planning and construction of an overheadl ine, that for exam ple for environm ental reasons, a length of cable has to beinserted.

The ear ly designs of phase com parison power l ine carr ier protec t ion were of thetherm ionic type and natural ly required non interru pted pow er suppl ies of m any tens

of VA. This necessitated in most cases the provision of large d.c. motor-generator



36 0 The appl icat ion o f pro tect io n to t ransmission systems

sets dr iven from the 110 V stat ion bat tery. Not only did these motor-generator setsrequi re a cons iderable amount of main tenance and room to accommodate them inthe substat ion bui ldings b ut , in order to provide as is custo m ary 6 hours s ta ndb y inthe event of the l .v.a.c, supplies fail ing, a larger station battery was needed. With

the advent of s ta t ic phase-comparison pro tect ion and the replacem ent of therm ionicprotect ion, a considerable reduct ion in these auxi l iary power supply requirementshas occurred. Nevertheless , the provision of adequa te non -interrupted powersupplies for the phase -com parison pro tec tion is sti ll necessary. The presen t practiceis to use 48 V bat tery suppl ies for this purpose as they are subjected to muchsmaller voltage fluctua tions in service than the 110 V b atte ry used for closing andtripping circuit breakers.

b) D istance p rotection:In UK applications the polar characterist ics of distanceprotec t ion are usual ly the convent ional c i rcular mho character is t ics , a l thoughmore com plicated shaped character is t ics are being introdu ced, where just i f ied, toreduce the problem of set t ing the third zone so that i t wi l l not maloperate underem ergency heavy load t ransfer co ndi t ions.

For both phase and ear th faul ts separate or common measuring elements perphase are used for zones 1 and 2, and separate measuring elements for zone 3. I t isconsidered that the extra re l iabi li ty and speed obtainable with a mu lt ie lemen tdistance relay com pared w ith a switched distance relay are justif ied for prote ctingfeeders in a major transmission system.

Stat ic designs of dis tance protect ion were introduced in mid 1960s and havesteadi ly taken over f rom electromechanical designs. The main advantages of s ta t icdistance relays are flexibil i ty of characterist ics, in particular the abil i ty to shape thecharacter is tic in the required m anne r, low VA consu m ption and high speed ofopera t ion .

The disadvantages of static distance relays are the need for auxiliarypow er supplies and the greater susceptibil i ty of the relays to ma loper ate or bedam aged by high frequ enc y interferenc e. The latter risks have in practice beenreduced to a negligible magnitude by careful design and extensive type testing.

Rec ent designs of distance protec t ion incorporate microprocessors as em bedde dcomponents in their design. This has resulted in great f lexibil i ty in characterist icsand logic sequences which can be easily changed without changing the design ofthe ha rdware. The ch aracter is t ics themselves can also be im proved, for exam ple,the o perat ing t ime can be ma de very fast (7 ms) provided there is no dis tor t ionin the input signals. In the presence of distor tion the m easuring t ime can be appro-pria te ly lengthened to ensure correct operat ion of the protect ion. Improved tes tfacilities are also available if required, e.g. built-in on-line testing. The latter facilityis included to improve the overall reliabil i ty of the protection in service.

If used in conjunct ion with phase com parison protec t ion as the other m ainprote ct ion, for diversi f icat ion reasons the dis tance protec t ion should preferably beaccelera ted ra ther than blo cked as phase-co m parison prote ction is in this respect

fundamentally similar to a blocking scheme, i .e. the local end must not be allowed



The app lication o f pro tec tion to transmission systems 361

to t r ip unt i l i t has wai ted to receive information from the remote end and thatinform ation has been com pared w ith local ly derived in form ation regarding thecurrent zero crossing points.

The m ain advantages of dis tance protec t ion are:(i) I t provides fast and discriminative pro tec tion for all faults occu rring on the

feeder on wh ich it is installed to within ab ou t 25% of the feeder length fromthe rem ote subs ta t ion .

( i i) I t provides some measure of back-up prote ct ion (z one 2 and zone 3) for otherfeeders and plant a t rem ote s tat ions and, of ten, on reverse reach of zone 3 ati ts own substat ion.

(i i i) I t does no t necessarily need an incom ing signal from a rem ote station in orde rto trip, even though the lack of an incoming signal may delay tripping.

The main disadvantages of dis tance prote ct ion are:(a) It requires costly voltage transform ers and if the pro tectio n is of the high

speed type, e .g . under 10 ms, i t normally requires vol tage t ransformers witha high fideli ty transient response; to obtain this response may be very costly.W ith som e designs i t ma y be difficult to o btain very fast clearance o f close-upthree-phase faults due to lack of polarising voltages.The p rotect ion m ay t r ip on load cu rrent in the absence of a vol tage inpu t , e .g .

due to the voltage transformer fuses being inadvertently left out after l inemaintenance. To overcome this problem very-high-speed vol tage t ransformeroutput monitor ing relays may be instal led to disconnect t r ipping before theprotec t ion can m alopera te , bu t the cos t and com plexi ty of th is mo ni tor ingequipm ent and the r isk of the monitor ing equipm ent m aloperat ing and dis-connecting tripping during a system fault has to be carefully assessed.

(b)

(c)

The available choice is betw een blo cke d or accelerated distance pr ote ctio n. B lockeddistance protect ion has the advantage that when the remote end circui t breaker isop en , faults are cleared qu ickly and discriminatively along the w hole length of theprotected feeder. On the other hand, for an accelerated protect ion with the remotecircui t breaker o pen there is no opera t ion of zone 1 pro tect ion at the end with theop en circuit break er and hence no acceleration signal is sent fro m that end. Fau ltson the feeder in zone 2, i .e . approximately 25% of the l ine length from the opencircui t breaker are therefore only cleared in zone 2 t ime, which may be too long tobe acceptable.

In the event o f a kn ow n failure o f the com m unica t ion channel the blocked pro-tect ion mu st be switched ou t o f service as i t wou ld be u nstable on a throu gh faul t .

An accelerated protec t ion however has the advantage that in the event of aknown loss of the communicat ion channel the protect ion can remain in service, butfaults occurring in zone 2 reach will on ly be cleared in zon e 2 t im e instead of thefaster accelerated t ime.

V arious designs o f b locking systems are available from m anufacturers w ithdi ffe ring per form ance charac ter is t ics and the m ost su i tab le sys tem has to be chosenfor a given app l icat ion. M any o f these syste m s include a fi rs t zon e t r ipping faci l i ty




independent of the rece ip t of any s ignal . The b locking or acce lera t ioncom m unica t ion channel for the d istance pro tec t ion shou ld idea l ly be of a d i ffe renttype f rom tha t used for o ther main pro tec t ion o n the feeder. O n the o ther hand inthe Uni ted Kingdom for a feeder, say, 30 km long us ing a voice- f requency phase-

compar ison pro tec t ion over a te lecommunica t ion company channel for the f i r s tm ain pro tec t ion , i t might som et imes be d i ff icu l t to jus t i fy the use of a car r ie rchannel for b locking or acce lera t ion o f the second m ain d is tance pro tec t ion . In fac t,i f a car r ie r channel could be econo m ica l ly jus t i f ied i t wo uld no rma l ly be be t te r touse the car r ie r channel for the pha se-com par ison pro tec t ion (and thus e l iminate theinherent ly long propagat ion t ime ov er a te lecom m unica t ion com pan y c i rcu i t o f thecom plex phase co m par ison s ignal ) and to use the channel for the simpler b lockingor acce lera t ion funct ion . Th e la t ter proposa l w ould n ot apply, however, w here , for

example due to a short length of cable in the car t ier s ignal path, the at tenuat ion bythe cable and, more important ly, the ref lect ions of the s ignal by the cable seal ingends resu l t in a car r ie r s ignal which may be d is tor ted and have an indeterminateeffect ive ph ase shif t in its prop agat ion . In such a case i t is preferre d to t ransm it andrece ive the re la t ive ly s imple b locking or acce lera t ion s ignal over the car r ie rchannel and re ly on the te lecomm unica t ion c om pan y channel to t ransmi t the morephase sens i t ive phase-compar ison pro tec t ion s ignal .

Where a car r ie r channel cannot be used , e .g . lack of room in the ava i lab le

f requenc y spec t rum or excess ive a t tenuat ion due to a shor t length o f cable in themiddle o f the long overhead feeder, doub le d is tance p ro tec t ion wi thte lecom m unica t ion com pan y s ignal l ing channels may hav e to be used . In th is case ,for d ivers i ty reasons , one of the main pro tec t ions should preferably be of theblocked type and the o ther acce lera ted . T his ensures tha t wi th a remote end c i rcu i tbreaker open fas t c learance t imes are provided by the b locked d is tance p ro tec t ionon the feeder. Also i f a l l communica t ion channels fa i l in an ex t reme emergency,e .g . due to widespread f looding of te lecom m unica t ion com pan y c i rcu i ts , the three-zone feature of accelerated dis tance protect ion could s t i l l be used to providediscr iminat ive protect ion for the feeder, a lbei t with some t ime delay.

In areas where high resistance earth faults are considered l ikely, the earth-faultdis tance prote ct ion can useful ly be supplem ented b y a direct ional com parison ear thfaul t pro tect ion m odule in order to provide the extra ear th faul t sensit ivity; theseparate signalling channel required for this facil i ty would normally be over thesame com m unica t ion l ink wh ich is provided for the associated distance pro tect ion .

7 6 1 2 Protection fo r a short overhead feede r: In this context , a shorter feede r isunder 30 km long wi th a negl ig ib le amount of cable in i t s route length . A widercho ice of protect ion is avai lable than for long feeders s ince the use of pr ivate pi lotsas wel l as te lecommunica t ion company channels become economica l ly feas ib le .Pi lot wire systems are descr ibed in detai l in Chapter 10.

The choice o f p rotec t ion is largely governed, in par ticular for very short feedersa few ki lom etres long, by the cost of providing pr ivate pi lot cables . As an extreme

example for protect ing an overhead l ine between two substat ions, say 4 km apart



The application o f pro tect ion to transmission sys tem s 363

in a city area, i t m ay be possible to util ise spare pairs in an existing pilot n etw ork atnegligible cost. If , how ever, new pilot cables had to b e laid, the cost of ope ningtrenches, crossing and resurfacing roads etc. , solely to provide pilot cables forprotect ion could well be prohibi t ive. Overhead pi lot cables suspended from the

towers or em bedded in the ear thw ire have rarely been used in the Un ited K ingdomowing to unsatisfactory service experience. Fibre-optic cable embedded in theearthwire may in future provide a more sat isfactory communicat ion l ink.

Often underground pi lot cables for protect ion and control purposes are instal ledin t renches on the same route as power cables , and hence are subject to induct ionand r ise of ear th potent ial effects in the event of faul ts on the power network.These induct ion problem s can be one of the main l im itat ions in using pr ivatepilots for protection. In a few cases i t has even been found necessary to install

isolat ion t ransformers every few ki lometres in an exis t ing pi lot in order to reducethe ind uced voltages impressed o n the pilot cores. Indu ced voltages, in add ition tocausing breakdown of the insulat ion of the pi lot cable , in extreme cases may causem aloperat ion of the different ial p rotect io n to w hich the pi lot cores are conn ected.Details of the prec aution s wh ich should be ta ke n in choosing pilots are given inChapter 7 .

W here for the f ir s t and second main pro tec t ion two phy s ica l ly segregated cablesare not avai lable for two private pi lot protect ion systems, i t is of ten economical ly

just i f iable to provide one pr ivate pi lot wire protect ion as the f i rs t main protect ionand a te lecommunica t ion company p i lo t wire pro tec t ion as the second mainprotect ion. In the past , metal l ic pi lots , with known resis tance and capaci tancecharac ter i s t ics , have been ren ted f rom te lecommunica t ion companies which aresui table for a longi tudinal different ial protect ion system and these are l ikely toremain in use for many years , espec ia l ly on very shor t routes . Where only voicef requency c i rcu i t s can be ren ted f rom te lecommunica t ion companies a pro tec t ionus ing a voice f requency s ignal l ing sys tem can be used , such as voice- f requencyphase-compar ison pro tec t ion or b locked d is tance pro tec t ion .

W here no pr iva te p i lo ts ex is t nor can be econo m ica l ly jus t i fied , one of the mainprotec t ion sys tems may be a te lecommunica t ion company p i lo t sys tem, and theother should be o f fund am enta l ly d i ffe rent des ign such as d is tance or, for impor tantcircui ts , power- l ine-carr ier protect ion. The la t ter two types of protect ion are cost lyand in som e cases cannot be jus t i fied ; in th is case two te lecomm unica t ion com pan ypilot protect ions with separately routed pi lots may be instal led.

For very short overhead feeders , depending on the impedance of the source offaul t curren t and the impedance o f the circui t to be protected ( i .e . the systemimpedance rat io) dis tance protect ion may be unsui table; most dis tance protect ionsystems have a m axim um s.i .r, of abo ut 50. In practice, most l ines in a t ransmissionsystem longer than about 6 km can be sat isfactor i ly protected by dis tanceprotec t ion , but i t may have to be of the b locked type .

17.6.1.3 Protection fo r an underground feeder:Most of the considerat ions which

apply to short overhead feeders apply equal ly to underground feeders . In view of



364 The app lication o f pro tectio n to transm ission systems

the high capital cost of e.h.v, cables, these feeders are normally under 30 km.How ever, pr ivate pi lots for pro tect ion purposes are muc h m ore l ikely to beavailable than for at , overhead feeder as usually the cheapest way to provide thenecessary c om m unica t ion channels betw een substat ions is to lay pi lot cables on the

same route as the e.h.v, cable, preferably in separate trenches to reduce inductionproblems. The latter particularly applies where individual e.h.v, cables are providedfor each phase of the feeder.

On some feeders and networks, the high capaci tance of the pr imary cables mayrequire shunt reactors to be connected to the cable circui ts e i ther permanently orthrough a circuit breaker. This can lead to severe switchgear rating and protectionproblems related to the high charging current of the cables and to the interrupt ionof highly ind uctive reactor curren ts. In general, separate fully discriminative pro-

tect ion systems are required for the cable and for any perm anen tly connectedshunt reactor to provide adequately sensi t ive protect ion for the reactor and toestablish quickly which is the faulted i tem of plant.

The main problem in protecting cable circuits is the high charging current whichmay be an appreciable fract ion of the load current ; this l imits the choice of mini-mum faul t current set t ing for the protect ion. In fact , a maximum current set t ing ofseveral t imes the s teady s tate charging current depending on the manufacturer 'srecom m end at ions m ay be necessary to ensure tha t the high t ransient discharging

and charging currents in the pr imary cable circui t under external faul t condi t ionsdo n ot cause the p rotect io n to m aloperate . Similar t ransient currents f low wh encable circuits are energised or de-energised. The frequ enc y an d m agn itude of thesetransient currents depend not only on the parameters , i .e . capaci tance, inductanceand resistance of the primary circuit being energised, but also on the characteristicsof the circuit breaker, e.g. the values of any switching resistors. These transientcurrents have, in pract ice, a f requency of the order of a few ki lohertz , and in viewof the low shunt impedance of the cable circuit at such frequencies appear asinternal fault currents. The protection has to be designed to be sufficiently in-sensit ive at these frequencies to prevent i t from maloperating.

If distance protection is used for protecting the cable circuit the characteristicangle of the dis tance relays should preferably be approx im ately the same low angleas that of the cable circui t being protected in order to obtain opt imum set t ings.The set t ing of dis tance protect ion on cable circui ts with crossbonded sheathspresents a particularly d ifficult p roblem as the effective imp edan ces o f the cablecircui t are dependent on the return paths of the faul t current , and may vary overa wide range of values depending on the fault location, resistivity of the cabletren ch back-fil l ing, etc.

As m entione d in Sect ion 17.2.1.1, where mo re than one inter t r ipping or s ig-nalling channels are required for dependabili ty they should be as diverse as possiblew ith regard bo th to the design of equipm ents installed and the types of com mu ni-cat ion channel used.

Problems may, however, arise as a direct result of providing this diversity. For

example i f i t is assumed that the o nly co m m unica t ion channels wh ich can be



The app lication o f pr ote ctio n to transmission systems 36 5

economical ly just i f ied for the protect ion and inter t r ipping of the double circui tt ransformer feeder shown in Fig. 17.3.1A are two separately routed pr ivate pi lots ,and th at each feeder is equipped with two main p rotec t ion systems with dupl icateintertripping for each feeder, then in order to reduce the risk of a failure to trip i t isevident that the f i rs t main protect ion for TFI should be associated with pi lot cableN o. 1 and the second main p rotect io n associated with pi lot cable No. 2 . Similar lythe f i rst inter t r ipping channel for TF I should be in cable No. 1 and the second inter-t r ipping channel for T FI in cable N o. 2 .

However, some forms of pro tec t ion may malopera te on load or through faul tcurrents in the event of the conductors in the pi lot cable becoming short c i rcui ted.Also w ith a s imple d.c . inter t r ipping sy stem a l ive con du ctor in the pi lot may beinadvertent ly connected to another conductor through a short c i rcui t , resul t ing in

malopera t ion o f the in ter t ripping equipm ent .One of the main causes of damage to pilot cables, particularly in urban areas, isbui lding o r road constru ct ion wo rk where, for exam ple, sheet pi ling may accidentallyresult in the severing and short circuiting of conductors in the pilot cable. In thisevent , i f the communicat ion channels for TF1 and TF2 were routed in theabove manner, bo th TF1 and TF2 might be inadver ten t ly t r ipped . On the o therhan d, i f a ll the protec t ion and inter t r ipping associated w ith TF 1 were rou ted in pi lotcable N o. 1 and th at for TF2 were ro uted in pilot cable N o. 2 the r isk of inadvertent

tripping would be confined to a single circuit .Where the protect ion or inter t r ipping for more than one pr imary circui t is

routed in one pi lot cable the r isk of more than one pr imary circui t being inad-vertent ly t r ipped can be reduced in var ious ways. F or exam ple, the use of s tar tingrelays for the pro tect ion will reduce the r isk of protect ion m aloperat ion and ther isk of inadvertent maloperat ion of the inter t r ipping can be pract ical ly el iminatedby using a voice frequen cy inter t r ipping system instead of a d .c . system.

In p ract ice, an engineering assessment has to be made of the consequences of

the loss of, or damage to a pilot cable or equivalent signalling channel and theappropriate measures taken within the economic constraints .

1 7 . 6 . 2 P r o t e c t i o n f o r a t r a n s f o r m e r

Details of the p rote ctio n system s available for pro tectin g tran sform ers are given inChapter 11.

Transform ers connected to a t ransmission system vary widely in size fromseveral h und red MV A do w n to a few MV A, the la t ter being mainly for providingauxiliary supplies at small power stations. It is usual, at least as far as the highvoltage connections are concerned, to provide similar protection for these smallt ransformers to that provided for the larger t ransform ers , as a s lowly or incorrect lycleared fault on a small t ransform er m ay be as disastrous to the o perat ion of thetransmission system as a similarly cleared fault o n a large transf orm er. H ence allt ransformers direct ly connected to a major t ransmission system require fast ear th-and phase-fault protection, although the risk of phase faults clear of earth occurringinside a t ransforme r tan k is extrem ely low.



36 6 The app lication o f pr ote ctio n to transmission systems

The risk of phase faults occurring on the h.v. or l .v. connections to a trans-former is, however, not low, especial ly i f open type conn ect ions are used. Flyingdebris can lodge on the conn ect ions al though even i f the conn ect ions are com pletelyphase segregated, safety ear ths inadvertent ly lef t connected af ter maintenanc e w ork

can result in three phase faults. As m entio ned in Section 17.2.1 , discriminativeclearance of phase faults may be difficult if inverse definite minimum time overcur-rent protect ion is the only phase faul t connect ion provided for the t ransformer.

Interposingtransformer

I m.p

].

-tl li i ..• I - " - " 1

E l . E E l

I , |

I d ' L

o R E I "

Interposingtransformer

_ I

Restraint coils

I

Operating coils

IF i g . 1 7 . 6 . 2 A Transformer overall differential protection



The appl ica t ion o f pro te c t ion to transmiss ion sys tem s 36 7

With regard to the protect ion of the t ransformer i tself , for double wound t rans-formers fas t ear th-faul t protect ion is usual ly obtained by unbiased different ia lpro tect ion sepa rately covering the l .v. and h.v. windings, and fast phase-faultprotect ion by a biased overal l different ia l protect ion system (Fig. 17.6.2A). For

econom ic reasons the ea r th- and phase-faul t p rotect ion systems usual ly share thesame c urrent t ransforme rs . In addi t ion to some form of inverse-t ime back-upovercurrent or ear th-faul t prote ct ion, Buchholz p rotec t ion, and on some t rans-formers oth er gas pressure operated devices , are provided. T he B uchholz p rotect io n,as well as providing back-up protection for phase and earth faults on the trans-former windings, a lso provides some measure of protect ion for inter turn faul ts onthe windings, and for core faults, i .e. failure of the lamination insulation result ing inhigh damaging circulat ing currents within the t ransformer core .

Most forms of t ransformer overal l different ia l protect ion use a harmonicrestraint feature to prevent the protect ion m aloperat ing in the presence of

B mm m m

I J 'Z_[ ] I

- - I -" 1 _ " 1

r .

F i g . 1 7 . 6 . 2 B Typica l example o f phase and ea r th f au l t d i ff e ren t i a l p ro tec t ion fo r ind iv idua lwind ing o f a t rans fo rmer



36 8 The appl icat ion o f pro tect ion to t ransmission systems

harm onics such as occur, for exam ple, in the t ransformer m agnet is ing inrushcurrent. With loads having a very high harmonic content, e.g. arc furnace loads, thisharmonic bias might resul t in very s low operat ion or even nonoperat ion of thetransform er overall different ial pr otect io n in the event of an internal fault occu rr ing

in the t ransform er. An al ternative w ay o f providing phase faul t pro tect ion for suchapp lications is sho wn in Fig. 17.6.2B where the pro tec tion for the l .v. windingsonly has been shown. Each winding is provided with a separate different ialpro tec t ion which avoids the t ransformer inrush and harmonic cur rent problems buti t requires current t ransformers to be mounted in the del ta winding connect ions.This may be difficult and costly in practice and can rarely be justified, bearing inm ind the very low risk of phase faults, clear of earth, occurring w ithin the trans-form er tank. A typical p rotect ion scheme for a doub le-wou nd transform er is

show n in Fig. 17.6.2C.If the h.v. or l .v. connections are very short , the separate protection can be

om it ted , bu t the main reason for providing this pro tect ion is to give discriminat iveindicat ion of the faul t locat ion which reduces appreciably the t ime required toestabl ish the exact locat ion of the faul t . I f , for example, the h.v. connect ion pro-tect ion has operated and there is an insulat ion co-ordinat ing gap in the protect ionzone covered by the h.v. connect ion protect ion, there is a high probabi l i ty that thefault w as caused b y the gap flashing over and th at i t is safe to restore the trans-

former to service without any tests being made. In some cases i t is permissible toini t ia te automatic reclosiag from the connect ion protect ion, arranging of coursethe prevent ion of automatic reclosing i f the t ransformer different ial protect ion"has also operated.

For phase faults , the unbiased different ial conne ct ion pro tect ion is a lso inherent lyfaster than the biased different ial t ransform er pro tect ion , th us providing fasterclearance of phase faults than wo uld be the case if the biased trans form er dif-ferent ial protect ion zone embraced the t ransformer connect ions.

The high set overcurrent protect ion in Fig. 17.6.2C fed from the current t rans-formers in the transformer h.v. bushings provides fast clearance of phase faults atthe upper end of the t ransform er h.v. winding. I t a lso backs up the t ransform eroveral l different ial and restr ic ted ear th faul t protect ion. The main diff icul ty whenapplying high set overcurrent protection is in establishing a suitable sett ing. Thesetting current must be sufficiently low to ensure that i t will operate quicklywith the m inim um faul t current available , yet i t m ust n ot be so low that i t will

respond to faul ts on the l.v. s ide of the t ransform er or operate w ith t ransformermagnet is ing inrush currents which, for some types of t ransformer, may be as muchas seven times th e full load curren t.

The back-up inverse t ime overcurrent protect ion operates in two stages toprovide discriminative clearance of faults beyond the transformer l .v. circuitbreak er in the first stage, and faults within the transf orm er or o n i ts 1.v. con nec tion sin the second stage. Both the high-set and inverse-time overcurrent protection areoften energised from current t ransformers in the individual t ransformer h.v.

bushings to perm it fu ture banking of a second t ransform er w itho ut having to m ake



The app l ica t ion o f pro tec t ion to t ransmiss ion sys tems 369

t i

/

/

ln v c - - - ' r- I

IIIII

IIIIIIIIII

1IIt_ . ._

i I t 1K C K,

_ _ , . . . . . . . . . I ~

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B

g

]m

m

~ - - I o ~ l ¸I

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l

I

I

II

FI LT l

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t " - -III

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Fig . 17 .6 .2C P r o t e c t io n s c he m e f o r a d o u b l e w o u n d t r a n s f o rm e r



37 0 The app//cat/on o f protect~on to transm ission system s

R

B L

A

[Delta ter t iary

ind ing

R. |7

. . . . .. ~ " ~ t _ . _ ¥

_ _ _ _ ] '

J

, , i v , , -

, k - ,

F i g . 1 7 . 6 . 2 D Different ial prote ction for an auto transformer

major changes in the pro tect ion of the exis t ing t ransform er.The unbiased different ial protect ion of an autotransformer is appl ied on a

phase-by-phase basis as shown in Fig. 17.6.2D. The protection is equally fast andsensit ive for phase and earth faults.

A typical pro tec t ion sys tem for an au to t ransforme r and i ts connect ions is sho wnon Fig. 17 .6.2E; i t is basically similar to that for a dou ble-w oun d transfo rm er. Oneof the differences between Fig. 17.6.2C and 17.6.2E is that the l .v. side in Fig.17.6.2C is assumed to be connected to a resis tance ear thed system (e .g. 33 kV)with only short conn ect ions b etw een the t ran sform er and the 1.v. c i rcui t breakerand hence the l .v. connect ion protect ion has been omit ted. In Fig. 17.6.2E, the l .v.s ide is assumed to be sol idly ear thed (e .g. 275 or 132 k V ) a n d there is separateprotect ion for the l .v. connect ions.

The ter t iary w inding on an auto transfo rm er m ay be used to provide, via ah.v. / l .v, t ransformer, the l .v.a .c , suppl ies in a substat ion; i t is a lso frequent ly themeans adopted of coupl ing react ive compensat ion plant into the t ransmission sys-tem. A typical scheme for-pro tect ing such a pr imary circui t arrangemen t is show nin Fig. 17.6.2 F. I t shou ld be noted that the te r t iary winding, wh en no t used for 1 .v.a .c . suppl ies o r react ive com pensa t ion co upling, of ten has one co rner of i ts windingsol idly connected to the neutral of the autotransformer to ensure that i t is properly

earthed and also included in one of the autotrans form er differential pro tect ion



T h e a p p l ic a t i o n o f p r o t e c t i o n to t ra n s m i s sio n s y ste m s 3 7 1

/1t I

~ - - TIIIIIIIIIiIIIIIiII

IIIIi

, 1ll

L . . . . . . 1

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. . . . . . . C

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m

D- = - = = - - ~ - ~ I P

II

III

e q u i p m e n tI~1

I II t

~ _ _ _ , ~

I -[ _ ~ . . . . ~ _ _ ~ - --~ .

I _1 _ _ _ ~ .

[1

I,'

U

F i g . 1 7 . 6 . 2 EProtection scheme for an autotransformer





The app lication o f pro tec tion to transmission systems 37 3

zones, (Fig. 17.6.2D) . W hen the ter t iary is conne cted to react ive compe nsat ionequipment and is ear thed as shown in Fig. 17.6.2F, the ear th connect ion on onecorner of the del ta ter t iary must be removed to avoid mult iple ear thing, and theear th-fault protec t ion for the ter t iary winding is then provided by the d ifferent ia l

ear th-fault protec t ion energised from the current t ransformers in the h.v. neutralof the ear thing/ l .v.a .c , supply t ransformer and in the bushings of the circui tbreaker control l ing the react ive compensat ion plant . Rel iance is placed on Buchholzand b ack-up overcurren t protec t ion for the very rem ote r isk of a phase faul t in thetert iary winding not involving earth.

I t should be noted that , for e conom ic reasons, there is no circuit breake r on thel .v. side of the l .v.a .c , supply t ransform er. Th e l.v. winding and conne ct ions o f thist ransformer are protected by different ia l ear th-faul t protect ion, but fuses provide

phase and ear th-faul t protect ion beyond the disconnector.

1 7 . 6 . 3 P r o t e c t i o n f o r b a n k e d t r a n s fo r m e r s

The protect ion requirements for banked t ransformers are inf luenced by:

(a)

(b)(c)(d)

the number and types of windings on the t ransformers to be banked, forexample s tar /del ta /s tar, in terconnected s tar, or autowh ether the t ransform ation rat io of each transform er in the bank is the samethe relat ive rat ings and impedances of the two t ransformersthe extent to which each transformer is required, to have i ts own discrimi-nat ive protect ion for operat ional and maintenance reasons.

If t ransformers of s imilar types are to be ban ked, and o ther factors perm it , i t m aybe permissible for a single transformer overall protection system to be sharedbetween more than one t ransformer (F ig . 17 .6 .3A) .

F ig. 17 .6 .3 A

,,,Overall -

/ differential ~1

l"trans. Trans.A ~ B

[1

Common overall different ial protect ive system for banked transformers



374 The app l icat ion o f pro tect io n to t ransmission systems

Care must be taken, however, to ensure, especial ly with biased t ransformeroveral l different ial prote ct ion systems, that not too ma ny current t ransformersare connected into the different ial system, otherwise the c . t . magnet is ing currentsm ay lead t o an unacceptable increase in the set t ing of the pro tect ion . In general,

for biased overal l different ial pro tect ion systems no t more th an tw o pow er t rans-formers can be protected by a s ingle system. I t must a lso be borne in mind thatwith biased systems the bias is usual ly obtained solely from current f lowingbetw een the h.v. and l.v. current t ransform ers , and i f it is possible for high throu gh-fault currents to flow between the l .v. current transformers, no bias will be availableto ensure s tabi l ity o f the pro tect io n. In such an appl icat ion it m ay be necessary toderive an addi t ional bias feature from the l .v .c . t , currents .

I f the b anke d t ransformers are o f differ ing rat io i t m ay be possible to em ploy a

co m m on overal l different ial system , bu t the magnet is ing currents of the addi t ionalmain and interposing c . t .s , i f any, may adversely affect the sensi t ivi ty and s tabi l i tyof the prote ct ion to an unaccep table ex tent . I f a small t ransform er is bank ed w ith amuch larger transformer, i t is essential that a separate differential scheme be pro-vided for the smaller t ransformer in order to obtain adequate set t ings for faul ts onthe l .v. side of the smaller transformer. Alternatively, if the transformer is relativelyvery small indeed, for exam ple, a 0 .3 MVA 33 /0 .41 5 kV auxi l iary t ransform erassociated w ith a 90 MV A, 1 32/33 kV tra nsform er, high breaking capaci ty fuses

m ou nte d as close as possible to the t ransform er 415 V winding wo uld usual lysuff ice. Faul ts in the auxi l iary t ransformer which produce faul t currents less thanthe set t ing o f the overal l different ial pro tect ion wil l be detected by i ts Buchholzprotect ion. However, for most t ransmission appl icat ions ful ly discr iminat ive pro-tect ion for each t ransform er an d i ts associated co nne ct ions is an operat ion alrequi rement , any common connect ions having the i r own pro tec t ion; an example i sshown in Fig. 17.6.3B. I t should be noted that very few changes are necessary toconvert the protect ion arrangements shown for s ingle t ransformers (Figs . 17.6.2Cand 17.6 2E) into the arrangements for a banked pair.

Disconnector auxi l iary switches are shown in the different ial protect ion bus-wiring to enable the h .v. connect ion pro tec t ion which is com m on to bot h t rans-form ers to rem ain safely in service wh en a transf orm er is ou t o f service formaintenance and i ts h .v. disconnector is open. A separate different ial protect ionrelay energised from c.t .s in the transformer h.v. bushing which remains in servicewhen the d isconnector i s open , may be provided , to pro tec t the h .v. connect ionsbetw een the ope n disconn ector and the t ransfo rm er h.v. bushings, thus al lowing thetransform er to be energised from the l.v. s ide only . This m ode o f ope rat ion m ay beneeded for example where react ive compensat ion plant is connected to the t rans-former tert iary windings and it is desired to retain this compensation plant inservice w hen the t ransfo rm er h.v. disconnector is ope n.

If certain l imitations are acceptable an arrangement such as is shown in Fig.17 .6 .3C m ay be permiss ib le, in which there is a com m on pro tec t ion sys tem whichini tia tes t r ipping, and tank ear th-fault a larms indicate wh ich o f the t ransform ers isfaul ty. The set t ing of the tank ear th-faul t indicat ion relay has to be high enough to



T he a p p l i c a t ion o f p ro t ec t i on t o t ransmiss ion sys tems 3 75

I

/itI

/l

p : c F ~II

- I

III

III

> _ _ _ l [

~ - -" ~ i '°< , ,sI I

~-- -~- ,~- - - r~o c o m p e n s a t i o n _ . _ ~ I

I , r - - T - - - ¢ i

III -

II ~ " - - - - J . . . .I

i.

, i _ HC K C K

- - - - - - , [ B B

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El

CK

,-- BB

[ 7I

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| 6 '/

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~ - - - ~ . v 1I

,, ,, ,o c l2S -. . . . 1 I H S

I_ I I. . . . J I

I - " " - - - - ~

~ ' ~ 1 " II

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k- RES

~ _ _ , . v• It E ~ s

- - - - S BLT I

I. . . . - - - - ' - 1

BB

I

] i

- ' [ ~ _ c JJ

F i g . 1 7 . 6 . 3 B Protect ionscheme fo r banked transformers




/

Trans.A i

I l i ~ -

_

Overalldifferential

|

i iI I

I I

TransB

F i g . 1 7 . 6 . 3 C Indicat ion o f fau l ty t ransformer by tank ear th faul t relays

ensure that t ransient currents which m ay f low th roug h the capaci tance of thetransform er windings under external faul t or switching condit ions are insuff ic ientto operate the relay. A typical set t ing used for a 275/132 kV 180 MVA transformerwas 200 A. I t should be noted that this arrangement does not permit discr iminat iveindicat ion of faul ts on the t ransformer connect ions and hence would not normallybe acceptable for control l ing automatic isolat ion of the faul ted t ransformer. Forthis , and other m aintenance and constru ct ion reasons this arrangemen t is now veryrarely used. T he arrangem ent m ay be acceptable i f operat ing condit ions perm it avisual inspect ion of the t ransformer and i ts connect ions to be made pr ior toreclosure.

17 .6 .4 Pro tec t ion fo r a t eed feeder

Some of the most diff icul t problems the protect ion engineer has to solve are

concerned with the pro tect ion of teed feeders . It is of ten very diff icul t to ob tainfault-clearance t imes, sensit ivity and stabili ty comparable to those obtainable fortwo-ended feeders, and in rare cases i t may even be impossible to provide a satis-factory pro tect ion scheme at a ll , thus necessita ting a change in the design of thepower sys tem.

Fo r m any appl icat ions e .g . relatively short teed feeders , some form o f differentialor phase compa rison p rotec t ion system is preferred, using pr ivate pi lots , power. l ine.carr ier or microwave communicat ion l inks. In present pract ice fast (under 40 ms)

differential p rotec t ion systems require a m icrowave l ink or s imilar wide band w idth



The appl icat ion o f pro tect ion to transmission sy ste m s 37 7

channel . Slower biased different ia l system s use pr ivate pi lot wire c i rcui ts. In pract ice ,in order to pro tect teed feeders especia l ly i f each leg is unde r 5 km in length, it isof ten necessary to sp l i t up the teed feeder in to two or more pro tec t ion zones byinser t ing current t ransformers a t the tee point as i l lus t ra ted in Fig . 17.6 .4A.

In the scheme shown, there a re th ree separa te d i ffe ren t ia l p ro tec t ion zones ,and when a fau l t occurs in one of the zones the pro tec t ion for tha t zone opera tesand in addi t ion the opera ted pro tec t ion i s a r ranged to uns tab i l i se the pro tec t ionfor the other two zones , thus ini t ia t ing t r ipping a t a l l three ends. Also, operat ion ofany one protect ion system is usual ly arranged to in i t ia te in ter t r ipping s ignals toa ll ends as the pro te c t ion a t the o ther ends m ay no t ope ra te even i f uns tab il i sed .Also direct in ter t r ipping of the c i rcui t breaker is of ten a fas ter means of c lear ing thefau l t than wai t ing for opera t ion of a loca l end o f a p ro tec t ion sys tem which has

been unstabi l ised.In addi t io n to the re la t ively long faul t c learance times obta ined the schem e

shown in Fig. 17.6 .4A is very cost ly, especia l ly i f overhead l ines have to be termi-na ted in a compound so le ly to enable pos t - type cur ren t t rans formers to be ins ta l ledat ground level .

1' i i '

iC

B

II , , ,o t l

F i g . 1 7 . 6 . 4 A Protection of teed feeder with two sets of current transformers at tee~ooint

One m etho d of avo id ing the need to insta ll cur ren t t rans formers a t the tee -po in ti s to ins ta l l a power d i rec t iona l compar i son sys tem in which the d i rec t ion of powerf low at each end of the pro tecte d feeder is me asured and t r ipping is inhibi ted a t a llends i f the pow er f lows ou t o f the feeder a t any one end (C hapte r 10) .

This system has occasional ly been successful ly appl ied but the fol lowing con-d i t ions mus t be met :

( a ) The sys tem conf igura t ions and fau l t in feeds mus t be such tha t fo r a ll in te rna l

fau lt condi t ions fau lt cur ren t can never f low out o f the pro tec te d feeder (F ig.




-12

Blocking signal~ = ~ sent to B and C

A

I - - ' - -II

C

Fig . 17 .6 .4B I n h i b i t i o n o f t r i p p i n g bM fa ls e b lo c k

A 2 1 B

l i . . . . /

v v

c

Fig . 17 .6 .4C F a u l t c u r r e n t d i s t r ib u t i o n

( b )

17.6.4B), otherwise the protect ion at the end where the faul t current is

f lowing out of the feeder wil l send a blocking s ignal to the other two endsand thus inhibi t t r ipping.If overcu rrent s tar t ing is used for l~he pro tect io n, the m argin between the

set t ings of the high set and low set re lays must be large eno ugh to ensure that ,for an external faul t , a high-set re lay a t one end cannot operate before alow-set re lay a t an othe r end has operate d, otherwise the pro tect io n will beunstable . I f the through-faul t current dis t r ibut ion is as shown in Fig. 17.6.4C,it will be seen that the high-set relay sett ing must be at least twice the low-set

re lay set t ing to ensure tha t a low-set re lay a t Stat ion B or C ope rates before



T h e a p p l ic a t i o n o f p r o t e c t i o n to t ra n s m i s s io n s y s te m s 3 7 9

the high-set relay at Sta tion A. To ensure that the low-set relays do no toperate w ith no rma l load currents , a min im um faul t set t ing o f a t least 200%of the maximum normal load current is necessary.

Often a more sat isfactory form of protect ion for a teed feeder, par t icular ly for alonger teed feeder, is accelerated or blocked dis tance pro tect ion . For diversityreasons one main p ro tec t ion should preferably be o f the b locked type and the o theraccelerated, a l though i t should be no ted some m ode rn system s use a m ixture ofaccelerat ion and blocking for different measuring elements in the same dis tanceprotec t ion .

There are very m an y factors to be considered in choosing the design and sett ingsof dis tance pro tect ion for teed circuits .

One of the most important is the absolute and relat ive lengths, and henceimpedances, of each leg of the teed circui t . The pr imary circui t configurat ionshown in Fig. 17.6.4D is for example much easier to protect than that shown inFig. 17.6.4E as in the la t ter case the dis tance prote ct ions at s ta t ions A and C canno tbe set to ope rate for faul ts near s ta t ion B w itho ut responding to faul ts in or beyo ndstations C and A respectively.

Another important factor in choosing sui table forms of dis tance protect ion arethe magnitudes and variat ions in the faul t current infeeds at each end of the teed

circuit . One aspect o f this is i l lustrated in Fig. 17.6 .4F . Assum ing a fault occursnear station C, i t is app aren t th at the distance pr ote ctio n at statio n A will see thisfaul t fu r ther away than i t actual ly is owing to the vol tage being injected at the teepoint f rom stat ion B. Similar ly, the dis tance pro tect ion at s ta t ion B will see the

A B

I

I

I

I

C

F i g . 1 7 . 6 . 4 D Sym m etr ica l eed c ircui t



38 0 The app lication o f pr ote ctio n to transmission systems

A B

- D i • - D - -I

IIII

ii

F ig. 17 .6 .4 E Te e d c i r c u i t w i t h u n e q u a l l eg s

A B

. . . . . .

I

I

I

Fig . 17 .6 .4F F a u l t o n a t e e d f ee d e r

fau l t fu r ther aw ay owing to the fau l t cur ren t in feed f rom s ta t ion A. The re lays a ts ta t ions A and B thus 'unde r reac h ' and , depend ing on the se t t ings wh ich can beg iven to these re lays, they m ay o r m ay n o t opera te in the i r second or th i rd zones .I t is there fore f reque nt ly necessary to a r range on such a t eed c i rcu i t tha t wh en the

pro tec t io n a t s ta t ion B opera tes i t in i ti a tes the sending of in te r t r ipp ing s ignals tos ta t ions A and C. Incide ntal ly , in general it i s usual ly p rud en t even whe re it is no tabso lu te ly essen tia l to a r range tha t the ope ra t ion of the pro te c t io n a t any ones ta t ion in i t i a tes the in te r t r ipp ing of the c i rcu i t b reakers a t a l l o ther s ta t ions on ateed c i rcu it . S om e o f the pa ram eters wh ich have to be ca lcu la ted and checked for atyp ica l acce le ra ted d i s tance p ro te c t io n sys tem appl ied to a t eed c i rcui t a re :

(a)

(b)

m ax im um permiss ible zone 1 se t tings

m in im um a t ta inab le z one 1 se t tings



The ap plication o f pro tec tion to transmission systems 381

c)

(a)( e )

09~ )

w ith all circuit break ers closed effective reach o f relays at each end as a resultof rem ote infeedsw ith one circui t breaker o pen , effective reach of relayswith two circui t breakers ope n, effect ive reach o f relays

m inimum zone 3 se t tingsload transfer l imitations.

The m ajor underreach problem w hich occurs with an accelerated dis tance p rotect ionsystem can be reduced by using a blocked form of dis tance protect ion system forwhich the reach of the t r ipping zone can be appreciably increased.

However, choosing appropriate set t ings for the relays performing the blockingfunc t ion can be difficul t . N ot on ly do the mag nitudes of the phase and ear th faul t

current infeeds affect the choice o f set t ing bu t a lso the direct ion of the currentscan affect op erat ion as show n in Fig. 17.6.4B.

Som e o f the calculat ions and checks that have to be made for a typical blockeddistance protect ion appl icat ion are:

(a)

( b )(c)

(a)

(e)

Check that the m inim um set t ing of the t r ip feature which is avai lable af terblocking has been removed reaches beyond the remote circui t end under theworst infeed condit ions.Calculate the minimum set t ing for the block-remove feature.Check that the t r ip feature af ter blocking has been remo ved or the block-remove feature do not reach be yo nd the reverse reach of the remo te-endblock init iate feature.Check that the block-remove feature does not have a sett ing that will causeope rat ion on reverse ear th faul ts.Establish the load transfer l imitations.

With some designs of complex blocked dis tance protect ion systems an assessmentof i ts perform ance for varying fault locat ions and differ ing types of faul t andsystem cond it ions can be very t ime consum ing and a diff icul t task requir ingextensive computat ion.~:In some cases in fact , the design of the protect ion systemhas to be ta i lored, with the aid of the m anuf acturer, to m ake i t sui table for apart icular teed circui t configurat ion, assuming cer tain maximum and minimuminfeed cond it ions at" each end of the teed feeder concern ed and cer tain assumedranges of posi tive, negat ive- and zero-sequence imped ances for that p ar t of theprimary network which is effect ively in paral le l with the teed feeder to beprotec ted .

After com plet ing the calculat ions, checks or tes ts, i t m ay well be found that theproposed teed circui t cannot be protected in a sat isfactory manner to meet a l loperat ional condi t ions, load t ransfer restraints being a par t icular ly common unac-ceptable l imitat ion. The s i tuat ion can be improved appreciably by inser t ing currentt ransformers at the tee point as shown in Fig. 17.6.4A. A cheaper arrangement , by

wh ich acceptable perform ance m ay sti ll be obtaine d is to inser t post- type current



38 2 The app lication o f pro tectio n to transm ission systems

- . . t 2 1

DI

' f : , , i , . v . ,i iI

[ ]_ ii C

F i g. 1 7 . 6 . 4 G

B

P r o t e c t i o n o f t e e d fe e d e r w i t h e x t r a c u r r e n t t r a n s f o rm e r s i n o n e l eg

I I

_ . , & , ?I

I , / II D A II

I..- .m.

II

I

B

I

F i g . 1 7 . 6 . 4 H P r o t e c t i o n o f t e e d fe e d e r w i t h i n d i v i d u a l p r o t e c t i o n f o r e a ch le g

t ransformers in only one leg of the tee as shown in Fig. 17.6.4G; in this case i tm ay b e technical ly b et ter and m ore cost effect ive to prote ct par t o f the teed feederby a diffecent ial or phase-comparison protect ion.

I f there i s a requi rement for au tomat ic opera t ion of d isconnectors a t the teepoin t to permi t au tomat ic res tora t ion of the hea l thy par t of a teed c i rcu i t , i t i sessent ial that each leg should be separately protected to enable the faul ted leg tobe ident i f ied and isolated as show n in Fig. 17.6.4H.

In spi te of a l l the techniques mentioned above there are some arrangements ofteed circui ts which cann ot b e sat isfactor i ly p rotected and the only op t ion lef t isto rearrange the pr imary system.



The applicat ion o f prote ct ion to transmission sys tem s 383

17.6.5 Protect ion for a t ransformer-feeder

For very short t ransformer-feeders , less than approximately 2 km long, i t may betechnical ly possible , current t ransformer lead burdens permit t ing, to include thefeeder in the associated t ransfo rm er different ia l pro tect ion zone in wh ich case theprotec t ion could be ident ical to that for a t ransform er. However, in order that atransformer-feeder may be switched back into service as soon as possible after afaul t , i t is necessary, for ma jor t ransmission sys tem appl icat ions, for the feeder andthe associated t ransform er each to have i ts own individual discriminat ive protect ion .

T h e feeder sect ion is in general protected in the same m anne r as descr ibed inSect ion 17.6.1, two ma in prote ct ion systems being provided for those transform er-feeders which form an important par t of the t ransmission system.

The relat ively high impedance of the t ransformer does, however, appreciablyaffect the appl icat ion of any dis tance pr ote ct ioh . The appl icat ion of dis tanceprote ct ion is eased where i t can be set to loo k into the t ransform er winding, i.e. theimped ance of the t ransform er can be used to enable the zone 1 reach set t ing to belonger than the feeder length, thereby permit t ing fast operat ion of the protect ionfor a l l feeder faul ts between the t ransformer h.v. terminals and the h.v. c i rcui tbreaker. On the othe r h and , this advantage does not a pply to any dis tancepro tec t ion ins ta l led a t the t ransformer end of the t ransformer feeder. Fur therm ore ,the imp edance of the t ran sform er l imits throug h fault currents to re la tively lowma gnitudes which necessi ta tes low set tings for any blocking relays. The t ran sform eris protected in the manner descr ibed in Sect ion 17.6.2.

The t ransform er protec t ion mu st t r ip bo th the h.v. and I .e . c i rcuit breakers .Also the feeder p rotec t ion at the t ransform er end m ay be very s luggish or even fai lto operate i f , as is of ten the case, the faul t current infeed through the t ransformeris low, and he nce two-w ay inter t r ipping is norm ally required ( Fig. 17.6.5 A).

For very impor tan t t ransmiss ion c i rcu i t s , two independent in te r t r ipp ing sys tems

are provided , norm al ly em ploy ing voice f requenc y s igna l l ing over pr iva teor te lecom m unica t ion com pan y p i lo t s. For depend abi l i ty reasons , the

l qI . . . I N T I N T ] ~ - - - - - - - i - . .. ]

I ~ II I II

' I 1 'I . e , , . . . . . . . - , = . - - - ~I I

' i ' " ' = i ' - PAI

L t i F 'oc . ji

F i g . 1 7 . 6 . 5 AProtection for short transformer feeder



3 8 4 T h e a p p l i c a ti o n o fpro t ec t i on t o t r a n s m i s s i o n s y s t e m s

I s M' B B I

/

. . . . . l u , . , i i i

/

I "I "

_ I - - - - ~ "

- I_ _ _ _ _ 1 "

p r o t e c t i o n

I ,o , , f l ,o~1

L V C . . . .

2

IIII

T

I

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I

' I

~ F - - - - , ~ -/

. j - - - - ~

tt[ ]

CK CK El

F g . 1 7 . 6 .5 BProtect ion for t ransforme r feeder forming p ar t o f a ma jor t ransmission system



T h e a p p l i c a t io n o f p r o t e c t i o n t o t ra n s m i s s io n s y s te m s 3 8 5

in te r t r ipp ing and pro tec t ion s igna l l ing equipments and communica t ion channelsshould b e as different as pract icable . In addi t ion , the inter t r ipping is back ed up byunstabi l isa t ion o f any d ifferent ia l , phase-com parison or block ed dis tance feederpro tec t ion , and acce le ra t ion of acce le rated d i s tance pro tec t ion . For less imp or tan tt ransmission c i rcuits , in view of the la t ter bac k up faci li ties , on ly one inter t r ippin gsys tem m ay be provided . The reason for the provision of th i s cons iderab le am oun tof red und ancy is to reduce to a m in im um , wi th in econom ic cons t ra in t s , the ri sk ofan uncleared, or s lowly cleared faul t on the t ransformer or i ts associated l .v. con-nect ions . Severe and cost ly damage, par t icular ly by f i re , can resul t f rom slowlycleared faults o n the tran sfo rm er or i ts l.v. cables; this m ay result in a very longoutage t ime before the t ransformer. feeder can be res tored to operat ional service ,wi th a conseque nt possible risk to con t inu i ty o f supply to consum ers .

A typ ica l scheme for pro tec t ing a t ransformer- feeder forming par t o f a majort ransmission sy stem is shown in Fig. 17.6.5 B.The p ro tec t ion a r rangem ents for t ransformer- feeders no t forming par t o f a major

t ransmission system are described in Sect ion 17.7.

17 .6 .6 Pro tec t ion for a double-busbar s ta t ion

Although the r isk of a busbar faul t occurr ing at most major t ransmission s ta t ions is

A i i

, E F - - o -

. , , , . , .

O O

nC+) (~12

F i g . 1 7 . 6 . 6 A Sim plif ied arrangement for busbar protect ion at a m ajor t ransmission s ta t ion





The application o f prote ct ion to transmission sys tem s 387

ext remely low, perhaps once in the l i fe o f the s ta t ion , the consequences to thesystem of a s lowly cleared busbar faul t are so severe that the expendi ture incurredin providing cost ly and com plex bu sbar pro tect io n can be ful ly jus t i f ied. A s imilarjust i f icat ion can be made for the provis ion of c i rcui t -breaker fa i l protect ion at

major t ransmission s ta t ions .I t i s therefore usua l p rac t ice a t 400 and 275 kV s ta t ions to provide the busbar

pro tec t ion and brea ker fa i l p ro tec t ion wh ich i s descr ibed in de ta i l in Cha pter 13.

I t should be no ted tha t a t an imp or tan t t ransmiss ion s ta t ion the design of thebusbar p ro tec t ion should be such tha t the fai lu re of any one re lay to opera te shouldnot resul t in an uncleared busbar faul t . I t i s therefore usual to provide somemeasure of redu ndan cy in the ' two out o f two ' re lay log ic, e .g . ex t ra per zone re laysm ay be connec ted in para llel wi th bo th the check and d iscr imina t ing re lays as

i l lus t ra ted in Fig. 17.6.6A.A typ ica l example of the pro tec t ion a r rangements a t a 400 kV double busbar

s ta t ion are shown on Fig. 17.6.6B.

17 .6 .7 Pro tec t ion fo r mesh s t a t ions

The pro tec t ion a r rangements for a typ ica l mesh type s ta t ion a re shown in F ig .

17 .6 .7A. The main d i ffe rences be tween the pro tec t ion a r rangements a t a meshs ta t ion compared to those a t a double-busbar s ta t ion concern the pro tec t ion pro-vided for the b usbars . At a mesh s ta t ion the num ber of c i rcui t breakers required tobe t r ipped in the event of a busbar faul t i s much smal ler than at a double-busbars ta t ion and the opera t iona l consequences of an inadver ten t opera t ion of the busbarpro tec t ion a re very m uc h less severe; hence there is no need for a ' two out o f two 'logic in the t r ipping sequen ce. The busb ar p rote ct ion at a me sh s ta t ion is norm allyof the high impedance different ia l type and is arranged in a 'one out of two' logic .

Auxi l iary switches on the feeder disconnector are arranged to disconnect thefeeder current t ransfo rme rs and associated different ia l re lay f rom the m esh cornerpro tec t ion buswiring whe n the feeder d i sconnec tor is open . Fas t d i sc rimina tivepro tec t ion for the connec t ion be tween the open feeder d i sconnec tor and thecurre nt t ransfo rm er housing is thus provided w he n the feeder is energised or lef t inservice f rom the rem ote end , e .g . for a teed feeder c i rcui t .

17 .6 .8 Pro tec t io n fo r com plex p r imary c i rcu it conf igura t ions

With the high cost of switchgear there has been an increasing tendency to designpr imary sys tems w i th the m in im um num ber o f c ircu it b reakers , e .g . by ban king andtee-connect ing ci rcui ts . A rela t ively s imple example is shown in Fig. 17.6.8A. I twil l be noted in this case that only a few changes are necessary to the protect ionarrange m ents sho wn in Fig. 17.6.3B. N evertheless , for some pr im ary ci rcui t con.f igura t ions the pro tec t ion , and a l so the au tomat ic swi tch ing a r rangements can

become very complex . An example i s shown in F ig . 17 .6 .8B and the assoc ia ted





The app lication o f pro tec tion to transm ission systems 38 9

I= | .

-+S y s t e m b a c k u ppro tec t ion

. 4

I . . . . C

CK

, - . ~I

I . r - . . . .

1 , - - - ' - -

, . , ( , ' , lI . . , I~ . . . .

, + - I i, , , . : ~ "'

< ; - L . i_. _ IHV Ci- : . ' - i 1 4 1

, , ~ t I '

m

"l"<Jcom pensat ion ~ N

l ( - 'v - i '- I ~ ,' L _ ~ " - - ? - - ' I t _ ~ LU

, , ~ - _ _ _ _ i ~

I B i t ,

t .L I ..... B , ." ' I

' - - - " I - .- - - - I - ~1 - - - - - - , - - - : - ~ i

[ ] }

l ,_+ ,+, l I " - '

Fig . 17 .6 .8A P r o t e c t i o n f o r b a n k e d t ra n s f o r m e r s a n d t e e d fe e d e r



39 0 The ap pl icat ion o f pro tec t ion to t ransmiss ion systems

F o r protect io n deta i l ssee schedulefor Fig. 17.6.8B

Station A

I

_ l _

I

L_.50 MVA

Stat ion C

[ ]

4 0 0 k V

275 kV

9 km to t e e point

To stat ion E275 kV

12 km

Sta tio n B /

2 x 750 MVA

26 km fromstation B

tee point

12 km

1

To station i.132 kV

I

240 MVA

- ,

.... 40 0 kV ci rcu it s---- " ----275 kV circuits

4 0 k m t o t ee p o i n t

Station D

400 kV

F i g . 1 7 . 6 . 8 B Complexprima ry circu it configuration



The ap plicatio n o f pr ot ec tion to transmission systems 391

Protection and automatic switching scheme for feeders shown in Fig. 17.6.8B

Station Voltage Circuit Protec tion

A 275 To stat ion s C & Dvia station B

A 400 To stations C & Dvia 275/400 kVtransformer atstation A

B 275 To station A

B 400 To stat ion s C & D

C 400 To stations A & D

C 400 To station s B & D

D 400 To stat ions B & C

D 400 To stat ions A & C

1st & 2nd Main: between stations A andB differential pilot wire feeder protectionof two different types

1st & 2nd Main: accelerated mho distanceprotection of two different types

1st & 2nd Main: differential private pilotwire feeder protection of two differenttypes

I st & 2nd Main: accelerated mho distanceprotection of two different types

1st Main: mho blocked distance protec-tion with PO signalling system to D2nd Main: accelerated mho distanceprotection

1st Main: mho blocked distance protec-tion with PO signalling system to D2nd Main: accelerated mho distance

protection

1st Main: mho blocked distance protec-tion with PO signalling system to C2nd Main: accelerated mho distanceprotection

I St Main: mh o b locke d dist ance protec-tion with v.h.s. PO signalling system to C2nd Main: accelerated mho distanceprotection

N o t e s1. Stations C and D are major power stations whereas station A is a relatively weak infeed.

The distance protection reaches from C and D are limited by the transformers at A and Band hence, there is no need to block the blocked mho distance protection from stations Aand B. Furthermore, it was found that reliable blocking could not be initiated fromstations A and B, if the circuit breakers at C or D were open.

2. lntert ripping is provided between all stations using a mixture of private and post officesignalling channels diversely routed and cascaded where necessary. Operation of anyprotection intiates intertripping of all circuit breakers on circuit concerned.

3. Automatic control facilities to avoid switching overvoitages and the effect of ferrores-onance are arranged as follows:

a} A- B-D circuitsFollowing the opening of the circuit breakers, the 400 kV switch disconnectors at B are

opened automatically. The autoreclose sequence is as follows:(i) close D 400 kV circuit breaker to charge line(ii ) close B 400 kV switch disconnectors to charge transformers and A cables

(iii) check synchronise then close C400 kV circuit breaker and A 275 kV circuit breakers.

b} A-C -D ci rcuitsFollowing the opening of the circuit breakers, the 400 kV switch disconnectors at A and Dare opened automatically. The D switch disconnector may then be reclosed immediately.The autoreclose sequence is as follows:

(i) close D 400 kV circuit breaker to charge line and D transformer(ii ) close A switch disconnector to energise transformer

(iii) check synchronise, then close C 400 kV circuit breaker, A 275 kV circuit breakerand D 132 kV circuit breaker.

The above closing sequences are also followed for manual operation.



39 2 The app /ication o f pro tec tion to transmission systems

schedule gives detai led protect ion arrangements for this par t icular pr imary systemconfigurat ion.

With such complex primary circuits i t is practically inevitable that, for protec-t ion and other reasons, there are operat ional l imitat ions, e .g . some circui t breakers

must not b e lef t c losed i f cer ta in other c i rcuit breakers are open.Also, the protec t ion arrangem ents for such com plex circui ts inevi tably result in

longer fault clearance t imes than are attainable for simpler circuits and these longert imes may not be acceptable as adjacent par ts of the system are developed andfaster faul t c learance t imes are required. These c om plex configurat ions m ay, fur ther-more, necessi ta te complex logic for the automatic switching equipments to avoidswitching overvoltages which might result from energising particular combinationsof overhead l ines and t ransform ers .

Nevertheless, especially for transit ional schemes, there are occasions when thesecomplex pr imary circui ts and their associated complex protect ion and automaticswitching arrangem ents can be just if ied on eco nom ic grounds.

17.7 Typical protect ion appl icat ions in a minor t ransmission system or majordis t r ibut ion system

For less im po rt a nt t ransmission system s and m ajor dis t ribut ion systems wherelonger fault clearance t imes are permissible and where the dependabili ty require-m ents for the protec t ion are less severe ma jor economies can just i f iably be m ade inm any cases by install ing less e laborate protec t ion schemes than those descr ibed inSect ion 17.6. O n the other han d, in the growth of a powe r netw ork in a co untr y i tis highly prob able tha t a system w hich starts as a transm ission system after severalyears or possibly decades wil l become part of a dis t r ibut ion system, and a cost-

benefi t analysis has then to be ma de o f w hethe r or not i t is wo rthwh ile to replaceexis t ing protect ion schemes on a par t of the network by s impler schemes. Majorfactors in this analysis are the availabil i ty and cost of spares for the old pro tec tionand cost of m aintaining i t .

1 7 . 7 . 1 P r o t e c t i o n f o r a f e e d e r

In m ost appl icat ions, two m ain protect ion systems cannot be just i f ied, and onema in protect io n and o vercurrent b ack-up protec t ion is usual ly adeq uate . Similarly,duplication of protection signall ing systems is rarely applicable; and for somefeeders dis tance prote ct ion o f the switched type with out accelerat ion or blockingmay wel l suff ice . The same factors apply in choosing a sui table protect ion systemas are described in Section 17.6.1 but for the majority of applications the cost ofpower- l ine-carr ier protect ion may wel l be prohibi t ive, depending on the cost of thecoupling equipm ent .



The application o f protect ion to transmission sys tem s 393

17.7.2 P r o t e c t i o n f o r a t r a n s f o r m e r

The protection scheme is often basically the same as described in Section 17.6.2,bu t there m ay n ot be sufficient justifica tion for providing overall differential

pro tec tion in all applications; h.v. and 1.v. differential earth fault pro tec tion , h.v.overcurrent protect ion, and l .v. overcurrent protect ion, of ten direct ional , maysuffice.

17.7.3 P r o t e c t i o n f o r b a n k e d t r a n sf o r m e r s a n d d u a l s e c o n d a r y t r a n sf o r m e r s

Som e o f the schemes descr ibed in the f irs t par t of S ect ion 17.6.3 m ay be acceptable ,using for example a common overal l different ial system for both t ransformers in aban k. In some substat ions dual secondary t ransform ers , e.g. 132 /11/11 kV may beinstalled and a typical pro tect ion scheme for such a t ransform er is show n at thetransform er end of the t ransform er-feeder in Fig. 17.7.5B. I t should be note d thatin this scheme instead of a t temp ting to provide t ransform er h.v. overcurrent pro-tect ion at the t ransformer end, which would introduce discr iminat ion problems, arelay is connected different ial ly w ith h.v. current t ransfo rm ers and current t rans-form ers in the 1.v. circuit bre akers to provide a plain balance scheme covering bo thtransformers .

1 7 . 7 . 4 P r o t e c t i o n f or a t e e d f e e d e r

Apart f rom there being no need for dupl icat ion o f the main pro tect ion , the samecon sidera tions app ly as described in Section 17.6.4. H ow ever, longer fault clearancet imes are of ten admissible in minor t ransmission systems and major dis t r ibut ionsystems which, in general , eases the problem of p rotect ing teed feeders .

17.7.5 P r o t e c t i o n f o r a t r a n s f o r m e r f e e d e r

Major economies can sometimes be made in the protect ion of t ransformer feederscompared to the schemes descr ibed in Sect ion 17.6.5; longer faul t c learance t imesare often permissible for feeder faults, and the cascade tripping situation i l lustratedin Fig. 17.7.5A may be acceptable . In this example, i t is assumed that the l imitedfault cu rrent infeed at the tr ans form er end is init ially insufficient to opera te the

uni t typ e feeder pro tect ion at that end. After the feeder pro tect ion at end A hasop erate d and the circuit b reak er Q has tripp ed , a higher voltage is available ats ta t ion A busbars to dr ive an increased faul t current through the t ransformers ats ta t ion B, thus enabl ing the feeder protect ion at the t ransformer end to operate andinitiate tr ippin g of circuit break er R. If the fault cu rrent infeed at station B forf e e d e r faul ts is insuff ic ient to operate the protect ion at that end and provided thatthe transformer feeder is unlikely to be energised or left energised from its l .v. sidewith the h.v. circuit breaker open intertripping can be provided to trip the I .v.

circuit breakers. Alternatively, provided the fault current infeed at the h.v. end is



394 The app lication o f pro tectio n to transm ission systems

Statio n ~ I-'P I Station

R- - -

"

l.'ault current prior to circuit breaker Q trippingFault current subsequent to circuit breaker Q tripping

Fig. 17.7.5A Seq uential ripping

high enough, some forms of different ial and phase comparison feeder protect ionhave an inherent inter t r ipping faci l i ty which may be suff ic ient ly fast . I f the t rans-

former feeder is protected by dis tance protect ion at i ts h .v. end and the h.v. systemis sol idly ear thed direct ional overcurrent pro tect ion on the 1.v. s ide of thetransform er can of ten provide ade quate pro tect io n for feeder faults , thus saving, a tthe t ransform er end, the cost of h .v. vol tage t ransform ers , current t ransform ers anda set of dis tance pro tect io n ( see Fig. 17.7.5B).

The direct ional overcurrent protect ion must be set to discr iminate betweenfaul ts on the h.v. system and faul ts on the t ransfo rm er feeder (see Fig. 17.7.5C).

If faul t cu rrent infeed thro ug h the t ransform er feeder 1.v. c i rcui t breaker is

s ignif icant for faul ts occurr ing bey on d the h.v. c i rcui t breaker, e i ther the direct ionalovercurrent pro tect ion m ust have an op erat ing t ime suff ic ient ly long to obtain dis-cr iminat ive operat ion, or the direct ional overcurrent re lay must be inter locked withthe dis tance pro tect ion at the h.v. c i rcui t breake r end via a pro tect ion s ignal lingchannel so that i t canno t o perate unless the dis tance protec t ion has opera ted. Thisis referred to as inhibi ted d irect ional overcu rrent p rotec t ion.

If the faul t current infeeds at the h.v. end of the t ransformer feeder are alwaysexpected to remain within a restr ic ted range, very s imple and relat ively cheap high

set overcurrent and restr ic ted ear th-faul t p rote ct ion m ay take the place of the h.v.end dis tance p rotec t ion as show n in Fig. 17.7.5D. ( See Chapter 12, Sect ion12.9.2(d)) . By choosing sui table set t ings for El , E2 and the h.s .o .c , re lays operat ionof the pro tect ion can be restr ic ted to faul ts occurr ing betw een the feeder pro tect ioncurrent t ransformers and the t ransformer h .v. neut ra l connect ion .

The high-set overcurrent must be set so as not to operate for faults on the l .v.s ide of the t ransformer. The s tar /del ta interposing current t ransformer el iminateszero sequence currents f rom the inputs to the E2 relays; the set t ing of these E2

relays mu st however take in to account , for fau l ts on the h .v. sys tem bey ond the



The application o f prote ct ion to t ransmission sys tem s 39 5

t

-%

]

/

t :~ m m ~ l

II

III

1

- u

" i . - J

i I " : " Ir I II I I ,,_,,' , I l l

~ - - - f f - ~ ,, , ~ - ~j L _ ~L , . - - . , .

r

1 3 2 k V

D

IIII

I'III

I ' IIIIII

- - i - I .

I

II

---- 41k-

II

I

,,

_; IL J

i

F i g . 1 7 . 7 . 5 B Protection scheme for transformer feeder with 132 /11 /11kV dual secondary

transformer



3 9 6 T h e a p p l ic a t i o n o f p r o t e c t i o n t o t ra n s m is s io n s y st em s

| | ,1--O------I,o.~

F i g . 17.7.5C Direct ion al overcurrent pro tec t ion on a t ransform er feeder

1 3 2 k Vbusbars

w

[ H S l

J l H s I[ - g ] _ _ _ [ o c I

E l E 2

: 0 0 -

0 0 - - - - ' - ' - -

Tr i p

F i g .17.7.5D High set overcurrent w i th res t r ic ted ear th fau l t pro tec t ion



The application o f protect ion to transmission sys tem s 397

feeder p ro tec t ion c . t. s, any nega t ive sequence cur ren t bac kfeed th rough the powert ransformer, inc lud ing the backfeed f rom s tored energy in any ro ta t ing p lan t on thel .v. ne two rk fed by the pow er t rans form er. Where necessary, the se t t ing of bo t h E1and E2 m us t a l so take in to ac count the cu r ren t d i s t r ibu t ion when para lle l t rans -

former feeders supply a co m m on l .v. busbar. The E l , E2 and h .s .o .c , re lays shouldal l be designed to be re la t ively im m une to the effects of d .c . t ransients to avoidunw anted opera t ion due to any d .c . com pon ent in the fau lt cur ren t and trans-former magnet i s ing in rush cur ren ts . To ba ck up the in te r t r ipp ing on a t rans forme r-feeder, i t i s of te n the pract ice for faul ts on the t ransfo rm er or it s associa ted l.v.connect ions to unstabi l ise any feeder uni t protect ion or accelerate the h .v. enddis tance pro tec t ion .

In some cases , to ba ck up the in te r t r ipp ing , the t rans forme r h .v. d i sconnec tor

may be opened au tomat ica l ly a f te r a su i tab le de lay. In the event o f the o ther meansof t r ipping the remote c i rcui t breaker having fa i led i f the current is very low, e .g .f rom a Buchholz opera t ion , the h .v. d i sconnec tor m ay success fu l ly b reak thecurre nt . I f i t fa ils to b reak the curre nt a local faul t wi ll be produce d b y the arcinga t the d i sconnec tor which wi ll resul t in opera t ion of the feeder p ro tec t ion a t theh .v. end . The ph i losophy of th i s t echnique is tha t the co nsequences of an unc learedfaul t on the t ransformer or i t s associa ted l .v. connect ions may be worse (e .g . at ransform er f i re) than i f an a t t em pt w ere no t made to open the d i sconne c tor.

For many appl ica t ions , par t i cu la r ly for long t ransformer- feeders , ins tead ofprovid ing h i red or p r iva te p i lo t in te r t r ipp ing i t may be cheape r and mo re d ependab leto ins ta l l a faul t - thro we r. At 132 kV, the presen t design of faul t throw ers is l imitedto a m ax im um fau l t l eve l o f 2500 MV A. I f fau lt th rowers a re ins ta l led , d i rec t iona loverc urrent pro tec t ion is nor m al ly used to t r ip th e l.v. c i rcui t break er for feederfaults .

1 7 . 7 . 6 P r o t e c t i o n fo r a d o u b l e b u sb a r a n d m e s h s t a t io n

The pro tec t ion for a double busbar and mesh s ta t ion i s usua l ly iden t ica l to tha tdescr ibed in Sec t ions 17 .6 .6 and 17 .6 .7 excep t tha t the am oun t o f redun danc y canbe reduce d, e .g . there is in general no need for m ore than one discr iminat ing zonerelay and check zone re lay to in i t ia te t r ipping a t a double busbar s ta t ion, i .e . i t canbe a s imple ' two ou t o f two ' re lay log ic scheme wi th no redundancy. For meshs ta t ions a 'one ou t o f one ' re lay logic scheme is norm al ly ade qua te .

1 7 . 7 . 7 P r o t e c t i o n f o r c o m p l e x p ri m a ry c ir c ui t c o n f i g u r a t io n s

The same cons idera t ions which a re ment ioned in Sec t ion 17 .6 .8 apply for choos inga protec t ion schem e, bu t the longer permiss ible faul t c learance t imes ease theproblem. Also the need for high emergency load t ransfer capabi l i ty is usual ly not assevere a prob lem as on a major t ransmiss ion sys tem.



39 8 The app l icat ion o f pro tect io n to t ransmission systems

17.8 Bibliography

General application

'Review of recent changes in protection requiremen ts for the CEG B 400 and 275kV systems' by J C W hittaker (lEE C onf. Publ. 125, 1975, pp.27-33 )'Rec ent developments in C EG B protection policy and practice' by J C W hittaker(lEE Conf. Publ. 185, 1980, Discussion Record, pp.l-4)'Cost o f power outages - the 1977 New York City Blackout' by W T Milese t a l .

(IEEE Industry Applications Society Annual Meeting, Seattle, 1979, pp.65-69)

Feeder pro tec t ion

'Ultra high speed relay for EHV/UHV lines based on directional wave detectionprinciples' by R P Carter (lEE Conf. Publ. 185, 1980, pp.166 -170)'The application of phase comparison protection to EHV transmission lines' byG Fieldinge t a l . (UM IST Proc. of Internat. Conf. on Feeder Protection, 1979)'Performance of distance fault detector relays' by W P L ewis (lEE Colloq. Dig1968/19 , pp239 -249)'Analysis of complex distance-relay characteristics taking load into account' by

L P Cavero ( lEE Conf. Publ. 185, 19 80, pp.192-194)'High speed protection for transformer feeders without pilot wires' by J Rushton(Electr. Energy, April 1958, p.132)'Pro tection aspects of m ulti-term inal lines' (IEE E Special Publ. 79, TH0056-2-PWR )'An evaluation of the comparative performance of distance and differential feederprotection systems' by W D Humpage and J Rushton ( lEE C olloq. Dig. 196 8/19,p .267)

Automatic switching

'A coherent scheme for the design and testing of automatic switching equipment'by C R Seym our ( lEE Conf. Publ. 185, 1980 pp.69-73 )



Tes ting , com m issioning andmanagement of protect ion

by E.C.Smith, revised by D.Hay

Chapter 18

18.1 In t roduct ion

This chap ter deals wi th protect ive gear thro ug ho ut i ts li fe , f rom i ts being specifon a scheme, th rou gh the t es t ing and co mm ission ing s tages in to rou t ine main tenaand faul t inves t igat ion. T he m ana gem ent of pro tect ive g ear covers a l l those aspeand more ; fo r example , keep ing records , a ssess ing per formance , and the avo idaof m is takes in tes t ing.

The ch ap te r does no t a t t e m pt to desc r ibe in de tai l ho w to comm iss ion aspeci fi c ty pe o f p ro tec t ion , bu t seeks to es tab li sh p r inc ip les wh ich app ly to schemes . S imi la r ly the recommenda t ions re la te to the t es t s which need to be dowi thou t spec i fy ing who sha l l do them.

18.2 Con trac tual obl iga t ions

The re is l i tt le d ou bt th at , w here i t is econ om ical ly poss ible , i t is be t ter for the uof the equ ipm ent to com miss ion i t h imse lf ; by so do ing he has a keen in te res tt he eq u ipm en t r igh t f rom the s ta r t and has t he op po r tun i t y t o ge t t ho roug hfami l ia r wi th i t be fore i t goes in to commiss ion . Th is knowledge wi l l s t and h imgood s tead dur ing su bsequen t m ain ten ance , and especia lly dur ing fau lt inves tiga t

However, fo r va r ious reasons i t i s cus tomary to spec i fy tha t the equ ipment shbe 'pu t to w ork ' by the co n t rac to r. Th is imm edia te ly in t roduces such d i ffi cu l ti e s'mar ry ing ' ex tens ions to ex i s t ing schemes , and connec t ing d .c . and a .c . supp l iesn e w e q u i p m e n t .

Such di ff icul ties can ar ise even w he n one co ntr acto r i s ent i re ly responsible a l l the new equ ipment ; bu t the d i ff i cu l t i e s a re worsened when severa l con t rac ta re invo lved . For example , the re may be d i ffe ren t manufac tu re r s fo r the h

swi tchgear, the 1 .v. swi tchgear, the t rans form ers and the m ul t i core cab les . Eacon t rac to r is r equ i red to p rove hi s ow n eq u ipm ent , bu t he can not a lways do



400 Testing comm issioning and m anagement o f pro tection

t e s t s som eth ing m ay fa il to func t ion ; the cause o f f a ilu re m ay n o t lie w i th tcon t r ac t o r ' s e qu ipm en t bu t m ay be due t o c ro s sed mu l ti co r e s o r w i r ing on a ssot ed equ i p m en t supp li ed on ano the r con t r ac t . I t is he r e t ha t s om e c o .o rd in a tinf luence i s necessary.

Th is co-ord ina t ion can bes t be done by the use r o r the consu l t ing eng ineeVery o f ten , in the in te res t s o f exped iency, the use r wi l l inves t iga te the t roub le h ise lf , bu t th i s is con t ra c tua l ly w rong . I f a co n t ra c to r is r e spons ib le fo r se t ting equ ipme n t t o w ork , t he u se r shou ld r e f r a in f r om so r t i n g ou t h i s t r o ub l e s f o r hHe shou ld , as fa r a s poss ib le , r equ i re each co n t ra c to r to p rove h i s ow n e qu ipm en

W hen severa l co ntra c tor s are involved o n a projec t i t is advisable to havegroup d i scuss ion be f ore han d to ou t l ine the t e s t s wh ich have to be done and cons ide r the e ffec t o f those t e s t s on the equ ipment as a whole .

A l thou gh many t e s t s may have t o be do ne t o p rov e a comp re he ns ive s ch emsom e a re usua lly mo re v i ta l than o the rs . T he use r m ay n o t be ab le to be repsen ted a t a l l the t e s t s ; he wi l l have to dec ide which he mus t do h imse l f , which wi l l par t ic ipate in or c lose ly supervise , which he wi l l le t the contrac tor do toprev ious ly d rawn-up p rogramme ( inspec t ing the t e s t r e su l t s a f t e rwards ) , and whii f any , he wi ll accep t ve rba l a ssurance f rom the con t rac to r a s hav ing been don eh im .

W hether the re is c lose pa r t i c ipa t ion o r w he th er the re is a lm os t casua l superv is ithe use r mu s t r em em ber one th ing ve ry c lea r ly : he , and he a lone , is r e spons ib le ensur ing th a t the new equ ipm ent has been cor rec t ly ins ta l led and tha t i t can sa fbe con n e c t ed t o t he sy s t em.

The con t rac to r m us t sa t i s fy the use r o f th i s . I t is co m m on prac t i ce , pa r t i cu lafor m ajor ins ta l la t ions , for the Bo ard ' s spe ci f ica t ions to ca l l for 't es t in g and se t tto work ' ; the spec i f i ca t ions a l so requ i re a 12-month main tenance pe r iod , dur

which any fau l t s caused by poor des ign o r bad workmansh ip a re pu t r igh t f reecharge b y t he m anu fac tu r e r.

18 .3 The men ta l approach to com miss ion ing t e st s

A fun d a m en ta l t hou gh t shou ld be kep t i n m i nd w he n ap p roa ch i ng any co mmsion ing t es t : the t e s t i s be ing do ne to p rove the equ ipm ent i s co r rec t ly ins ta l ledm a y n o t b e .

Throughout the t e s t s , c r i t i ca l a t t en t ion shou ld be pa id to a l l the aspec t s o f work ; no th ing shou ld be ' g lossed over ' wi thou t a fu l l exp lana t ion be ing sought . Ion ly b y pay ing minu t e a t t en t i on t o d e t ai l t h a t a h igh s t an da rd o f pe r fo rm ance be ach ieved .

1 8 . 4 C o m m i ss io n i n g t es ts

18 .4 .1 Reason s fo r com miss ion ing t e st s

C i i i d



Testing comm issioning and managem ent o f prote ction 401

a )

b )c )

(a)

Tha t equ ipm ent has n o t been dam aged in t rans it and tha t i t can be sa fe ly acon f iden t ly conne c t ed t o t he sys tem .Th a t the spec if ied e qu ipm ent has been cor rec t ly ins ta l led .To prove character is t ics of the protect ion which are based on calcula t ions , example the p r imary se t t ings o f h igh impedance pro tec t ion sys tems .To obta in a se t of tes t f igures for fu ture reference i f and when necessary.

Com m issioning tes ts , as wel l as cover ing protect ive gear, co ntro l and a larm ci rcua l so cover much of the p r imary p lan t wi th which such equ ipment i s assoc ia tS ince, how ever, th i s boo k dea l s w i th pow er sys tem pro tec t ion , no th ing m ore wi lsa id o f p r imary p lan t commiss ion ing excep t tha t , on any pr imary c i rcu i t be i

com m iss ioned , i t is a good plan to consider i t as being bui l t o f uni ts such as busbbusbar protect ion, c i rcui t breakers , t ransformers , cables , d isconnectors , protectgear, in te r t r ipp ing , and so on . Each such un i t should be p roved before a t t empt iconjunct ive tes ts on the complete c i rcui t .

1 8 . 4 . 2 P l an n i n g o f c o m m i s s i o n i n g t es t s

I t i s des i rable to p lan com m iss ioning tes ts in a logical ord er, ensur ing that no e qum ent is over looked and , once tes ted , is no t unnecessari ly d i s tu rbe d , thus requ ira re . tes t . A commiss ioning programme should be planned as fol lows:

S t u d y o f D i a g r a m s

(a) B loc k diagrams:I t is helpful to have (or to prepare i f non e is av ai lab le)b lock d iagram of a sub s ta t ion tO show the l ines , t r ans formers and busbars and

c . t . and v. t . posi t ions . The var ious types of protect ive gear may be indicated block out l ine wi th th in sol id l ines between them and the c . t . s , and dot ted l inbetween the re lays and the c i rcui t b~eakers they t r ip .

A block diagram o f th is k ind (Fig . 18.4 .2A) presents a very c lear p ic ture of p ro tec t ion and con t ro l l ayout a r rangements o f a subs ta t ion .

(b) Circuit diagra m s:The manufac ture rs wi l l o f ten supply c i rcu i t d iagrams , buif no t the y should be ex tracted f rom the m ain wir ing diagrams. Sep arate c i rcdiagrams can be prepared for d .c . t r ipping , c los ing, a larm and indicat ion c i rcuand a .c . d iagrams for var ious typ es o f pro tec t ion, vol tage and sync hronis ing c i rcuEach c lass of c i rcui t can be shown on a separate d iagram so that each can be s tudand tes ted wi thout confusion. A typical c i rcui t d iagram is shown in Fig . 18.4 .2

Tw o wa rnings are necessary w i th regard to the use of c i rcuit d iagrams; bow arnings involve com pletene ss o f the d iagram. I f an eng ineer is ext rac t ing his oci rcui t d iagram, he would be wise to make qui te sure that he has included the wh

of the c i rcuits co nn ec ted , say, betw een any given fuse and l ink; b y d oing a fdiagram he can be conf ident that no poss ible ' sneak ' c i rcui ts are over looked



402 Te st ing omm issioning and management o f prote ct ion

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t ha t i t shows eve ry haza rd in t he c i r cu i t . Fo r example , a t one t ime o . c .b , r ack - inp lu g a n d s o c k e t c o n n e c t i o n s w e r e n o t a l w a y s s h o w n o n tr ip p in g a n d c lo s inc i rcu i t d iagrams. I f c i rcu i t d iagrams a re be ing drawn by the user, he should makethem as s imp le a s poss ib l e , i gno r ing the phys i ca l l ayou t o f t he equ ipmen t andmak ing h i s d i ag ram a pu re e l ec t r i ca l d i ag ram, avo id ing unneces sa ry c ros sove r s ande n su r in g a s m o o t h f lo w o f t h e e y e f r o m s i d e t o si d e o r t o p t o b o t t o m w i t h o u t a nunn ecessa ry ' t o and f ro ' m o t io n . C i r cu i t d i ag rams o f a s emiphys i ca l t ype , w h ich a rd r a w n s o t h a t th e n u m b e r o f m u l t i c o r e s n e c e ss a r y b e t w e e n t w o p o i n t s c a n breadi ly assessed , a re a lmos t as d i ff icu l t to fo l lo w as a w i r ing d iagram, th rou gva r ious co n tac t s and co i l s , un t i l t he c ir cu i t f rom pos i t i ve t o nega t ive is co m ple t eby w h ich t ime (un le s s t he eng inee r has an unusua l ly r e t en t ive me m or y) a s t a t e o

pa r ti a l con fus ion w i l l r esu l t.Good c i r cu i t d i ag rams a re i nva luab le du r ing commiss ion ing t e s t s and f au l t



404 Te sting ommissioning and management o f protection

From a c i rcui t d iagram i t i s of ten poss ible to d iagnose the reason tbr a c i rcui t fabefore do ing any conf i rmatory t es t ing .

P l a n n i n g t h e p r o g r a m m e

(a) Overal l review:The equ ipm ent to be comm issioned m ay vary f rom a singaddi t iona l c i rcu i t co nnec ted to a low vol tage busba r, to a com ple te 40 0 k V doubusbar subs ta t ion w i th t rans form ers , genera to rs and l ines.

In e i ther case the m eth od of ap pro ach should be th e same. Fi rs t of a l l, a should be prepared i temis ing a l l the tes ts i t i s necessary to do; for example , inst ion res i s tance , secondary in jec t ion , p r imary in jec t ion , con t ro l , ind ica t ion , inlocks , a larms, t r ipping, end- to-end, auto-reclose , synchronis ing, on- load tes ts e tcsom e cases i t m ay be helpful to have sheets available hea ded , for exa m ple , 'd iscne cto rs ' or ' c i rcui t break ers ' or ' t ransfo rm ers ' , an d on these sheets have a l isi t ems tha t have to be inspec ted o r t e s ted before tha t pa r t i cu la r p iece o f equ ipmcan be sa id to have been checked.

Fig . 18.4 .2C gives the schedules in rough ou t l ine of som e of the tes ts usuadone on d i ffe ren t p ieces o f equ ipm ent . The schedule m ay be e labora ted necessary for a par t icular c i rcui t conf igura t ion.

Fig. 18.4.2C Typical schedule of protect ion commissionir g tests

In spec t ion p r io r t ode t a i l ed t e s ti ngTerm ina l t igh tnessCheck of fe r ru le numbersInsula t ion res is tance tes ts

Examina t ion o f aux i l i a ryswitches

O rcu i t b reakersTiming tes tsCheck of c . t . pos i t ions

D i s c o n n e c t o r sOpera ti ng m echan i smControl c i rcui ts

Vol tage t r ans fo rm ersR a t i o

Back-energisingPhasing

Trans fo rmersPhasing a t 415 vWinding t emp e ra tu r e

indicatorsBuchholz device

Cur ren t t r ans fo rmersM agnet ic curvesPola r i tyR a t i o

Tripp ing tes t sTrip c i rcui t supervis ion

Pro tec t ion gear to t r ipp ingrelay

Tr ip in te r locks

Tr ipp ing re lays to b reakersIn te r t r ipp ingAcce le ra t ionAutorec lose

A l a r m s c h em eShor t ing o f in i ti a t ing

con tac t s

Proving f rom all in i t ia t ingdevices

Pro tec t io nSecondary in jec t ionPr imary in jec t ionOn-load tests

Whenever poss ib le the re shou ld be a pa t t e rn in the approach to commiss ion



Testing comm issioning and management o f prote ction 40 5

gear and the d.c. circuits as feedingo u t f r o m the protect ive gear to the t r ippingre lays and c i rcu i t b reakers they t r ip . Th is m etho d of w ork ing breaks the job up eas i ly manageable sect ions a t an ear ly s tage and enables faul ts to be quickly c lew i tho u t the r isk o f losing t rack i f a com ple te sequence o f t e s ts has been a t t em p

al toge ther.Having proved each sect ion individual ly, tes ts to check the overa l l funct ion

can b e don e fa i rly qu ick ly . The cause o f any fa i lu re to op era te sa t i s fac to r i ly easi ly be p in -po in ted , a s the fa ilu re m us t be in the unproved conn ec t ions be twthe a l ready proved sect ions .

(b) The com m issioning log :A comm ission ing prog ramm e can be ca r ri ed th rou gm ore e ff i c ien t ly and e ffec t ive ly i f p rop er records a re kep t . One conven ien t w ato m ake up a fo lder, so m et im es ca l led the com m iss ioning log, con ta ining s tencisheets for recording"

the p rog ram me o f t es tsbr ie f d ia ry o f day- to -day progressthe var ious tes t resul tst empora ry cond i t i ons

ou ts tand ing i t ems .

The commiss ioning log should a lso include copies of the c i rcui t d iagrams to be udur ing the t es ts , and an y o th er shee t s tho ugh t necessa ry.

Such a commiss ion ing log enab les a comprehens ive record to be kep t o f twhole job , which wi l l be o f va lue no t on ly dur ing the t es t ing p rogramme bu t any future faul t inves t igat ion.

( c ) P rogramming :Tes t s need to be ca re fu l ly p rogrammed, no t on ly to ensurtha t equ ipm ent is r eady on t ime , bu t as regards p lan t ou tages , pe rmi t s , a l loca tionstaff , e tc . T he tes ts should a lso be pro gram m ed to take place in a logical ae ff ic ien t o rder so tha t no th ing a l ready tes ted is d i s tu rbed dur ing subsequ en t t e st s

In p repar ing an overa l l p rogramme of manufac tu r ing t ime , de l ive ry, e rec t ioinspec t ion , t e s t ing and commiss ion ing , adequa te t ime mus t be inc luded fo r the lthree i tems. Engineers engaged on the f inal commiss ioning tes ts are doing thei r p

of the job a f te r m os t o f the o the r peop le have f in i shed the i rs ; the re is o f ten ipa t i ence to ge t the new equ ipm ent in to com miss ion , and the comm ission iengineers are l iable (usual ly unjus t i f iably) to receive cr i t ic ism for any delays making a l ive , and are somet imes urged to cut shor t the i r tes ts . Commiss ionitest s m ay reveal severa l de fec t s which need cor rec t ing , and a reasonab le am ou ntt ime for such rec t i f ica t ion should be included in the overa l l p lanned t ime.

18 .4 .3 In spec t ion p r io r t o t e s ti ng



406 Te sting omm issioning and management o f protec tion

(a) Check ing for t ightness a ll co nn ec t ions in re lay panels , co ntro l panels , m arshl ing kiosks , c .t . te rm inal bo xes , e tc .

(b) Checking that fuses and l inks are pro pe r ly wired. In the case o f rewirafuses , the wire should be t inned copper -n o t a so f t l e ad compound wh ich candeform and e i ther b low at a current less than i t s ra t ing or become loose af ter sevweeks . The fuse wire should not be taut , as i t i s l iable to break when the fusebeing inser ted in i t s holder. Car t r idge- type fuses should be inspected to ensure tthe correct rat ing is used for the circuit .

I f the l inks a re o f the type w hich a re wi red wi th s t i ff wi re they sho uld inspected to ensure that the wire s t i ffness i s not d is tor t ing the jaws and thus pven t ing adequa te con tac t p ressure .

(c ) Clearances should be checked be tw een re lay studs and the edges o f the hoin the s tee l panel , making sui table a l lowance for poss ible dust accumulat ion othe years .

(d ) Ensure tha t a ll the secon dary equ ipm ent in ou tdo or subs ta t ions is wea th

proof , but that there is provis ion for vent i la t ion and heat ing ( i f required) to prevcond ensa t ion , in accordance w i th the Board ' s spec if ica t ion .

(e) P ar t icular a t ten t ion should be paid to auxi l iary swi tches . These appe ar a lm ost every c i rcui t - for exa m ple , t r ipping, c los ing, indicat ion, a larm, synch roning, autorec lose , e lec tr ica l bol t in ter locks , busb ar p rote ct io n, a nd occasional lyc . t. c i rcu it s o ther than busbar p ro tec t ion .

P oints to loo k for are s ilver p la t ing w here specif ied , co ntac t pressure , c leanl ineadequa te 'w ipe ' and fo llow th roug h . The dr ive should be exam ined to ensure ti t is posi tive , no t l iab le to j am or be com e d i sconne c ted f rom the m ain m echanisand tha t i ts ac t ion c or responds w i th the m ain swi tch . On auxi li a ry swi tches used busbar protect ion c . t . se lect ion, i t i s essent ia l that they are dupl icated, and that aux i l i a ry swi tches make before and break a f te r the assoc ia ted pr imary con tac t s ,as to ensure tha t the seco ndary c ircu it s a re com ple te before any pr imary unba lancan occ ur du e to pre-con tact arc ing or a h igh res is tance in one par t of the pr im

circui t. T es ts shou ld be devised to prove tha t the auxi l iary swi tches m eet trequ i reme nt . The imp or tance of cor rec t ope ra t ion o f aux i l ia ry swi tches cannot overemphasised, as fa i lure of an auxi l iary swi tch can defeat the bes t des ignschemes.

( .I") Before energising new sec on da ry wiring i t should b e ch eck ed as far reasonably prac t icab le tha t the re a re no bare , dangl ing , uncomple ted leads a

where on s i t e which wi l l be made a l ive a t the same t ime as the equ ipment untes t . Any such leads should be t e rmina ted , d i sconnec ted or (as an abso lu te mi




1 8 . 4 . 4 T h e t e s t s

We can a s sume tha t t he t e s t s t o b e done h ave a l r ea dy be en s chedu l ed , p r imci rcu i t by p r imary c i rcu i t , p robab ly wi th t a rge t da tes . Before beg inn ing t es t in

shou ld b e con f i rmed tha t the pa r t i cu la r pa r t o f the c i rcu i t und er t e s t is com pand tha t no one is l ike ly to in te r fe re w i th i t . The t e s ts to be do ne shou ld be l iin chrono log ica l o rder to ge the r w i th an y spec ia l p recau t ions to be t ake n . The twi l l f al l und er four b ro ad head ings :

( a )(b)(c)

(a)

Tes ts be fo r e m ak ing s econda ry eq u ipm e n t a li ve .Te s ts b e f o r e m ak ing p r ima ry e qu i pm e n t a li ve .Syn chron i s ing t e s t s where the new eq u ip m en t is a live, bu t no t ye t in pa raw i t h e x i s ti n g e q u i p m e n t .On- load tes ts .

Schedu les fo r r ecord ing t es t r e su l t s shou ld be compi led in advance , when p rept ions can be m ade a t a t im e f ree f rom tes t ing s t ress . The in i ti a l and com prehenadvance p lann ing i s a b ig he lp in making the t e s t s run smooth ly.

Be lo w a r e li s ted som e o f t he m or e u sual te s t s , w i t h c o m m e n t s .

(a ) Insula t ion res i s tance tes ts :Thes e t e s ts s ho u ld b e d o ne w i th a 5 0 0 V M eggeca re b e ing t aken t ha t t e l ephone t ype p i l o t s a r e no t i nc luded . To en su re t ha t tvo l tage does no t appea r ac ross rec t i f ie r s in the even t o f a wi r ing e r ro r, o r ac rcapac i to r s a n d so li d s t a te e qu ipm e n t , t he y shou ld be sh o r t ened ou t u n t i l M etes t s a re co m ple te . Each c i rcu it shou ld be t e s ted in tu rn w hi l st a ll the o the rs e a r t hed t o en su re t ha t t he r e i s no c onn ec t i on be t w ee n tw o s upp osed l y s ep a r

c i rcu it s . Th is po in t is pa r t i cu la r ly im po r tan t and needs s t ress ing .I t is d i ff icu l t to adv ise w ha t va lue o f insu la t ion res is t ance shou ld be ob ta ined .pane l and o th e r indo or w i ring one w ould exp ec t a f igure in excess o f 100 M f~ou tdo o r w i r in g i n a m ed ium-s ized s ubs t a t i on , 1 0 M ~ w o u ld b e a f air f igu r e; an da ve ry la rge subs ta t ion , w here wi r ing loops in to m an y au x i l i a ry swi tches j unc t i on b oxes , 1 M f~ w ou ld no t be un rea son ab l e . Th e w ea the r can a ls o a f f ec tresu lt s ; a w arm hu m id d ay t en ds to lower the va lue , w hereas a co ld d ry d ay ghigh values.

Of ten the M egger po in te r wi l l swing tow ards ze ro wh en m egger ing beg ins oc i rcu i t . Th i s i s because the co nn ec ted wi r ing has capac i t ance ; the p o in te r wi l l c ltow ards in f in i ty as the wi r ing bu i lds up i ts charge . I t is adv i sab le no t to tou ch sw i r ing i m m ed i a t e ly a f t e r s t opp ing w i nd i ng

(b) Seco nda ry in jec t ion tes ts :These t e s t s cons i s t o f a . c . in jec t ion in to the re laycoi ls to prove th a t the re lay ca l ibra t ion i s corr ec t . Even i f the re lays have be

tho rough ly t e s t ed i n a l abo ra to ry o r i n t h e m an u f ac tu r e r ' s work s , s ome k in d check sh o u l d be made when t hey a r e m ou n t ed on t he p an e l t o en su re t h ey ha ve n



408 Te sting commissioning and management o f protection

In genera l , h igh-accuracy secondary in jec t ion t e s t s on s i t e a re no t necessaL ab ora to ry s t andard s a re n o t be ing sough t . What is need ed is a se t o f f igures , r ed u c i b le l a t er f r o m t i m e t o t i m e o n r o u t i n e t e s t s, w h i c h b e a r a k n o w n r e la t io n s h i paccura te f igures p rev ious ly ob ta ined in a l abora to ry. I f , however, the p ro tec t ive g

c o n t a i n s t u n e d c i rc u i ts w h i c h h a v e t o b e ca l ib r a te d b y s e c o n d a r y i n j e c ti o n o n cmiss ion ing , due regard shou ld b e pa id to the f re que ncy and w ave shape o f the s u p p l y t o e n s u r e t h e r e a r e n o h a r m o n i c s p r e s e n t .

There a re several ty pes o f r e lays , and these a re d i scussed b r i e f ly under thappropr ia te head ings .

( i ) General :Whatever type of re lay i s be ing tes ted , i f i t i s f i t ted wi th a p lug br idthe t e s ts shou ld be don e o n the lowes t t ap so tha t the w hole o f the co il is inc ludC a l ib r a t io n c h e c k s m a y b e d o n e o n o t h e r t a p s i f d e s ir e d , b u t t h is s h o u ld n o tnecessa ry. I t is , how ever, w ise to check the c on t inu i ty o f each co i l t ap . Th i s cand o n e b y m o v i n g t h e s e t t i n g p l u g f r o m t a p t o t a p a n d c h e c k i n g t h a t c u r r e n t f l ow h e n t h e t e s t s u p p l y is sw i t c h e d o n . T h e p lu g b r id g e s h o r ti n g c o n t a c t s h o u l dproved w hen the se t t ing p lug is w i thdra w n. Th i s shou ld be don e wi th the tcu r ren t swi tched on , and i t shou ld be obse rved tha t wi thdrawal o f the p lug does

open-c i rcui t the re lay coi l .Al tho ug h the t e s ts m en t ion ed a re o f an e lec tr i ca l na tu re , due regard shou ld

pa id to the mec han ica l cond i t ion o f the re lay. Dus t and o the r fo re ign bod ies shob e r e m o v e d w i t h a f e a t h e r, o r b y a b l o w e r ;n o t b y t h e b r e a t h b e c a u s e t h i s c o n t a i n sm a n y d r o p l e t s o f m o i s t u r e . T h e r e l ay m o v e m e n t s h o u l d b e c h e c k e d f o r c l ea r a n cegaps , con ta c t a l ign m ent , and f reedo m of op era t ion . I f the re lay is f i tt ed w i thm echan ica l fl ag , the f lag shou ld op era te a s , o r jus t be fo re , the con tac t s m ake ,

the f l ag mechan i sm shou ld no t in te r fe re wi th the opera t ion o f the re lay.

( i i ) Ins ta ntaneo us re lays:These wi l l be ac tua ted by cur ren t o r vo l t age and wiu s u a l l y b e o f t h e a t t r a c t e d - a r m a t u r e t y p e . T h e c u r r e n t s h o u l d b e i n c r e a s e d s l o wu n t il o p e r a t i o n o c c u r s. T h e c u r r e n t v a lu e i m m e d i a t e l y p r io r t o o p e r a t i o n s h o u l dno te d because o f the change in r e luc tance o f the m agne t i c pa th o f the re lay (Chap te r 6 ) . The cur ren t o r vo l t age shou ld then be dec reased s lowly, and the varecorded a t which the re lay rese t s .

( ii i) Inverse def in i te m in i m um t im e ( i .d .m . t . ) r e lays :I t shou ld be checked tha tthe con tac t jus t makes on ze ro t ime mul t ip l i e r ( t .m . ) . Also the rese t t ing t ime a t t1 - 0 s h o u l d b e m e a s u r e d a t z e r o c u r r e n t . F o r r e c o r d p u r p o s e s , t h e m a x i m u m c u r rwh ich wi ll a l low the re lay to r e se t fu l ly shou ld be asce r t a ined .

I t shou ld no t be necessa ry to p rove the re lay curve , bu t a t l eas t two po inshou ld be ch ecked on i t; one to f ind the m in im um cur re n t , w i th t .m . = 1 43

wh ich the re lay w i ll jus t ope ra te (usua l ly r e fe r red to a s the c reep va lue ), and o the r to check the t iming a t , s ay, four t im es cur ren t o r vo lt age se t t ing , a



Testing comm issioning and management o f prote ction 409

(iv) Biased relays:Such re lays have a t leas t two co il s; one t endsto c au se o p e r a t i o nwh i l st the o the r ( s ) r es t ra ins . The m eth od o f t e s t wi ll va ry w i th the re lay typ e app l ica t ion , b u t i t is usual to ch eck the b ias curve by m easur ing the cur ren t neein the opera t ing co i l to cause ope ra t ion w he n d i ffe ren t va lues o f separa te ly in jec

cur ren t a re passed th rou gh the b ias co il . Care shou ld b e t ak en no t to overhea t r e lay by the fa i r ly l a rge opera t ing cur ren t necessa ry to overcome a s t rong bcur ren t . Only th ree o r four po in t s on the curve need be checked .

Wi th s om e t ypes o f end -to -end p ro t ec t i o n , it may be n ece s sa ry t o c a r ry os imu l t aneo us i n jec t ion t e s ts a t b o th e nd s t o che ck , f o r e xa m p le , t he b ia s cha r a cis tics of a teed feeder p i lo t prote c t io n sch em e. Severa l w arnings shou ld be i ss

hero:I f the (dead) overhead l ine i t se l f is used as pa r t o f the second ary in jec t i

equ ipmen t , t h e g r ea t e s t c a r e shou ld b e exe r c i s ed w i th t he eq u ip m en t bec auseinduced vo l tage f rom o the r l ines on the sam e towers o r in the v ic in i ty. Suvo l tages can be dangerous ly h igh , and the p ro tec t ive equ ipment and ' l ive ' pa r t st he s ec ond a ry i n jec t ion t e s t equ ipm en t s ho u ld no t be t ouch ed u n le ss t em pora rea r t hed . Su c h w ork shou ld o n ly be car ri ed ou t w i t h th e a u th o r i t y o f a s anc ti on -ftes t card .

End - to -end t es t s using the p r im ary l ine as an in te rco nn ec to r a re dep reca tbecause o f the danger, bu t a re m ent ion ed h ere fo r the record .

I f s imul taneous secondary in jec t ion t e s t s a re to be done a t the two ends ustwo se ts o f t e s t equ ipm ent and separa te supp l ies a t each end , it is e s sen ti a l tthe tw o supp l ies a re in phase o r in an t i -phase , depen d ing on the typ e o f p ro tec t iI f the phas ing i s kn ow n, we l l and goo d . I f no t , i t wi l l have to be es tab l i shed . c a r e f u l n o t t o r e l y t o o m u c h o n t e l e c o m m u n i c a t i o n c o m p a n y p i l o t s a s a m e a n s

phas ing ou t te s t supp l i e s (w h ich sho u l d , o f co u r se , be t r an s fo rm ed t o a s a f e vo l tbe fo r e a pp l y i n g t hem to t he p il o t s) , b eca use t h e i n duc t a nc e an d cap ac i t ance o f p i l o ts c an i n t ro duce app rec i ab l e phase - sh i f t . I n on e c a se , 30 V s upp l i e s a t e ach ew e r e p h a s e d o u t o v e r a p a i r o f t e l e c o m m u n i c a t i o n c o m p a n y p i l o t s b y e n g i n e ewh o ha d n o t a p p rec i a t ed t he pos s ib i l i t y o f phas e - sh i f t . Th e i r re su l t s, w h en p lo tou t , i n d i c a t ed ap p ro x im a te ly 30 ° d i f f e r en ce in phase be tw e en t he s upp l i e s a t t wo ends . To compensa t e f o r t h i s ' d i f f e r enc e ' t he t e s t s w e r e don e p ha se - t o -n eu t ra t one end and phase - t o -phase a t t he o t he r, w hen t he tw o s upp l i e s w e re i n f a c tphase . N eed l e s s t o s ay, m os t i n t e r e s t ing r e su l t s we r e o b t a ined

This p ro b lem of phase -sh i ft in p i lo t s can be overco m e once i t is kn ow n, bu t isa fe r to phase o u t t e s t supp l ies aga ins t a kn ow n vo l tage re fe rence a t e ach end .

W i th end- to .end in jec t ion the re is wide scope fo r e r ro r s , and i t is on ly too eato o ver look a 180 ° sh i f t th ro ug h a t r ans form er.

End- to -end t es t s by secondary in jec t ion shou ld no t be re l i ed on en t i re ly fes tab l i sh ing p ro tec t ive gear po la r i ty. At bes t , they a re on ly a gu ide and the resu

shou ld a lw ays be co nf i rm ed la te r by on- load t es t s .I f a b iased re lay is used fo r the overal l p ro tec t io n o f a pow er t r ans form er i t m



410 Testing comm issioning and management o f protec tion

co i l s t ab i l i ses the re lay aga ins t unwanted opera t ion on magne t i s ing in rush cur rea t t he i n s t an t t he pow er t r an s fo rm er is sw i t c he d o n t o t he s y s t em . Th e ca li b r ato f the tuned c i rcu i t a s soc ia ted wi th th i s co i l shou ld be p roved , and here tf reque ncy o f the t e s t su pp ly shou ld be conf i rm ed as 50 H z , o the rwise inaccurac

m ay be p r e s e n t .

(v) Distance pro tect io n relays:Cer ta in d i s t ance re lays a re o f the d .c . pe rm ane n tm agne t mov ing co il t ype , in wh ich t h e c o il m ov es ax i al ly o r r ad i a ll y. Som e t imes re lay has on ly o ne co il f ed f rom the d .c . b r idge w hich com pares the ma gni tude sthe cur ren t and vo l tage and so is a m easure o f the im ped ance to the fau l t . ano ther typ e the re lay possesses two co il s on the one fo rm er ; one co i l is f ed f r

the rec t if i ed c ur ren t and t end s to cause o pera t ion , whi l s t the o th e r co il is f ed frthe rec t if i ed vo l t age , thus t end ing to res t ra in .

In secondary in jec t ion t e s t s on these re lays , cu r ren t and vo l tage o f va rym agn i tude ( to s imula te d i ffe ren t va lues o f f au l t im ped ance) a re fed in to the ptec t ive gear to p rove tha t the eq u ip m en t i s in ca l ib ra t ion .

(vi) Directio nal relays:Di rec t i ona l r e l ay s a r e f r e qu en t l y o f t he i nduc t i on t ypewh ere the d i sc o r cup is ac ted upo n b y a cur ren t co i l and a vo l tage co i l. Dep endon th e phase ang le be tw een vo l tage and cur re n t , the re lay wi l l e i the r opera tec lose i ts con tac t o r r em ain w i th i ts con tac t s o pen .

Di rec t iona l r e lays usua l ly ac t in con junc t ion wi th o the r r e lays . For exampin a d i rec t iona l overcur ren t r e lay the sens i t ive d i rec t iona l e lement con t ro l s ope ra t i on o f a l ow-se t ove rcu r r en t e l emen t . I n many t yp es o f d i s t a nc e p ro t ec tthe d i rec t iona l r e lay ac t s as a s ta r t ing re lay and con t ro l s the ope ra t ion o f m aauxi l i a ry re lays assoc ia ted w i th the m easur ing e lem ents .

In t e s t ing d i rec t iona l r e lays i t is usua l to p rove tha t th ey w i ll no t opera te o r ec reep in the p resence o f e i the r ac tua t ing qu an t i ty a lone . I t is a lso necessa ry to fthe phase ang le be tw een the cur ren t and app l ied vo l tage to g ive ze ro to rqu e - ti s, the po in t a t which the re lay changes d i rec t ion . I f the app l ied vo l ts a re thm o v e d 9 0 ° f r o m t h a t p o s i ti o n , m a x i m u m t o r q u e s h ou ld b e o b t a i n e d . I n tpos i t io n f igu re s may be t aken o f t h e m i n im um c u r r en t r eq u ir ed t o c a use ope ra ta t r a ted vo l t s , and o f the m in im um vo l tage requ i red to cause op era t ion a t r acu r r en t .

In the case o f d i rec t iona l ea r th - fau l t r e lays , cons ide r w ha t ha ppen s wh en ea r th fau l t occurs on a so lid ly ea r thed sys tem . Th is con d i t ion is dep ic ted vec to r iin (a) of Fig . 18 .4 .4A in which a red phase- to-ear th faul t i s assumed. The re layfed w i th res idua l cur ren t and w i th a vo lt age de r ived f rom op en .de l t a t r ans form eth i s i s sh own byVOD in (b ) and the fau l t cu r ren t is show n byIRE . The re lays fo rt h is app l i c a ti o n a r e de s igned t o ope ra t e w i t h m ax i m um t o rqu e wh en the a pp lcur ren t and vo l tage a re in phase , and i t is , the re fo re , necessa ry to m ove one vec

or the o th e r to ach ieve th i s in -phase cond i t ion und er fau l t co nd i t ions . The vo ltvec to r i s the one m os t con ven ien t ly m oved , and (c ) and (d ) show how the requ i



Testing comm issioning and management o f pro tection 411

o r d er o f 6 0 ° t o 7 0 ° .Measu remen t o f the co mpen sa t ion ang le may be m ade by sing le -phase in j ec t io

on , s ay, t he r ed p r imary wind ing o f t he aux i l i a ry v. t . s and by then measu r ing theopen de l t a F O B ) , capac i to r ( I / c ) , and r e l ay co i l ( I /REL) vo l t ages .

The secon da ry in jec t ion tes t s on d i rec t ion a l ear th- fau l t re lays should b e car r ieou t i n a s imi la r ma nne r t o t he i n j ec t ion t e s t s on d i r ec t iona l ove rcu r ren t r e lays , dua l lowance be ing made fo r t he d i f f e r ence in max imum to rque ang le and sens i t i v i t y.

V R

V B Vy ~

I F

V B

V R IR E Si , . , f

V O D

(a) V oltage and current phasors (b) O pen-delta voltage andfor Red phase earth fault on residual current in relaysolidly-earthed s y s t e m

V O D ~ ~ 7 v C

- - , v '--n compensa t ion

1 _ _ 3 1 lI _ ] - R e l ay -

V O D

( c ) A u x i l i a r y v. t . ' s ,open-del ta andvol tage compensat ion circui t ( d ) I 'h a s ~ r s s h o w i n g a n g l e o f

compensa t ion

- I B a t unity power f a c t o r l o a d

= I R E S

0

I ( e ) R e d v o l t a g e f u s eremovedJ a n d l im b shorted.

Blue c . t . shorted and disconnected.- V R Note similarity to( b )= V O D

Fig . 18.4 .4A Proving o fe a r t h - f a u l t d i r e c t i o n a lrelays

( c ) P r e l im ina ry t es t s on cu r r en t t r an s f o rm er s :Many th ings shou ld be checkedbefore the s tage of heavy-cur ren t in jec t ion i s reached . These a re

i ) Over lap o f c .t .s: W here the c . t . s a re a r ranged to ov er lap the c i rcu i t b reak er sotha t a f au l t w i th in t he b reake r i s cove red by bo th ad j acen t zones o f p ro t ec t ion , i t




A

v

P o s t - t y p e c . t . ' s

¢ ,I I

I Iz I)

Te s tt r a n s f ( ) r m e r

H e a v y t e s t c u r r e n t is p o s s i b l e

2 4 0 V s u p p l y

F i g . 1 8 . 4 . 4 B

Te s t c u r r e n t s e v e r e ly l im i t e d b y im p e d a n c e o f t r a n s f o r m e r

E f f e c t o f c. t. p o s i t i o n o n m a g n i t u d e o f t e s t c u r r e n t

t h e b r e a k e r c o u l d n o t b e c l e ar e d b y e i th e r s e t o f p r o t e c t i o n .T h e p r o v i n g m a y b e d o n e b y v i s u a l i n s p e c t i o n , t h o u g h t b e i n g g i v e n t o t h e e

presen t poss ib i l i ty o f c rossed fe r ru le numbers . I f v i sua l inspec t ion i s imposs ib led i f fi c u l t, t h e n a M e gg er o f c o n t i n u i t y i n d i c a t o r m a y b e a p p l ie d b e t w e e n th e p ro pr ia te r e l ay(s ) and ea r th a t the re lay pane l s , wh i ls t a rov ing ea r th is ' da bb ed '

a n d o f f t h e s e c o n d a r y t e r m i n a l s a t t h e c . t. s t o p r o v e t h e y a r e o n t h e c o r r e c t sid et h e b r e a k e r.

( i i ) The co r rec t c . t . s :Often the re a re several se ts o f c . t .s in the o ne bush ing and i s v i t a l to p rove tha t these a re r e spec t ive ly connec ted to the cor rec t p ro tec t iSo m et im es a ll the c . t. s wi ll be o f the sam e ra t io b u t wi l l have ve ry d i ffe rcharac te r i s ti c s . A t o the r t imes the c . t . r a t io wi l l be d i ffe ren t bu t c lose eno ug hcause confu s ion un less g rea t ca re is t ake n .

Wi th p os t - typ e c . t. s o r c . t .s in an o .c .b , bus h ing the p rov ing o f the cor rec t r a t io c a u se s l i tt le d i f f ic u l t y b e c a u s e h e a v y t e s t c u r r e n t c a n b e p a s s ed t h r o u g h t hb u t w h e r e s u c h c . t. s a re i n th e b u s h in g s o f a p o w e r t r a n s f o r m e r, t h e p o s i t io nm u c h m o r e d i f f i c u l t b e c a u s e o n l y s m a l l t e s t c u r r e n t s c a n b e p a s s e d . T h e t w o c aare indica ted in Fig . 18 .4 .4B.

( ii i) M a g n e t i s a t i o n c u r r e n t t e st s :These t e s t s a re done :

(a)(b)

t o p r o v e t h e r e a r e n o s h o r t e d t u r n s i n t h e c . t . ,



Testing com m iss ioning and mana geme nt o f pro tec t ion 41 3

c )

is tha t po in t on the m agn e t i sa t ion curve w here a 10% inc rease in app lvol tage wi l l resul t in a 50% increase in magnet is ing current ) ,to es tab l i sh the capabi l i ty of the c . t . s . (This i s a l l ied to the point (c) ( i ipage 412 and i s a way o f p rov ing tha t the d i ffe ren t types o f c . t . s can

d i s t ingu i shed f rom each o the r. ) M agne t i sa t ion curves a re ob ta ined by p ly ing a s inuso ida lvol tage to the secondary wind ing o f a c . t . and measur inthe m agne t i s ing cu r ren t f lowing fo r d i ffe ren t va lues o f app l ied vo l t age .typ ica l cu rve is show n in F ig . 18 .4 .4C. There is no need to con t inue th e tb e y o n d t h e k n e e p o i n t . T h e se te s ts m u s t b e m a d e w i t h t h e p r i m a r y o f t h e ope n-c i rcu i t ed . W hen ca r ry ing ou t m agn e t i sa t ion t e s t s on c . t .s , it i s m oim po r tan t tha t the app l i ed vo l tage is p rogressive ly reduced to ze ro be foi n t e r r u p t in g t h e s u p p l y ; o th e r w i s e t h e v e r y h i g h r a te o f c h a n g e o f f lu x minduce a vo l t age su ff i c i en t to damage the secondary insu la t ion .

K n e e p o i n t v o l t a g e is t h a t v o l t a g ea t w h i c h a 1 0 % i n c r e a s e in v o l t a g e

5 0 0 r e s u l t s i n a 5 0 % i n c r e a s e in c u r r e n t

I 0 % ,

4 0 0

0>~ , 3 0 0

2 0 0

I 0 0

I s o II II IIJ I K n e e p o i n t v o l t a g e 4 2 0 V

I ] c . t. r a t i o 5 0 0 / i

I II II II II II II II II I

I II II I

0 5 0 i 0 0 1 50 2 0 0

C u r r e n t ( m A )

F i g . 1 8 . 4 . 4 C A t y p i c a l c . t. m a g n e t i s a t i o n c u rv e

( iv ) F l ic k tes ts :M u c h p r e l im i n a r y c h e c k i n g c a n b e d o n e w i t h t h e a i d o f a 1 .5 ba t t e ry and a vo l tm e te r. All the c . t .s in a g roup can be chec ked in tu rn to p ro



414 Te sting omm issioning and management o f protection

sensi t ive d .c . range, should be c l ipped across (and lef t across dur ing the tes ts) re lay or c . t. seconda ry whi l s t the 1 .5 V ba t te ry is touc hed across the bar p r ima ryeach c . t . in tu rn . Grea t ca re should be t aken to ensure tha t the ba t te ry l eads avol tmeter are appl ied in the same way each t ime. Fig . 18.4 .4D shows the arran

m ent . I t is im por ta n t to no te tha t the d ef lec tion o f the vo l tme te r is the same et im e the b a t te ry is con nec ted , and each t im e i t is d i sconn ec ted .

A f l ick t es t can be don e prof i t ab ly w i th c . t. s in a t rans form er bu sh ing , becauthe d .c . cu rrent can t raverse the t rans form er windings w i th li tt le op po si t ion. Ieven poss ible to do rough operat ion and s tabi l i ty tes ts by excluding or including ba lanc ing neu t ra l c .t . w i th in the pa th o f the ba t te ry cur ren t .

Vo l t m e t e r l e ft

c l i p p e d a c r o s s r e la yd u r i n g t e s t

I V 2 V -c e l l

, , , , , .

- - I . o r w a r d k i c k o n a p p l i c a t i o n o f t e s t l e a d .

T h i s l e ad f l i c k e d B a c k w a r d k i c k o n r e m o v a l o f t e s t l e ad .

o n a n d o f f

F i g . 1 8 . 4 . 4 D A r r a n g e m e n t s f o r f li c k te s ts

(d ) Prima ry in/ection tests:Primary in ject ion tes ts are required to prove that c . t .a re co rrect ly con nec ted to re lays . These tes ts wi l l usual ly include a check o f po la r i ty o f the th ree c . t .s in a g roup , and o f ten the c om par i son o f the g roup w

another. The sens i t iv i ty o f the p ro tec t ion , in t e rms of p r imary amps , may a l sode t e rmined .

At th is s tage a s imple d iagram should be d raw n ( to fo rm pa r t o f the commsioning log) show ing c .t . locat ions , the tes ting t ransfo rm er an d the pos i t ion of tes t leads in the c i rcuit for eac h tes t o f op era t ion, a nd s tabi l i ty, phase- to-phaphase- to-ear th , e tc . Typical d iagrams are shown in Figs . 18.4 .4E, F, and G, but wbe referred to again la ter.

Pr imary in ject ion tes ts can be a prol i f ic source of er ror. The fol lowing fashould be borne in mind:

( i ) The pr imary equ ipment mus t be dead . The eng ineer conduc t ing the t esa l tho ug h po ss ibly no t in charge o f the safe ty aspects o f the tes t , w ould be wadvised to sa t is fy him self that a ll equ ipm en t to be w orke d on has bee n isola ted locked off f rom a ll live equ ipm ent .

( ii ) The w ork wi ll pro ba bly have to be don e un de r a sanct ion-for- tes t car.





416 Tes ting ommissioning and management o f protection

C o n n e c t i o n a p p l i e d t oeach phase in tu rn

. . . . . . . . . 0 L2 4 0 V s u p p ly

/ f ~ : O N/ // / [

/ / I Here for/ & s t a b i l i t y

/ H e r e f o ro p e r a t i o n

Amm~

F i g . 1 8 . 4 . 4 F Prov ing o f c . t. s in t r an s fo rme r bus h in g by i n j e c t io n f rom 2 4 0 V a .c . s up p l y

( iv ) Tes t con nec t ion s wi ll have to be app l ied , and m us t m ake good con tac t in vof the low vo l tages and heavy cu r ren t s w hich a re used .

(v ) The t es t cu r ren t wi l l somet imes f low th rough c . t . s o the r than the onesw hich one i s spec i fi ca lly in te res ted a t the tim e . P recau t ions wi l l , the re fo re , be n eeto p reven t the t r ipp ing o f ad jacen t c i rcu i t s and the remote ends o f c i rcu i t s .

(v i) I t is possib le to p roduce w ha t i s tho ug ht to be an ope ra t ing con d i t ion , w heas , in fac t , the con d i t ion m ay be o f one s t ab i l i ty. Th i s can occur by acc id en tapassing cur ren t in the wro ng d i rec t ion th ro ug h the c . t . ; the ske tch o f the pos i t ions and the t e s t con nec t ion s wi l l he lp to p reven t th i s .

(v ii ) E ar th . fau l t re lay coi ls, s tabil i sing res is tors , non- l inear res is tors and th e l ikeof ten sho r t . t ime ra ted and so may be dam aged un less su i t ab le p recau t ions a re t ak

(v ii i) M ul t ipurpose ins t rum ents m ay be dam aged un less the y a re on the cor rrange fo r the cond i t ions be ing app l ied . Before swi tch ing on the t e s t supp ly, thabo u t t h e wh o le j ob and m ake sur e th a t t he n ec e s s a ry p r e ca u t io ns have been t akThe p recau t ions t aken shou ld be l i s t ed so tha t when the t e s t s a re comple te eveth ing c an be r e s to r ed t o no rm a l w i tho u t t h e r i sk o f a ny t h in g be ing f o rg o t t en .

I t is advisable to sw i tch on a t a low value o f curr ent and increase i t s low ly in c

the re a re an y c . t . s open-c i rcu i t o r inc or rec t ly con nec ted . I f the t e s t r e su l t s d i fapp rec i ab ly f r o m those ex pec t ed , r e duc e t h e c u r r en t a nd s w i tc h o f f wh i l e the m a



Testing comm issioning and managem ent o f prote ction 41 7

w . . . ~

\

IIII Te m p o r a r y

ea r th app l i edI to each phasel in turn

4 1 5 V 3 ph ase t e s t sup p lyapp l i ed he re

Fig. 18.4.4G Proving of po la r i t y o f c.t .s in t rans form er bushings by in jec t ion o f 41 5 V three-phase supp ly on L v. s ide o f pow er t rans form er

A wo r d o f wa rn ing abou t t he b u rdens o f t e s t i n s t rume n t s w h en connec t edc i rcu i t on the miUiamp range . On th i s r ange the ins t rument wi l l have qu i t e a himp edan c e , a nd t h is m ay a f fec t the d i s t ri b u t i on o f cu r ren t s in the s econ da ry c i rund er t e s t . Th i s can be even m ore confus ing i f on ly one in s t rum ent i s ava il ab le i t has to be t r ans fe r red f rom phase - to -phase .

I t is wise to p rove op era t ion f ir s t - tha t i s, p rove tha t th e p ro tec t ion wi ll ope rf rom a t l east o ne se t o f c . t. s . If the c . t. s a re con nec ted to fo rm a c i rcu la ting-cursche m e, s t ab i l ity can be p roved by go ing th rou gh the tw o se ts o f c . t .s in se rThere shou ld be no , o r l i t t le , cu r ren t in the re lay.

F ig . 18 .4 . 4H(a ) shows some t yp i c a l p r ima ry i n j e c t i on t e s t e qu i pmen t . Th e tcu r r en t w i ll u sua l ly be be tw een 10 0 a nd 4 0 0 A . F o r t he s a ke o f c l a ri ty o n thed i ag ram s sho w ing t he d i f f e ren t t ypes o f t e s t , t h e i n j ec t ion e qu i pm en t ha s no t b

d r a w n a g a in , b u t t h e t w o h e a v y. c u r r en t le ad s l ab e ll ed ( 1 ) a n d ( 2 ) a r e t e m p o r a rconn e c t ed t o t he c . t. s be ing t e s ted .F ig . 1 8 .4 .4H(b) shows th ree c . t. s r e s idua l ly connec ted to a re lay. The ra t io

the c . t . s mus t be p roved , a l so the cor rec t connec t ions o f the c . t . s in the g roupthere i s m ore th an one g roup o f c . t .s conn ec ted to a re lay, a s in busb ar p ro tec t(F ig . 1 8 . 4 .4H ( c ) ) , t h en no t on ly m us t e a ch g ro up be p rove d as a g ro up b u t egroup mus t be ba lanced aga ins t the o the rs .

F ig . 18 .4 .4H (d) show s how a ra t io t e s t i s do ne . T he t es t cu r ren t i s pass

th rough one c . t. ( say R ed ) and p r im a r y an d s e conda ry cu r r en t s a re measu red . I fc . t. i s c o r r ec t ly m a t ched t o t he p ro t ec t i on , t h e r e la t ionsh i p be tw e en t h e t w o sho



418 Te sting omm issioning and management o f protec tion

L

a. ~ Varia: ~ ;

N( a ) P r imary in j ec t ion t e s t ing equ ipmen t

R Y B R Y B

M e a s u r i n g c . t . a n d a m m e t e r

r , w , ~ , ( ~ ) R Y B _

~ t e s t i n g t r a n s f o r m e r• < Q - _

(b) Group of c.t.sc o n n e c t e d to a r e l a y

R Y B

(c) S e v e r a l g r o u p s o f c . t . s c o n n e c t e d t o a r e la y

R ~ Y ~ B1 / f

(d) R a t i o t e s t

R j Y BA

Zero

(e) B a l a n c e t e s t R to Y

Group A

|

.

G r o u p B

v v _

G r o u p C,

(f)Balance tes t group agains t group

F i g . 1 8 . 4 . 4 H C o n n e c t i o n s f o r p r i m a r y i n j e c t i o n t e s t s

To p r o v e t h e e . t .s i n t h e g r o u p , i n j ec t in t o t h e R e d c . t . a n d o u t o f th e Ye l l o w a ss h o w n i n F i g . 1 8 . 4 . 4 H ( e ) . T h e r e s h o u l d b e l i t t l e o r n o r e a d i n g e v e n t h o u g h t h e s a m et e st c u r r e n t is f l o w i n g a s b e f o r e . I f t h e s e c o n d i t i o n s a r e s a t i s fa c t o r i l y a c h i e v e dc u r r e n t s h o u l d b e i n j e c te d i n t o Y e l l o w a n d o u t o f B l u e , w h e n s i m i la r r e su l ts s h o u l db e o b t a i n e d . T h e c o r r e c t b a l a n c e o f t h e e .t .s i n t h e g r o u p h a s n o w b e e n p r o v e d , a sh a s a l s o t h e r a t i o o f a l l t h r e e .

F i g . 1 8 . 4 . 4 H ( f ) s h o w s s ev e r a l g r o u p s o f e . t .s i n p a r a ll el o n a r e la y. O n e g r o u pm u s t b e p r o v e d f o r r a t i o a n d b a l a n c e a s a l r ea d y d e s c r i b e d , a n d th e t h r e e c . t. s i ne a c h o f a l l th e g r o u p s m u s t b e b a l a n c e d a g a i n st e a c h o t h e r. I t is n o w o n l y n e c e s s a r yt o p r o v e g r o u p a g a i n s t g r o u p . T h i s c a n b e d o n e i n s e v e r a l w a y s a n d o n e i s d e s c r i b e d

t e s t c u r r e n t is in j e c t e d i n t o ( s a y ) t h e R e d c .t . o f g r o u p A a n d o u t o f th e R e d c . t . o fg r o u p B . I f t h e p o l a r i t ie s a n d r a t i o s a r e c o r r e c t t h e r e s h o u l d b e l i tt l e o r n o s p il l



Testing comm issioning and managem ent o f pro tection 41 9

group B is the same as in g roup A; and s ince it has a lready been p roved th a t th ree c . t . s in each g roup b a lance , g roup A m us t ba lance g roup B . In a s imi la r wgroup A (o r g roup B) m ay be b a lanced aga inst g roup C , e tc . Where the re lay isthe h igh impe dance typ e , the t e s t am m ete r (A) in F igs . 18 .4 .4H(d ) , ( e ), and shou ld b e co nne c ted in pa ra l le l w i th th e re lay ins tead o f in se ri es .

R ever t ing to c . t .s in t r ans fo rm er bush ings w here i t is imposs ib le to in jec t ss t an t i a l t e s t cu r ren t , a te s t can be d one by app ly ing a 240 V a .c . l ead to each hbush ing in tu r n wh i ls t the l .v. s ide o f the t r an s form er has a th ree -phase shor t -c ircac ross i t . I f des i red a 415 V th ree -phase supp ly can be app l ied to the th ree bush ibu t th i s wi l l no t p rove the n eu t ra l c . t . as no cur ren t wi l l f low th rou gh i t . The o f the s ing le -phase supp ly enab les the neu t ra l c . t . to be inc luded in the t e s t

des i red as ind ica ted in F ig . 18 .4 .4F . I t wi l l be app rec ia ted th a t the p r im ary cur ref lowing wi l l be smal l so the secondary cur ren t s wi l l on ly be o f the o rder o f miamps . However, such a t e s t can p rove cor rec t connec t ions .

I t i s poss ib le to ob ta in a l a rge r t e s t cu r ren t by app ly ing the th ree -phase 415supply to the l .v. s ide o f the power t r ans former. Th i s i s shown in F ig . 18 .4 .4which a l so shows how a t empora ry shor t on the h .v. s ide can be app l ied in d i ffe rpos i t ions to p rove s t ab i l i ty on each phase . R e lay sp il l cu r ren t s m us t be checkl a te r when t he t r an s fo rm er i s on l oad .

M any t rans form ers a re p ro te c ted by overal l b iased d i ffe ren t i a l p ro tec t ischemes incorpora t ing h .v. and l .v. r e s t r i c ted ea r th - fau l t r e lays . Al though heacur ren t p r im ary in jec t ion t e s t s can be m ade f rom c . t. s to re lays , i t is d i ff i cu ltp rove the h .v.c . t . s aga ins t the l .v. c . t . s because o f the phase sh i f t th rough the t r afo rmer. S uch b a l ance can be p roved w h en t h e t r an s f o rm er h a s gone on lo ad , buwo rd o f w arn ing is necessa ry ; i f the t r an s form er i s f i t ted w i th a fau l t th rowsw i tch , t em po ra r i ly rem ove the t r ipp ing f rom the overa ll b iased d i ffe ren t i a l p

tec t ion un t i l the p ro tec t ion has been p roved cor rec t . A case is on record wh ere po la r i ti e s w ere wron g , so the p ro tec t ion in te rp re ted load cur ren t a s an in te rna l f athe p ro tec t ion ope ra ted and c losed the fau l t th row ing swi tch on to the live 132 l ine.

On g ene ra to r p ro t ec t i on t he zon e o f p ro t ec t i o n m ay i nc lude bo t h t he gene r aand i ts s t ep-up t rans fo rm er. The conn ec t ions f ro m a l l the c . t .s to the re lays mbe p ro v ed b y p r ima ry i n j ec ti on , bu t t h e b a l anc e o f t he h . v . c . t . s w i t h t he l . v. chas to be p roved w hen cu r ren t is f lowing th rou gh the genera to r and trans fo rmThis i s ach ieved by us ing the m ach ine i t se lf a s a source o f t e s t c u r ren t . Three -phphase- to -phase , and phase - to -ea r th t empora ry connec t ions a re app l ied in tu rn to h .v. s ide o f the t r ans former, and the mach ine i s run on low exc i t a t ion whi l s t co r rbehav iour o f the p ro tec t ion is check ed . These te s t s , in con junc t ion w i th tp r ima ry i n je c t ion t e st s a lr e ady don e , p r ov e the m a ch i ne p r o t e c t i on .

Because the gene ra to r t r ans fo rm er is fi t ted w i th t ap chang ing gear, some scur ren t m ay appea r in the overa l l p ro tec t ion w hen the m ach ine is run on

ex te rna l th ree -phase shor t -c i rcu i t . Th i s t e s t shou ld , the re fo re , be done in i t i a l ly t he nom in a l- r a ti o t ap o f the t r an s f o rm e r, an d r epea t ed o n t he t w o ex t r em e



4 2 0 T e s t i n g o m m i s s io n i n g a n d m a n ag e m e n t o f p r o t e c t io n

no need for th i s par t o f the tes t to be hur r ied .Unba lanced cu r r en t s i n a mach ine cause ro to r hea t ing , so ca re mus t be t aken to

ensure tha t th i s i s no t exces s ive . B efore the un ba lanced cur ren t i s app l ied i t in e c es s a ry t o k n o w t h e a m o u n t o f u n b a l a n c e , in o r d er t o d e t er m i n e f o r h o w l o nt h e u n b a l a n c e c a n b e t o l e r a t e d , a n d w h a t r ea d in g t h e se c o n d i t i o n s w i ll g iv e o n t hm a c h i n e a m m e t er s . A s a n e x a m p l e , c o n s i d er t h e c a s e o f a 6 0 M W, 0 . 8 p . f. , 11 - 8 kg e n e ra t o r c o n n e c t e d t o t he 1 3 2 k V s y s t e m t h r o u g h a 7 5 M V A d e lt a / s ta r t ra n sf o rm e r. O n fu l l l o a d c .m . r , t h e 11 . 8 k V a n d 1 3 2 k V c u r r en t s w i ll b e 3 6 6 0 A a n32 7 A , r e spec t ive ly, and the cu r r en t s in t he de l t a w ind ings o f t he t r ans fo rmer w i lb e 2 1 2 0 A . T hi s is s h o w n i n F i g . 1 8 . 4 . 4 I ( a ) .

2 1 2 0 IG e n e r a t o r 6 0 M W IR 3 6 6 0 R 1 R

o . , 2 11 .8 k V I y 3 6 6 0 Y Y

7 5 M VA B 3 6 6 0 B Bv

1 1 . 8 / 1 3 2 k V

( a ) 3 - p h a s e f u l l l o a d .--'

3 2 7i|

3 2 7i|

3 2 7

2 1 2 0

I y 0 Y Y .

B 2 1 2 0 ~ . B B . . ~ ,

Z 3 2 7 3 2 7(b) Phase - to -ea r th f au l t --" ---

2 1 2 0J

I y 2 1 2 0 Y y

2 1 2 0 B

( c ) P h a s e -to - p h a s e f a u ltF ig . 1 8 . 4 . 4 I

I f i t is dec ided to pass the fu l l load cu r ren t of 327 A throu gh to phase- to-

ea r th ' sho r t' on the 132 k V s ide o f t he t r ans fo rm er, t he cu r ren t s f low ing in thet h re e p h a s es o f t h e g e n er a t o r w i ll b e 2 1 2 0 A , 0 A , a n d 2 1 2 0 A as s h o w n i n F i



Tes t in g c o m m is s i o n in g a n d m a n a g e m e n t o f p r o t ec t io n 421

I f the same cur ren t o f 3 27 A i s no w passed th rou gh a phase- to -phase ' shor t ' onthe 132 kV s ide of the t ra ns form er, the cu r ren ts f low ing in the th ree phases o f theg e n er a to r w i l l b e 4 2 4 0 A , 2 1 2 0 A , a n d 2 1 2 0 A as s h o w n in F ig . 1 8 . 4 . 4 I ( c ) .

To assess the e ffec t o f these unba lanced cur ren ts on the genera tor, re fe rence

s h o u l d fir st b e m a d e to t h e s e q u e n c e c o m p o n e n t s s h o w n in F ig . 1 8 . 4 . 4 J . F r o mthis i t w i l l be seen tha t a phase- to -ear th ' shor t ' imp oses a nega t ive phase sequen cec o n d i t i o n o f 5 7 . 7 p e r c e n t o f c . m . r.

1R

I ,R I12 R1R

i l B ~ y

I 2 I 2 J L - -I B Iy y B ,,~'-2 . ~ 12BI I

P o s i ti v e N e g a t iv e A d d i t i o no f p o s i t i v ea n d n e g a t i v e

12 IR 4240 2120: 5"/.7% CMRR - - ~2 2

S e q u e n c e c o m p o n e n t s f o r p h a s e - t o - ph a s e fa u l t

I1 R IR

12 ' ~ l R f " I1 2 y R 1 2 y~ 12 R

" , , , , " - / 1 , - " , <12 B 1B

P o s i t iv e N e g a t iv e A d d i t i o n o f p o s i t iv ea n d n e g a t i v e

12 = I.RR = 2120 = 1220 = 33.3% CM RR ~ 7~

F o r p h a s e - t o - e a r t h f a u l t

F i g . 1 8 . 4 . 4 J S e q u e n c e c o m p o n e n t s o f g e n e r a t o r c u r r e n t s f o r f a u l t s o nh.v.. s i d e o f g e n e r a t o r

Fig . 1 8 .4 .4K g ives the n .p . s , w i ths tan d f igures in percen tage of c .m. r, ra t ing fo2 2ma chin es o f v ar ious 12 t va lues . A ma chin e w i th an 12 t va lue of 15 could w i ths tan

the ear th -f au lt con d i t i on for 130s and the phase- f au lt con d i t i o n for 47s , w he rea2

a mach ine w i th an 12 t va lue o f 7 cou ld on ly w i th s t and the se con d i to ns for 60s an2 I s , respec t ive ly.

Reve r t ing t o t he phase - to -phase f au lt co nd i t i on w h ich imposes cu r r en t s o f

4 2 4 0 A , 2 1 2 0 A , an d 2 1 2 0 A o n t h e m a c h i n e , t h e m a n u f a c t u r e r m a y b e u n w i ll into permi t th i s s l igh t over load on on e phase ( the c xn . r. r a t ing is 36 60 A ) . I f so



422 Te sting ommissioning and management o f protec tion

C u r v e

1 2 2 t

2 0 0

1 0 0

50

2 0

10

10

A B C D E - G H J

2 0 1 5 1 3 1 2 1 0 7 3 2 . 5

A

J H G

2 0 3 0 4 0 5 0 7 0 I 0 0

N P S a s a p e r c e n t a g e o f C M R

2



Testing com missioning and management o f protec tion 423

the read ings to be t aken . W hatever cur ren t va lue is t aken and w hatever the to f m achine , re fe rence to the approp r ia te curve will show for how long the cdi t ion can be w i ths too d. I t is the n advisable to a l low a factor o f safe ty by halvthese t imes .

( e ) Test s on com m iss ion ing o f ca rr ie r p ro tec t ion (R efe r a lso to Sec t ion 18Th e ban d of f requencies a t p resent em plo ye d in the UK for power- l ine-cars ignall ing is 70 -70 0 kH z, a l thou gh cer ta in par ts o f th is ban d are not avai lablecontinuous carrier s ignall ing. This restr ict ion is necessary to avoid causing inference wi th radio service including those used for aeronaut ical and marine nagt ion p urposes . H owever, w i th the exce p t ion of a band des igned to p ro tec t in te rna t iona l d i st ress f reque ncy , no rm al ly qu iescen t power-l ine-car rie r sys tem s m

be used anywhere in the remaining avai lable f requency range provided that t ransmiss ions do not exceed two seconds and that there is an in terval of a t leas t minutes between success ive tes t t ransmiss ions . Carr ier protect ion consis ts of s ta r t ing ne tworks , the e lec t ron ic equ ipment , the l ine coupl ing equ ipment and lt raps , and the power supply un i t and power packs . Tes t s a re made to p rove eachthese i tems.

The s ta r t ing ne tworks can be checked by secondary in jec t ion . Before s ta r t

work on the e lec t ron ic equ ipment the power suppl ies should be checked . Theseusua l ly p rov ided b y d .c . /d .c , con ver to rs w hich invert a ba t te ry supply to p rov idea.c . supply which is then rect i f ied and s tabi l i sed to provide the appropr ia te vol tleve l(s ) requ ired b y the equ ipm ent . Ar rangem ents vary w i th the type of equ ipmand the manufac ture r ' s commiss ion ing ins t ruc t ions should be consu l ted fo r de tpar t icular a t ten t ion being paid to fuse ra tings , polar i ty of suppl ies and vol tvalues.

Cer ta in specia l tes t equipment i s necessary for use on the e lect ronic equipm

and on the l ine coupl ing equ ipment . Th is t es t equ ipment compr i ses a ca thode osci l loscope, a valve vol tm eter, a n osci l la tor a nd am pl i fier cover ing the range 70700 kHz. The pro tec t ion equ ipment cons i s t s o f a t ransmi t te r and rece iver a t eend of the feeder, and tes t s have to be done on these to ad jus t the ou tpu ts asensit iv i ties and to check the perform anc e of f i lters.

Line-coupl ing equipment provides a means of in ject ing h . f . carr ier s ignals onan h .v. po w er l ine and i t preven ts these s ignals being diss ipated in the su bsta t iontyp ica l se t o f l ine -coupling equ ipm ent is show n in F ig . 18 .4 .4L . The e qu ipm ecom prises a l ine t rap and an h .v. capac i tance s tack , s tack tuning coi l ( 'ser ies armand shun t a rm assoc ia ted w i th each o f the tw o phases used for the transmiss ioncar rie r frequenc ies . The ' shunt a rm s ' a re con nec ted ac ross the ea r thy end of se r ies a rms to p rov ide a low impedance to ea r th fo r 50 Hz . The shunt and se ra rms con s t i tu te a band pass f il te r. A num ber o f s tandard ba nds a re ava ilab le, typ iones be ing show n in the t ab le be low .

On co m m iss ion ing , the tun ing of the l ine t raps , the se r ies a rms and the shun t a

has to be ad jus ted or checked . The se r ies and shunt a rms should tune to the miband f r equency, ad jus tmen t be ing p rov ided on the s e ri e s a rm co i l s. The shun t a




Band F r e qu ency r ange kHz M id b a nd ( g e om e t r ic m e a n ) f r e q u e n c y k H z

1 70 - 81 752 80 - 95 873 9 0 - 11 0 9 9 . 54 100 -125 1125 110-140 1256 120-158 1387 130-175 1518 1 5 0 ~ 1 4 1 7 9.59 1 8 0 ~ 8 0 22 5

10 2 5 0 5 0 0 3 5 411 3 5 0 G 0 0 4 9 5

The tun ing o f the l ine t r aps is ach ieved by su i t ab le cho ice o f shun t capac i tOn e Br i ti sh man u fac tu r e r r e com m e nd s t un i ng t he m to t he car ri er f r equenw h e r ea s a n o t h e r m a n u f a c t u r er r e c o m m e n d s t h a t t h e y b e tu n e d t o t h e m i d b af r equency. T he method of tun ing wi l l be the same in bo th cases and i s desc r ibebe low.

Fo r these tes ts the osc i l la tor, am pl i f ier and valve vo l tm eter are used . Signal levin exces s o f t he m ax im um ou tpu t no rm a l ly g iv en by t e s t o s c il la to r s a re requ i redmask poss ib le in te r fe rence , thus the ampl i f i e r shou ld have an open-c i rcu i t ou tpuabout 50-100 V. For a l l t e s t s the ea r thy s ide o f the ampl i f i e r ou tpu t and vavo l tm e t e r sh ou ld be connec t ed t o t h e s t a t i on ea r t h .

When check ing the tun ing o f the va r ious resonan t c i rcu i t s in the l ine coup l

e qu ipm en t , g r e a t c a r e mus t be t ak en t o s ee t ha t e l e c t r i c a l conn ec t i on s be t weuni t s do no t in t rod uce capac i t ance in pa ra ll el wi th the un i t u nde r t e s t . S t ray captance shou ld a lso be red uced to a m in im um . Wi th these p o in t s in v iew i t is r ecom end ed tha t i so la tion o f the line tr aps shou ld be ach ieved by ea r th ing the line tron the l ine s ide wi th por tab le ea r ths and d i sconnec t ing the copper connec to rs the s t a t ion s ide , the d i sconnec t ion be ing m ade a t the l ine t r ap end o f the conn ec toR efe rence shou ld be m ade to F ig. 18 .4 .4M w hich shows the te s t con nec t ion squ i red to tune the l ine t r aps , the se ri es a rms and the sh un t a rms .

Line traps: Because the l ine t ra p is a para l le l -con nected wide ban d res ona nc i rcu i t wi th h igh impedance a t r e sonance , the va lve vo l tmete r r ead ing wi l l bem ax im um o ve r t he ban dw id th a t r e so na nce . To ob t a in go od s e lec ti v it y a h iimpedance source i s required . This i s achieved by connect ing the l ive s ide of am pl i f ier o u tp ut v ia a h igh res is tance (a ha l f -w at t 30 k~2 carb on res is tor is su i tabto the s t a t ion s ide o f the l ine tr ap . T he valve vo l tm ete r is con nec ted w i th i ts ls ide to the l ine- t rap s ide of the res is tor.

Series arm: The se r ies a rm i s a c i rcu i t wi th low impedance a t r e sonance , and thva lve vo l tmete r r ead ing wi l l be a min imum a t the resonan t f requency o f the se r



Testing comm issioning and management o f pro tect ion 42 5

Line traps

Busbars

0 -,

i0- -

Seriestuningunit

Capacitor

v.t.h.v.chambers

--I

t ilter

cubicles

m

I

Couplingcapacitors

L i n e

~ ShuntL tuningunit

- 2Coaxial cableto indoor equ ipment

L i li i





Testing commissioning and management o f pro tection 42 7

The low er end o f th e se ri es a rm shou ld b e i so la ted as i t en te r s the cap ac i to rh . v. c h a m b e r. O n a c o m b i n e d c o u p l i n g c a p a c i t o r / c a p a c i t o r v. t . a s s e m b l y, capac i to r v. t . mus t be inc luded in the c i rcu i t . To ob ta in good se lec t iv i ty a i m p e d a n c e s o u r c e is r e q u i r e d a n d a n a m p l i fi e r o u t p u t i m p e d a n c e o f 7 5 S2 is

f ac to ry. Th e live s ide o f the am pl i f ie r and va lve vo l tm e te r a re con ne c ted tol o w e r e n d o f t h e s er ie s a r m .

Shun t a rm: The sh un t a rm i s a pa ra l l e l -conn ec ted reson an t c i r cu i t . It shou ld tt o t h e m i d b a n d f r e q u e n c y o f t h e c o u p l i n g f il te r.

A ll c o n n e c t i o n s s h o u l d b e r e m o v e d f r o m t h e u n i t a n d th e liv e s id e o f t h e a mf ie r s h o u l d b e c o n n e c t e d t o t h e u n e a r t h e d s id e o f th e u n i t v ia a 3 0 k ~ r e s is t o r.v alv e v o l t m e t e r r e a d i n g w il l b e a m a x i m u m a t t h e r e s o n a n t f r e q u e n c y o f t h e sa r m .

S ince a t t enua t ion o f the ca r r i e r s igna l occurs a long the p r imary l ine an ende n d m e a s u r e m e n t o f th i s a t t e n t u a t i o n m u s t b e m a d e . A g a in u se is m a d e o fo s c i ll a to r a n d a m p l if i e r, w h i c h a r e c o n n e c t e d t o t h e h . f. c a bl e a t o n e e n d o f th ewhi ls t the rece ived s ignal level i s measured a t the o ther. I t i s usual to make th iso v e r a r an g e o f f r e q u e n c ie s o n e i th e r s id e o f th e i n t e n d e d w o r k i n g f r e q u e n cp o s s i b l e t h r e e s e t s o f t e s t s s h o u l d b e d o n e : o n e w i t h t h e l i n e d e a d a n d e a r t h et h e s t a t i o n s id e o f t h e l in e t r a p s , o n e w i t h t h e l in e u n e a r t h e d , a n d o n e , l a te r, a

the l ine has been made a l ive and i s ca r ry ing load . A curve o f some typ ica l r eis i l lus t ra ted in Fig . 18 .4 .4N .

P r e c o m m i s s i o n i n g t e s t s o f m o d e r n p h a s e - c o m p a r i s o n c a r r i e r p r o t e c t i o n s hinclude tes ts on t ransmit ter levels , rece iver sens i t iv i t ies and t r ip angle se t t iT h e s e a r e b e s t a c h i e v e d b y f o l l o w i n g t h e m a n u f a c t u r e r ' s r e c o m m e n d e d c o m

32

3 0

2 8

2 6

-~ 24

.£

= 2 0

< 18

16

14

12

10

N o te " C u r v e d r a w n t h r o u g hm e a n o f p o i n t s o b t a i n e d

I 0 0 1 20 1 4 0 1 6 0 1 80 2 0 0 2 2 0 2 4 0 2 6 0 2 8 0 3 0 0 3 2 0

I ' r e q u e n c y ( k H z )




s ion ing ins t ruc t ions fo r the par t i cu la r equ ipment , ca re be ing t aken to ensure tadequate margin exis ts between a larm and t r ip level sens i t iv i t ies . The bui l t - in tesfac i l i t i e s which , by compar i son of the in jec ted cur ren t s a t the two ends , pe rrou t ine s tab i l i ty and t r ipp ing t es t s to be ca r r i ed ou t , shou ld a l so be checked o

and phased ou t .The l a tes t des igns o f normal ly qu iescen t power- l ine -ca r r i e r p ro tec t ion have h

thei r bui l t- in tes t fac il it ies mod if ied to c om ply w i th the p erm it ted l imi ts o f cart ransmiss ion t ime and a re the re fore capab le o f app l ica t ion th roughout the baHo wev er, dur ing co m m iss ioning tes ts w hene ver carr ier t ransm iss ion to l inerequ i red ca re m us t be t aken to com ply w i th these cond i t ions if opera t ing a t quenc ies wi th in the res t r i c ted bands . Ind iv idua l manufac tu re r ' s commiss ionins t ruct ions provide deta i led informat ion on th is aspect speci f ica l ly for thei r eqmen t .

To assist in check ing th e t r ip angle and sensi t iv i ty of the p rote ct io n, fac i l ia re p rov ided fo r d i sconnec t ing the ou tpu t to line and inse rt ing a du m m y loadtha t the equ ipm ent is t e rmina ted in 7512, the reb y enab l ing these t es ts to be ca rou t w i thou t t r ansmission o f ca rr ie r to l ine .

When a l l these tes ts have been done sa t is fac tor i ly the l ine may be put on lo(see Sect ion s 18.4 .5 and 6 ) af ter w hich an end- to-end po lar i ty tes t m ust be m a

This can be done by the t emporary d i sconnec t ion and shor t ing o f the same phc . t . s a t the two ends o f the l ine to ob ta in an ou tpu t . Opera t ion o f re lays mus t s imula ted i f load cur ren t is be low the se tt ing o f the eq u ipm ent , and the phasethe carr ier s ignals checked on the ca thode ray osci l loscope. Fig . 18.4 .40 shows t r ip and s tab i l i ty zon es o f a typ ica l phase-com par i son equ ipm ent .

E n d A E n d B

U o n d i t i o n ( a) D e f i n i t e t r i p p i n g

I

E n d A E n d B

. . . . ,Jldllllllllll

llillllil ll llllllllli lillr ' ' ° 'it ' o n d i t i o n ( b) D e f i n i te s t a b il i ty

F 3 0 ° g a p s " 7

g lll li l ll li l. . Ig g lll l( l l , , . .. , . lH IIlIlII li lI I l lJ i ll li l liet . ' o n d i t i o n (c ) M a r g in a l t r i p p i n g / s t a b i l i t y

E n d B c u r r e n ta s i n cond i t i , ~n ( h ; End A c u r r e n t

F ind B lead ingend A as incond i t i , ~n ( c )

S t a b i l i t y

E n d Al e a d i n g e n d B

T r i p p i n gzont~

I ' h as o r r e p r e s e n t a t io no f t ri p p in g a n d st a b i l it y z o n e s



Testing comm issioning and management o f protec tion 429

(f) Trip angle tes t u ni t :One m ake of equ ipm en t has bui l t- in faci li ties for checkinthe t r ip angle se t t ing, but for another an external t r ip angle tes t uni t i s requirThe fundamenta l requ i rement i s to modula te the ca r r ie r f requency s igna l in o rto prod uce var iable gaps and so dete rm ine the t r ip angle se tt ing.

This device produces pulses of var iable durat ion which are used to modulatco nsta nt carr ier s ignal f rom the local osci lla tor in order to es tabl ish the s hor tes t w hich w i ll cause t r ipping.

D.C. pulses , adjus table in magni tude over the range 50 mV to 30 V anddu ra t ion f rom 20 to 75 e lec tr ica l degrees a re p roduce d . F ixed dura t ion pu lse180 e lectr ica l de grees are a lso available by sw i tch se lect ion. R eference shou ldmade to the manufac ture r ' s ins t ruc t ions fo r su i tab le t es t po in t s fo r connec t ion

the tes t un i t ou tpu t .Fig . 1 8.4 .4P sh ow s the c i rcui t d iagram of a t r ip angle tes t uni t consist ing of fo l low ing m ain pa r ts :

(/)

~ )

Fig. 18.4.4Q

( iiO

i )

P o w e r p a c k : This is a s imple ful l wave push-pull rect if ier circuit producinapprox im a te ly 30 V ac ros s C1 .Phase shif ter : This is com posed of a pow er t rans form er T r l , r es is to rs RVR 1 and capac ito r C2. R es is tance R 2 is h igh com pared wi th R 1 o r V R

consequ en t ly R 1 , VR 1 and C2 can be cons ide red t o pass a com m on cu r rand the vo l tage ac ross R 1 and VR 1 wi ll be 90 degrees in advance of tacross C2. The vector re la t ionship of these vol tages is shown in Fig . 18.4 .f rom w hich i t can be seen th at the locus o f the voltage a t point D is equalm agni tude to one ha l f o f the secon dary vo ltage o f T r l whi le it advancesphase re la t ive to tha t vol tage as VR 1 is redu ced .

Locus of point D

Q CommonSecondary voltage of Trl

Voltage phas or relationship

Phase compara to r :This is a logical N AN D circui t, w here the col lec tors ot ransis tors T1 an d T2 are bo th sho r ted to c om m on i f a posit ive vol tage presen t on e i ther base . Th erefo re , as i llus t ra ted in Fig . 18 .4 .4R a pulse p roduced on th i s common co l lec tor once per cyc le , i t s dura t ion be ing d

term ined by the phase di fference betw een the vol tages a t B and D (idepe nden t upo n the s e tt ing o f VR 1) .f





Testing comm issioning and managem ent o f pro tectio n 431

is f ed t o t he p r e se t po t en t i om e te r VR 2 w h ich p rov ides ad ju s tm en t o f t h e oupu t v ol tage level .

(v) Ou tpu t swi tch:In s om e cases it is necessa ry to have 180 deg ree pulses andth e se a r e ob t a ined by r emov in g the a c t i on o f t r an s is t o r T1 f rom the com

para to r c i rcu i t by shor t ing D3 v ia an ou tpu t se lec to r swi tch .The com po n e n t s c an be conv en i en t l y b u i l t i n to a sma l l m e t a l b o x w i t h t he p o t enm e te r c o n t ro l and sw i t ches loca t ed on t o p fo r e a sy o pe ra t i o n .

Vol tage a t @

/ iI i \ /~t i X / ~I I ~ I

Volta ge a t ( ' ~ I \ I /¢ Ix , _ ) i \ t /

i I \ I / i i

Vol tage onT I base

II

Vol tage onT2 base

I I II I I: I I

, t , /I 1I I

t / -

Vol tage on I LT l co lle ct o r ....

Vo l tage a t 1 Lo u t p u t

F ig . 18 .4 .4 R Vo l t a g ewaveforms

(g) Tripping a nd closing tests:I t i s not poss ib le to prove the t r ip and c lose wir ingof a new c i rcu it u n t i l the fuse and l ink have been inse r ted ; ye t if the re is any th i




a precau t ion agains t any thing u ntow ard hap pen ing, it is wise to che ck w i thvol tmete r ac ross the top and b o t to m jaws o f the ho lder before the fuse is inse r tand again for the l ink. A zero reading on the m eter proves that the fuse or link co nf id en t ly be inser ted. A reading on the v ol tm eter is an indicat ion that there

pa th f rom pos i t ive to nega t ive th rough the vo l tmete r and some par t o f the c i rcuSuch a vo ltage may be qu i te h ea l thy - fo r exam ple , the re m ay be a pa th th rousome c i rcu i t sup erv i s ion equ ipm ent o r ind ica ting lam p; bu t the reason for tha t pshould be es tablished be fore inser ting the fuse or l ink.

As ment ioned ear l ier, a copy of a l l ext racted diagrams should be included in com m iss ioning log and as each c on tact , fuse , l ink, changeo ver swi tch e tc . , is provi t should be t icked of f on the app ropr ia te d iagram. I f the same diagram is used

several c i rcui ts , d i fferen t ly co loured t icks m ay be used for the di fferent c i rcuits . re lay has two separate contacts , one t r ipping a c i rcui t breaker d i rect and the ott r ipping a t r ipping re lay which then t r ips the c i rcui t breaker, each should checked separa te ly.

M ost presen t-day re lays are f i tted w i th me chanical flags , and i t should ca re fu l ly checked tha t these a re no t l ike ly to in te r fe re wi th the opera t ion of tre lay e lem ent . I f ser ies e lectr ica l flags are em ploy ed i t should be proved th at twothem ope rate i f thei r associa ted co ntacts are bo th c losed in para lle l. Unde r t

condi t ion the f lag current wi l l be roughly halved, and i t i s des i rable to check tthe re i s an adeq ua te marg in fo r opera t ion .

Tr ipping tes ts should be proved a t 100% and a t leas t one lower value e .g . 6ba t ter y vol ts. The la t ter is no t an e nt i re ly theore t ica l tes t because i f a c i rcbreaker i s c losed onto a faul t requir ing the breaker ' s immediate t r ipping, the voavai lable f rom the bat tery wi l l be reduced by the dra in of the c los ing solenoHowever, i t should be noted that a requirement of Br i t i sh Standard Specif ica t

5311 and 3659 is for the ful l faul t breaking capabi l i ty of c i rcui t breakers to available w hen the vol tage a t the t r ip coil is w i thin the range 80-120 % o f nom iand care should be tak en in the des ign stage to ensure that 80% b at te ry vol tagavailab le a t the t r ip co i l under the m os t on erous cond i t ions o f opera t ion .

Tr ip c i rcui t supervis ion re lays should be proved, and i t should be checked tthe a la rm su pply is no t fed f rom the b a t te ry be ing superv ised , o therwise no a lacould be given i f the supervised supply fa i led . Tr ip-heal thy lamps and res is tshou ld be proved and the lamp s shou ld be shor ted-o ut to prove that the ser

resistance is adequate to prevent t r ipping. In doing this test , i t is advisable to shoou t the lam p several t imes in quick success ion whi ls t som eb od y l is tens adjacenthe swi tchgear to make sure that the associa ted t r ip coi l p lunger i s not movingcase has been exper ienced where the current passed by the res is tor was suff ic ito l if t the p lunger and m ove the t r ipp ing m echanism s l igh t ly bu t no t e noug h to tthe b reaker. A f te r severa l opera t ions , how ever, the t r ipp ing m echanism had ' inchso far that t r ipping occurred.

I f , dur ing any of these tes ts , i t has been necessary to d isconnect the dr ivesbanks of aux i l i a ry swi tches to opera te them in bo th pos i t ions , the d r ives mus t

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Testing com m issioning and managem ent o f pro tection 433

(h) Indica tions an d alarms:These tes ts involve checking tha t each indica t ion andalarm funct ion s co rrec t ly an d the y are la rge ly se l f -explana tory. Th ey are , how everim por ta n t and the opp or tu n i ty shou ld be t aken o f ins t ruc t ing the opera t ing s t a ff it he func t ion ing o f the va rious con t ro l s and o f g iv ing them the op por tun i ty ocancel ling the a larms.

18.4.5 Phasingtest.,

I t i s necessary to prove correc t phas ing before a new piece of eq uip m en t can be puin paral le l w i th exis ting equ ipm ent . For exa m ple , a 13 2/33 kV grid t rans form eis genera l ly f i t ted wi th l inks in the 33 kV s ide which enable i t to be connected t

vec tor group Yd I or Yd I 1 , and tes ts should be do ne to es tabl i sh tha t the new t ransform er phas ing is the same as tha t wi th wh ich i t is to be c onn ecte d in para lle l.This can be done before the t ransfo rm er is ma de a live f rom the sys tem by

applying 415 V three-phase to the h .v. s ide , me asur ing vol tages be tw een the 132 kVand the 33 kV s ides , and recording them in a schedule s imi lar to the one be low.

33k V Side 132 kV Side

Rt Yl Bl

R 2 " ' " ' " . . . . " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Y 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . . . . . . . . . . .

B 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Vol ts should be m easured f rom R~ to R 2, ]"2 and B2, f rom Y~ to R 2, Y2 and

B2 , and f rom B~ to R 2 , Y2 and B2. I t is essentia l tha t the 132 kV and the 33 kVsides are temporar i ly commoned a t some poin t for th is tes t , o therwise mis leadingvalues wi l l be obta ined as there i s no meta l l ic re turn pa th for the vol tmeter currentW hen the result s have been o bta in ed , the vec tors can be p lo t ted f ro m w hich thephas ing of the t ransformer can be es tabl i shed .

On lower vol tage sys tem s, for exam ple up to 11 kV , i t is possib le to em ployphas ing s t icks . These are insula ted s t icks which can safe ly be inser ted in to swi tchgear spouts and used in the sam e wa y as a vol tm eter to phase out the R, Y and phases of one c i rcui t wi th the R, Y and B phases of a c i rcuit a l read y proved.

I t is im prac t icab le to use phasing s t icks above this vol tage, so for 33 kV , 66 kV132 kV, 275 kV and 400 kV sys tems, phas ing tes ts a re usual ly car r ied out be tweenthe 110 V s ides of vol tage t ransfo rm ers conne cted to such sys tems. Phas ing tes tcan , however, be done be tw een a 110 V supp ly and one o f 240 V o r 415 V. Hereagain there must be a common poin t of re ference be tween the vol tage suppl iewh ich are to be com pare d . A 'nul l ' m etho d is insuff ic ient - for exa m ple , if vol t

w e re o n l y m e a s u re d R - R , Y - Y a n d B - B , n o r e a di ng w o u ld b e o b t a i n e d i f t hecompared vol tages were ident ica l in magni tude and phase . Such a tes t could indi

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4 3 4 T e s t i n g , o m m is s io n i n g a n d m a n a g e m e n t o f p r o t e c t io n

V. T . ~ lo . 1 V. T . N o . 2

R

m e t e r c u r r e n t Y

B E a r t h e d v . t . y e a r t h e d v . t .

B

F i g . 1 8 . 4 . 5 A D a n g e r o f n u l l m e t h o d o f p h a s in g R - R , Y- Y, a n d B - B

obta ined because the tw o vol tage supp l ies had no co m m on p oin t o f reference . Itwould thus be possible for the suppl ies to be out-of-phase, yet no vol tage readingwould be obtained to indicate that fact . Fig. 18.4.5A i l lustrates this .

I f the two suppl ies have no common reference poin t , one must be a r ranged

ei ther by temporari ly ear thing the unearthed supply on one phase or a t the neutral ,or by temporar i ly s t rapping the tw o sys tem s together a t one po in t .

To ge t a pos i t ive check , the red phase of one supply should be compared wi thred , ye l low and b lue of the o ther ; then the ye l low phase of the supply should becom pared w ith the other three phases, and f inal ly the b lue. T he resul ts m ay berecorded on a schedule as show n in F ig . 1 8 .4 .5B.

M e a s u r e d v o l t a g e s Pha .~ l} r~ ph , t t ed A c t u a l p h a s i n g

E x a m p l e IR2 Y2 B2

RI 2 109 I 1 0YI 109 2 I I IB I I 1 0 I I I 1 .5

E x a m p l e 2

R2 Y2 B2

RI 34 9 0 123

Y I 124 33 90B I 9 0 1 2 . } 3 4

I- :xample 3

R2 Y2 B2

RI 210 240 310YI 350 240 310BI 210 240 130

RIR 2

BI B2 Y I Y2

R 2 Hi

BI ~ ~ y Y2

IB2

R I

R 2

Y 2

R IR 2

R I k 2

B - Y IB 2

R I

<

R 2




W hen a circui t is being paral leled for the f i rst t im e, the v. t . associated w ith thac i rcui t must be energised f rom a known source to prove tha t the v. t . connect ionare correc t . Af ter th is has been done the vol tmeter should be le f t connected , anthen wh en the new v. t. is energised f rom an unk no w n source and synchronism

indica ted , the two pr imary suppl ies must be in phase .Al thou gh a phase r o ta t io n m eter m ay be used in the case of a genera tor, i t is

more def in i te check to employ two vol tmeters connected red to red and b lue tb lue be twee n the genera tor v. t. (ak ead y proved) an d a v. t. on the sys tem. The twvol tmeter readings should beat up and down together a t s l ip f requency, thuindica t ing tha t the phase ro ta t ion i s correc t , and they should both be a t zero whethe synchro scope needle i s a t 12 o 'c lock. Having checked th is , the synchroscopcan then be rel ied upon for closing in.

1 8 . 4 . 6 C l o s in g u p

The set t ings of overc urren t relays should be set dow n for paral lel ing or for switching in new equ ipm ent to ensure th a t i f i t is faul ty, the faul t wi ll be d isconn ectedw i th the m inim um dis turbance to the sys tem. In th is conn ect ion i t is moreim po r tan t to se t dow n the t im e m ul t ip l ie r than the p lug set t ing . I f the la t te r is sedow n then i t mus t be resto red to no rmal be fo re load in excess o f the t emp ora ryrelay set t ing is picked up.

1 8 . 4 . 7 O n - l o a d t e s t s

Having got the new equipment on load i t is advisable to do certain tests . These havbeen touched on earl ier, but are here given in more detai l .

(a) C urrents in relays:A spl it p lug and a m m eter i s a conven ient wa y of checkingcurrents in re lays f i t ted w i th p lug br idges . The amm eter m ust be on a su i table rangand a spl i t plug should be inserted on the same, or higher, tap as the relay set t ing -otherw ise there is the possibi l i ty tha t the relay wil l op erate i f the circui t is heavilloaded.

I t should be check ed tha t there is negl igible spil l cur ren t in an y residual lyconnected re lays and in any protec t ion working on the d i fferent ia l pr inc ip le . Beforme asuring curr ent in the c . t . c ircui ts of different ial relays i t is advisable to removthe appropr ia te t r ip l ink .

Directional relays(b ) Phase-fau lt relays:On4 oad tes ts of d i rec t ional re lays are requi red to prove thepolar i ty of the v. t . connect ion wi th respect to the c . t . connect ion .

For these tes ts i t cannot be s t ressed too s t rongly tha t the d i rec t ion of the MW

and MVArs must be pos i t ive ly known, a lso the power fac tor. Any load var ia t ionduring the tests m us t be closely supervised - especial ly a change from a lagging t




Prima ry in jec t ion tes ts and seconda ry w iring checks should a l ready havestabl ished that the correct currents and voltages are associated with theappro pr ia te e lemen ts . I f proven ins t rum ents are a l ready available on the pr im arci rcuit con cerne d, the load character is tics wi l l be kn ow n; i f no t , i t wi l l be n ecessar

to do 'wat tmeter tes ts ' . In these tes ts a se t of vectors i s p lo t ted by which theposi t ion of the current re la t ive to the vol tage can be seen. A cent re-zero w at tm eteis used and the current-coi l inserted in each phase in turn.

The w a t t f u l com pone n t o f t he re d cu r ren t is measu red w i t h R - N , Y -N and , f ogood m easure , B - N vol ts . Som e of the readings wi ll be pos it ive and som e negat ivThese readings, plot ted along the voltage vectors in the appropriate direct ion, widetermine the pos i t ion of the current vector. F ig . 18 .4 .7A shows a typica l resul t .

v R

l Vo l t s Wa t t t' u l c o m p ( ) n e n t

+1 1.2 ~ IR R -- N ÷ i.2

Y 2 0

i i ~ .N o t e :. . . .

( I ) P l o t th e t h r e e v o l ta g e p h a s c) rsV R , V y, V B , an d t h e i r c o n t i n u a t i o n

i n t h e r e v e r s e d i r e c t i o n .( 2 ) W i t h t h e w a t t m e t e r c u r r e n t c o ili n t h e R e d p h a s e , m e a s u r e t h e w a t t sfo r R - N . Y - N , and B - N vo l t s .( 3 ) P l o t t h e se as s h o w n ' t h e p o i n t

V B V y w h e r e t h e t h r e e p r o j e c t i o n s m e e tg i v e s t h e p o s i t i o n o f t h e R e d c u r r e n t

F i g . 1 8 . 4 . 7 A W a t t m e t e r te sts o n d i r e c t io n a l r el ay s t o d e t e r m i n e l o a d a n d p o w e r f a c t o r

A ll tha t now remains is to check th at ea ch phase-fault direc t ional relay is bhaving in the wa y i t should . To check th is i t wi l l be necessary to know the type di rect ional connect ion (30 ° , 90 ° , or 90 ° - 45 °) appl ied to the re lay. I t w i l l a lsonecessary to kn ow the p olar characterist ics o f the relay; i f th ese ha ve not beefurnished by the ma nufacture the y should ha ve been obta ined dur ing secondarinjec t ion tests by ap plying to the relay voltages o f va rying phase angles relat ive the in ject ion current , and f inding a t wh at angle i t has m inim um torque .

Thus with al l this information available, i t wil l be possible to predict exacwhat the di rect ional re lay should be doing for any load condi t ion; and i t must nobe proved tha t i t is doing the r ight th ing for the p ar t icular load a t the t ime. No rm aa phase-fault dire ct iona l relay is connected so tha t i t w il l op erate for a f low o f fapower ou t f rom the busbar.

I f separa te phase-faul t d i rec t ional e lements are used for each phase they shoul

a l l behave in the same way under the same condi t ions , but i f the three d i rec t ione lements d r ive on to a com m on d isc and sp ind le mo vem ent , each shou ld be p rove



Testing commissioning and management o f protec tion 43 7

I t canno t be stressed to o strong ly that phase-fault direct ional relays have nobeen proved unless the fol lowing condit ion s have been sat isf ied:

(i)

(ii)(iii)

(iv)

The type of d i rect ional connect ion should be known.

The relay characterist ics shou ld be kn ow n - that is, i ts polar curve.The di rect ion and power factor of the load must be kn ow n w ithou t thsl ightest doubt.Each phase-fault element in turn must then be seen to operate in the expected way for that load condit ion. This establishes that the c. t . and v. tpolarities are correct.

(c) E arth-fau lt directional relays:Directional earth-fault elements are moredifficult to prove in respect of c.t . and v.t . polarities because the polarising voland am ps will not be there under heal thy load condi t ions . Faul t condi t ions , therfore, have to be sim ulated.

On the load test i t must f irst be checked that there is no open-delta voltage ores idual current under th is condi t ion. The most convenient way of proving that 70 ° relay will correct ly respond to an e arth fault on or be yo nd the pro tected l in(as in (b) of Fig. 18.4.4A), is by short-circuit ing and disconnecting the blue phas

current t ransformer, removing the red phase v. t . fuse, and strapping the dead sido f the fuse to the neu tral conn ectio n. S ection (e) of Fig. 18.4.4A indicates thresultant vectors and similari ty to (b) . The relay should operate for load-currenphase-angles betw een abo ut 9 0 ° lag and 60 ° lead.

I t is not s tr ict ly necessary to make the test cyclical ly round the phases, s inczero ou tpu ts for balanced input have a l ready been checked; the other phases couldhowever, be proved for good measure.

1 8 . 4 . 8 M o d i f i c a t i o n s t o e x i s t in g s u b s t a t i o n s

All the tests so far enumerated have applied to new substat ions. Although possiblcom plex, the comm iss ioning of a new substa t ion is s t raightforward comp ared w i tmajor modificat ions to an exist ing si te . In the lat ter case, much of the equipmenhas to be kept in comm iss ion dur ing reconst ruct ion which m ay involve reallocatioof m ult icore cables, the conn ecting in or reconne cting o f c. t.s and al terat ions t

relay panels . Considerable ingenuity is needed here and much improvisat ionEssential t r ipping and closing supplies have to be m aintained during wholesalreorganisa t ion, and much temporary cabl ing and wir ing may have to be run toeffect a sm oo th changeover.

I t is impossible to st ipulate a ny p roced ure for this class of wo rk be yo nd stressinthe need for part icular care and advance planning.

1 8 .5 R o u t i n e m a i n t e n a n c e t es ts




precision if a fault occurs on i ts associated prima ry equ ipm ent. Perhaps a m odiff icul t requirem ent of the protec t ion is tha t i t must remain inopera tive on nearbextern al faults . R outine main tenan ce tests on protect ive gear are done to ensure ththe equipment is always ready to perform in a ful ly discriminative manner the du

required of it .

18 .5 .1 Caus e s a nd e f f ec t s o f de t e r i o r a t i on

Protective gear does not norm al ly deter iora te wi th usage, but can be harm ed badverse condi t ions . For exam ple , a re lay subjected to pron ounc ed cont inuouvibrat ion can suffer damage to i ts pivots or bearings; dam pness in jun ctio n box

and circuit-breaker kiosks can low er the insulat ion resistance of the mult i-cocables and wiring; a polluted atmosphere may have a detr imental effect on rell igaments, relay conta cts and auxil iary switches; the heat p rodu ced by con tinuouslenergised coils can often age the insulation; electrolysis may cause 'green spoleading to open-circuited coils or contacts .

Measures to al leviate or prevent many of these causes are taken in the desigs tage of modern equipment . Outdoor terminal boxes are vent i la ted , heated, anproof against vermin. Access to terminal boxes is made easier by the provisioof hinged or s l ide-off doors, instead of bolted covers; new relays are subjected tv ibra t ion type tes ts to prove that they wi l l s tand reasonably heavy i l l - t rea tmenwith out ma lopera t ion or damage; insula ting mater ia ls are chosen to s tand heat damp wi thout deter iora t ion; and e lec t ro lyt ic corros ion can be prevented on d .c i rcui ts by us ing bias ing equipment on the bat tery, thus keeping the whole of thd.c. wiring at a negative potential relative to earth.

1 8 .5 .2 F r e q u e n c y o f r o u t in e m a i n t e n a n c e

When determining a program me, the f requenc y of rout ine maintenance inspect ionand tests will dep end on the fault history an d fault l iabi l ity of the eq uipm enThere i s no point in over tes t ing equipment ; the programme should be planned sthat faults in the equ ipm ent are ant ic ipated, not precipi ta ted . The f requency o f tewil l , however, vary widely with the type of equipment. Certain i tems are continuously m oni tore d, some are checked severa l t imes a day, whils t o thers m ay be tes temonthly, year ly or even every two years . Typical examples of the f requency antypes of equ ipm ent are"

(a) C ontinuou s mon itoringPilot supervisionTrip circuit supervision

Relay voltage supervisionBattery earth-fault supervision



Testing com missioning and management o f prote ction 439

(b) DailyRelay flags inspected on every shiftCarrier protect iontests -e i th er man ual ly every shi ft or by c lock-tes t every 4 hours

(c) MonthlyWater level of l iquid earthing resistances

(d) Two-monthlyIntertr ipping channel tests without t r ipping any switches

(e) Six-monthly

Tripping tests

(f) YearlyCh eck o pe rating levels, sensitivites, trippin g angle and reflex te st facilit ies of phacom par ison carr ier protec t ionSecondary in jec t ion on the more complex forms of protect ionInsulation resistance testsCheck of bat tery b ias ing equipmentInject ion of gas and oil actua ted relays with air or oilCalibrat ion check on l iquid earthing resistances

(g) Two-yearlySeconda ry in jec tion tes ts on the m ore robust re lays

18.5.3 Insp ectio ns and tests

The main features of the various inspections and tests are briefly indicated below:

(a) Inspections: O noutdoor equipment , check that heaters are on and that ventsare not blocke d. Check there is no evidence o f vermin in relay panels and switchgear terminal boxes; check relay set t ings against schedules or cards; insperelay conta cts and l igam ents for c orrect posi t ioning and corrosion; inspect relayfor foreign bodies such as dust, specks of iron f 'ding in magnet gaps, flaking opla ted par ts ; vol tmeter check to prove that bat tery b ias ing equipment i s in ordeetc.

(b) Intertripping tests:These can be a prolif ic source of error especial ly on threeor four-poin t circuits . Part icular care is need ed to be on the correc t panel . A detai leprocedure should be established for carrying out these tests , the test l inks, i f anbeing changed over at al l points of test before hand. Care must be taken to restor

the l inks when the tests are completed.




protect ive relays to the t r ipping of the circui t breaker. The relays are usual lope ra ted m anu al ly, but wi th par t icular ly del ica te re lays there i s some advantage ac tuat ing them elec t r ica l ly. When re lay covers are removed pr ior to these tes ts , andust sho uld be w iped o ff the top of the cover before removing i t so tha t dus t do

not fa l l in to the re lay. The oppor tuni ty should be taken on these tes ts to prove ththe relay contacts are clean and secure, and to observe the contact wipe. The f lashould be checked for pos i t ive opera t ion , and the appropr ia te a larms should bproved a t the same t ime.

(d) Insu lation resistance tests:These ma y be done wi th a 500 V Megger, on onesecondary c i rcui t a t a t ime wi th a l l the o thers ear thed. The resul ts obta ined ma

vary wi th the weather, but i t i s the t rend ra ther than the absolute value which iimpor t an t .

(e) S eco ndary injection tests:Any var ia tion of opera t ing or rese t t ing t imes orvalue could indicate a cracked or s t icking bearing. Before secondary inject iom aintenan ce on re lays , the termina l boards and re lay s tems should be brushedown wi th an insula ted brush or insula ted vacuum cleaner nozzle .

( f ) Re cording o f tes t resul ts and progress:I t is im po r tan t to keep a record o f theprogress of the resul ts of rout ine maintenance tes ts . The methods adopted wi l l varto sui t the par t icular organisa t ion , but in pr incip le should show the f requency oany tes t , w hen i t i s due , wh en i t was done , and i f i t becom es overdue. The testresul ts can best be re cord ed on test sheets specif ically m ade up for the differenkinds of tes ts and protec t ion . One method in use has a complete b inder of such tesheets per s tat ion , an d each sheet has enou gh spaces to cover several ye ars ' test in

so that the resul ts of past tests can be seen at a glance and compared. The tessheets are m ade ou t in schedule form , in such a way tha t the sheet is no t on ly record b ut a guide to w hat should be done o n the tests . Any defects fou nd in theq uip m en t should b e recorded . Defects of a des ign nature should be repo r ted tthe manufacturers so tha t they can improve the i r des ign.

(g) General. In th is sec tion on rout ine m aintena nce tes ts , no a t te m pt has beenm ade to describe any tes t in de ta il ; the ph i losoph y o f such tes ts has , how ever, beeful ly discussed. Several furth er im po rta nt po ints can be m ade to con clude thisec t ion: one should be famil iar wi th the equipment on which work is to be doneand fu ll use should be m ade of app ropr ia te d iagrams and tes t schedules . It m ay bhelpful to p repare a l is t of item s dis turbed dur ing the tes ts so tha t the e qu ipm encan be quickly res tored to normal wi thout anything being over looked. There i s l i t t lpo in t in m ain ta in ing equ ipm ent in pe r fec t cond i t ion i f i t is pu t back in to commision on the wrong se t t ing or wi th a con nect ion lef t off . In th is respect , i t should b

noted tha t the d isconnect ion points provided in cer ta in wir ing c i rcui ts are in tendefor special faul t invest igat ion pu rposes o nly an d sho uld n o t be used as isolat io




18.5.4 Maintenance o f b u s b a r p r o t e c t i o n , b a c k - t r i p p i n g a n d c i r c u i t - b r e a k e r failp r o t e c t i o n a t d o u b l e - b u s b a r t y p e s u b s t a t i o n s

The consequences of busbar protect ion fai l ing to operate are very serious becau

clearance of the faul t would then be depen dent upo n the opera t ion o f remodistance, ov ercurrent or earth-fault prote ct ion . These rem ote relays wou ld bunable to discriminate betw een the hea lthy and fau lty busbar sect ions result ing com plete d isconnect ion of the substa t ion wi th consequ ent d isrupt ion of the sys temNot only would any generat ing plant at the faulted stat ion be lost , but i t is alspossible that due to the long fault-clearance t ime involved other generators in thclose vicini ty would lose stabil i ty and that low-frequency protect ion would opera

causing fur ther in ter rup t ion of supply. Therefore i t is of param oun t im portance thbusbar prote ct ion is m aintaine d regularly to prove i ts integri ty. Owing to diff icult iinvolved in test ing busbar pro tect io n, and the conseque nces of inad verten t opert ion during such test ing, i t has been the pract ice to take the protect ion out oservice when carrying out maintenance. However, for the reasons stated above lonoutages of the protect ion can no longer be tolerated and techniques have beeevolved for test ing the prote ct ion in service exce pt when chec king insulat ioresistance of d.c. circuits.

From a study of Chapter 13 i t wil l be seen that busbar protect ion, back-tr ippinand circuit-breaker fai l protect ion are closely interrelated and test procedures muoverlap all three facilit ies. It is usually permissible to take back tripping out service to simplify test procedu res, because the r isks to the system are no t so greas when busbar protect ion is out of service. Circuit-breaker fai l protect ion can btested when a circuit is out for routine maintenance.

The tes t procedures which fol low are based upon the 400 kV busbar protect ion

back-tr ipping and circuit-breaker fai l schemes shown in Chapter 13, but thepr inciples de m onstra ted can be adapted to sui t o ther protect ion arrangem ents arequired.

(a) Basis o f busb ar p rote ct ion & ba ck tn 'pp ing tests: T hebasis of the testingprocedure is to work on one zone of protect ion a t a t ime re ly ing upon the 2 out o2 feature to prevent inadvertent t r ipping, and then ideal ly proving operat ion bsimultaneous secondary inject ion of the check zone and a discriminating zone tt r ip each sect ion of busbar in turn with i ts associated circuits . The tr ip outputs othe back-tr ipping receive tr ip relays must be isolated to ensure that the tr ippinis ini t iated from the circuit busbar protect ion relays and not from the zone relayvia the back-tr ipping system. Operat ion of the back-tr ipping system up to thtr ip relay contacts is proved at this t ime and then the circuits are selected to thopposite busbar and the zone relays manually operated to prove the correct select ion of the back tr ipping system via the busbar selector auxil iary switches.

If system cond it ions are such that i t is not ac ceptable to t r ip a sect ion of busbatoge ther with i ts associated circuits a possible al ternat ive is to t r ip only bus-coup le




of busbar. As in the ideal method described in the previous paragraph al l circuback tr ipping relays should be operated via both busbar isolators to prove thcomplete scheme. For th is tes t procedure t r ipping, in ter t r ipping and protect iounstabil isat ion or accelerat ion fro m the circuit busbar pro tec t ion tr ip relays, an

interlocked current t r ip relays, must also be removed. Tripping of individual circuican then be proved by manual opera t ion of the la tching type c i rcui t t r ip re layduring individual circuit maintenance.

In carrying out the tes ts i t i s recommended that the fo l lowing precaut ionshould be observed:

( i) No switching, earthing or s imilar wo rk should be al lowed within the stat iofor the dura t ion of the tes ts.

( ii ) W eather condi t ions should be good a t ou tdoo r type substa t ions .( i ii ) An agreed test ing schedule should be prepared and adhered to.( iv) A minim um of two engineers exper ienced in the protec t ion should carry ou

the tes ts and each should check the ac t ion of the o ther.(v) Autoreclose equipm ent a t the remote feeder ends should be taken out o

service.(vi) Test points should be clearly identif ied and cau tion notices displayed o

equipment no t under t e s t .(vii) The test supply source should be isolated, earth free and of high imp edan c

with a substant ia l tw o pole swi tch betwe en the in jec t ion supply and the tepoint .

(b) T est procedures fo r bu sbar prote ction and back tripping

O ) Insulation resistance:Insulat ion resistance to earth of the current t ransformercircuits should be measured using a 500 V megger. The instrument should bapplied across the appro priate earth l inks before th ey are open ed and should not bremo ved unti l the earth l ink is reclosed. The insulat ion resistance of d.c. circuimust b e measured w i th the appropr ia te supply l inks and fuses wi thdraw n and t ripping circuits isolated. Wiring associated with individual circuits should be testeduring routine circuit outages.

(ii) Secondary in/ection:All a.c. and d.c. tripping relays should be visuallychecked to ascer ta in that the y have rese t before and af ter in jec tion of each a .relay. Inject ion tests should be carr ied out on on ly one phase of a discriminating ocheck zone at a t ime, increasing in small voltage steps and recording operat invalues of supervision, circuit and zone relays in turn noting also that only the relayassociated w ith the injected phase and zone o pera te. The dur at io n of curre nt injec t ion should be kep t to a minim um to prevent damage to re lays and equipm en

It is not necessary to repeat these tests with the circuits connected to the oppositbusbar to prove correct select ion of a .c . wiring via busbar selector auxil iar



Testing com m issioning and managem ent o f pro tection 44 3

(iii) Tripping tests:The p rotect ion should be opera ted for each busbar zone inturn, thereby tr ipping the associated circuits and operat ing the back tr ippinreceive tr ip relays. Operat ion should be by simultaneous a.c . inject ion of the apprpriate discriminating and check zone relays. The injected voltage should b

approximately 150% of the set t ing value obtained in the earl ier tests . I t is noadequate to operate the a.c . zone relays manually for these tests as the manueffort achieved may exceed the electr ical energy available for operat ion. Thprocedure wi l l have es tabl ished correct opera t ion of the busbar protect ion but thc i rcui ts should be connected to the opposi te busbar and the zone busbar protect ioa.c. relays operated manually, to prove correct select ion of the back tr ipping receivtr ip relays by the busbar selector auxil iary switches wh ich are no t m on itore d

Tripping of the circuit breakers from their back tr ipping receive tr ip relays can bproved dur ing c i rcui t maintenance by manual opera t ion of the la tching type re lays

(c) Basic test proce dure fo r circuit bre aker fail p rotection:As described inCh apter 13, circuit-breaker fai l protec t ion ope ns, via the back tr ipping system athe circuit breakers selected to the same busbar as the stuck circuit breaker. Aftproving the back tr ipping system by the test described in Section 18.5.4(b) thcircuit-breaker fai l protect ion can be tested during individual circuit maintenancwithout r isk to other circuits , by removing the l inks which isolate individual circuifrom the back tr ipping bus wires. With these l inks removed the circuit-breaker faprotect ion can be tested down to i ts own back tr ipping receive tr ip relay. The tesshould include insulat ion resistance, a .c. seco ndary inject ion o f curre nt checrelays, d.c . sequence and t iming tests .

18.5.5 Maintenance and test ing of intertr ipping and protect ion signall ingequ ipmen t

Typ e T 40 in ter t ripping and T15 protect ion s ignal ling equipm ent (see Chapter have achieved thei r fas t opera t ing t imes a t the expense of s impl ic i ty, many mocom pon ents being used than in earl ier equipm ents : the c i rcui try a lso is m uch m ocom plex. The quest for h igher speed has a lso m eant that the c i rcui t ry is m uch m osusceptible to air-borne or c able-borne interference and precautions have to b

taken to ensure that in ter ference does not produce malopera t ion or inhibi t or delatr ipping.

Al though a ll m odern equipm ents are n ot necessari ly to T40 or T15 specif ica t ionthey tend to be based on the same design using similar techniques and requir ing thsame precautions against interference.

(a) Commissioning:A full l is t of commissioning checks wil l usually begiveninthe m anufa cturer ' s hand boo k b ut the fo l lowing aspects , which m ay not bincluded, should also be checked to ensure that :



444 Te sting ommissioning and management o f proW ction

(ii)(iii)( i v )

(v )

( v i )(vii)

(viii)

( i x )

( x )

( x i )

all connections are t ight and of the correct polari tythe cabling is of the correct size, and that twisted pairs are used if specifiedthere are no spurious earths on the cabling, or on the cubicle battery suppleadsthe intertr ip send relay does not operate with less than the specif ied minmu m opera te cur ren tthe equ ipm ent , including mo dules , has no dry soldered jo intsthe e lectr ica l noise fed back to the bat ter y f rom the equip m ent i s wi thin thspecif icat ion l imit . A typical value for this is one not greater than 5 mVpeak to peak over the ba ndw idth d . c . - 1 MHz. I t is im porta nt to realisthat such noise on the b at ter y can adversely affect equ ipm ent conn ected t

i t , d.c.-d.c, po w er supp ly units being the m ost usual source of noiseif Post Office line isolation is required this has been fitted and has not iany w ay been bypassed .no spur ious t r ipping operat ions occur when wi thdrawing or inser t ing thpower supply fuses. This should be checked by using a device which carespond to pulses as short as 1-2 ms and a suitable device using a reed relais described in Section 18.5 ~ (d )the output relay operat ion is as specif ied for the protect ion scheme; foinstance is the delayed autoreclose faci l i ty required? Check also that threlay is operated for the minimum specif ied t ime, i rrespective of the durat ion o f operat ion of the sending relayend-to-end signalling times are in accordance with the specification. It iim porta nt to follow the se t ting up procedure as g iven in the m anufacturer 'handbook for the commiss ioning tes ts . End- to-end t r ipping tes ts should bcarried out f irst with the battery voltage normal and then with i t at the

specif ied extremes, i .e . typical ly on the 48 V battery at 44 and 56 V, ineluding tests with high volts at one end and low volts at the oth er.

(b) R ou t ine main tenance and tes ting: T heequipm ent hand boo ks g ive adv ice onthe f requency and type of rou t ine t est s bu t the recom mend ed f requency of t est invar ies between equipment types . For some equipments even weekly tes ts arsuggested, albeit level tests carried out with the equipment remaining in service. Ipractice the f requency o f tes ting is very m uch a comprom ise betw een the econom iuse of resources and the need to keep the pro babi l i ty of an unde tected fault as lowas possible.

A commonly accepted maintenance programme is for the equipment to besubjected to routin e tests every tw o mo nth s. How ever, i f the increased rel iabili tant ic ipated f rom the more modern equipment mater ia l ises i t should be poss ible tex tend the period be tw een rou t ine tes ts to s ix m onths wi thou t de t r iment . Threcommended programme would then be for rout ine tes ts to be carr ied out a t th

end of the f i rs t , second, four th and s ix th months af ter commiss ioning and thereafter at intervals not greater than six months.



Testing comm issioning and management o f protec tion 44 5

m ent bu t in any case are deta iled in the m anufac turers ' ha ndbo oks . The y covesuch i tem s as the checking and adjustm ent o f pow er supply voltages, osci l latofrequencies, monostable t imers, s ignal levels at various test points etc. Final ly thequipment should be end- to-end tes ted and t imed.

Signal t ransmission t ime is cr i t ical ly important for intertr ipping and protect iosignall ing equipment. For example when protect ion signall ing equipment is used foblocking, on the occurrence of a through fault a s ignal must be received from thremo te end in about 20 mil li seconds f rom the detect ion of the faul t , o therwise thprotect ion wi ll malopera te . Co nsequ ent ly, i t is recom m ended that the equipm ent t imed dur ing rout ine m aintenance per iods . Push bu t to n tes t ing, wh ereby the senbutton is operated and the received signal observed at the remote end, does not giv

a real indicat ion of s ignal t ransmission t ime.Timing can most easi ly be achieved by a ' ref lex ' or ' round-the-loop' test . In thithe send relay at end A is ope rated by a push ke y, a con tact o f whic h also startsdigital t ime r . At the rem ote end B, the receive con tacts are arranged to ope rate thsend relay and return the signal to end A, the receipt of wh ich stops the t ime r. Thtest is com parat ive ly simple if switches are specially provided for this faci l ity on thinter t r ipping/pro tect ion s ignal ling equipm ent or on the re lay panel . Absence osuch switching arrangements would make the test more diff icult and entai l thdisconnect ion of p i lo t cores f rom the equipment terminals .

The t ime recorded for the test is the total for the signal t ransmission in bo tdirect ions: half this t ime is norma lly a sat isfactory est imate of the t ime for eacchannel . If the recorded t ime is outside the equip m en t specif ication furthe r investgat ion is required to locate the trouble. I t should be remembered that the measuret ime also includes the l ine transmission t ime but this is unlikely to be greater thaabo ut 1 ms for 22 km of cable length, which is the t ime for s ignal propa gatioalong standard loaded cable. The propagation t ime for carr ier routes should bm uch less.(c) F au lt investigation:The recommended fault invest igat ion procedures for sol idstate equipment as outl ined in Section 18.6.3 are applicable to these types of equipment .

(d) Trip mo nitor device:This dev ice ment ioned in Sec t ion 18 .5 .5 (a ) ( ix )may beused during the commissioning and test ing of intertr ipping equipments, and is easi l

con struc ted. The circuit diagram is show n in Fig. 18.5.5 A.The tr ip receive contacts which can ei ther be clean i .e . clear of power supply, o

supplied from 48 (or 110) V d.c. bat ter ie s are con necte d to terminals A-B. A 48 power supply is connected to terminals C-D. (In ei ther case the polari ty is imma terial because of diode bridges D1-D4 and D5-D8). If the tr ip relay con tacts arconnec ted to a ba t tery then key swi tch KA should be se lec ted to the posi tioshown in the diagram. When the tr ip receive relay operates, relay RA is energiseand locks in over i ts ow n co ntac t to the second coil. The audible alarm is given anthe l ight emit t ing diode (LED) is l i t . When the tr ip contacts have reset the alarm

b b i l i h k




i

~ i )10

.k_a 4~/I Io v

4 8 / i l 0 V KA

Clean

1

Alarmsilence

A I ) ~ C I

I

KC"Rese t

O A_--,.-,d

Clean I r ip__ COl|I~lC|S

KA

B

O r "

d.c.

O D

r .L:~ l ~oo I-I° [ 772

F i g . 1 8 . 5 . 5 A Tr i p m o n i t o r d e v i c e

If long te rm supervision o f a t r ip is required the alarm silence k ey can be ope rateto disconnect the audible alarm: a t r ip receive wil l s t i l l be indicated on the LED.

If the rese t k ey is changed to the opera ted posi t ion, the a larm will autom at ica l lreset when the tr ip contacts restore. With f leet ing tr ip condit ions the audible alarsounds and the l .e .d, l ights only during the slow operate t ime of relay RB.

When the t r ip contacts are c lean, key KA should be opera ted . The c i rcui t topera te RA is then: posit ive terminal C or D; d iode br idge D5-D8; key KA operatedterminal B; tr ip contacts ; te rminal A ; d iode bridge D1-D4; RA coil ; key KAoperated; negative via diode bridge D5-D8 to terminal C or D.

The equ ipmen t is com pac t and can be f i tt ed in a box typ ica l ly 1 10 x9 0 x 55mm

18.6 Fault investigation

Fau lt invest igat ion fal ls under two ma in headings: the invest igation of prim ar

faults and the invest igat ion of faults on the protect ive equipment.

18.6.1 Primary faults

In this con tex t the cause of prima ry faults is ignored, and on ly the effects of thafault on the protective gear and second ary eq uipm ent are considered.

Faults which are cleared correct ly by the protect ive gear normally need l i t t linvest igat ion. The protect ive gear engineer should, however, sat isfy himself that thprotective gear opera t ions are com me nsurate wi th the n ature of the pr im artrouble If there is any doubt he should also sat isfy himself that the protect ive gea



Testing comm issioning and management o f protection 447

has no t been damaged in any w ay by the fau l t cond i t ions to w hich i t has beesubjected ,

A suspected m alope ra t ion of protec t ive gear shou ld , how ever, be inves tiga ted ithe greatest detai l .

G reatest care sho uld be tak en to co l lect and con sider al l the relevant evidencand to assess i t object ively .

Much can be g leaned f rom record ing vo l tmete r s and ammete rs , s topped c locksau tom at ic faul t- recording osc i llographs , repor ts of f lashes , v ibra t ion , noises, a larmand dips on the l ights .

1 8 . 6 . 2 F a u l t s o n t h e p r o t e c t i v e e q u i p m e n t

Many faul ts on protec t ive equipment are s imple and se l f -evident , such as lowinsula t ion res is tance , bad ly adjus ted f lags , and re lays ou t o f ca libra t ion . In term itent faul ts , or fai lure of a swi tch to t r ip when t r ipping tes ts are being done are of tem ore d i ff icul t to d iagnose .

I t i s im po r tan t tha t the inves t iga t ion o f the faul t should be carr ied out in such wa y as no t to d es t roy any ev idence .

The most l ike ly sources of t rouble should be looked for f i rs t . In the case of c i rcuit break er fa il ing to t r ip , these should inc lude low ba t ter y vol tage , faul ty p luand socket conn ectors , auxi l iary swi tches , bu rnt -o ut t r ip coi ls , local ] rem ote se lec toin the wron g p osi t ion , and some k ind of m echanical fa i lure . I f a quick v isual inspet ion reveals non e of these fa ilures, then e lec tr ica l tes ts should be m ade. Such tes tshould cover checks of the t r ip supp ly, fuses and l inks and the con tac t res istance othe aux i l ia ry swi tch and the loca l / r emote se lec to r, and the co n t inu i ty o f the t r icoi l c i rcui t . These points should be methodical ly checked wi th an ohmmeter ovo l tmete r - n o t by pul l ing out fuses or d is turbing an y contac ts . By checkingthrough the c i rcu i t wi th an oh m m ete r o r vo l tmete r the fau l ty pa r t can be iden ti fi ewi th ce r ta in ty. The t roub le can then be rem edied , and one is then ce r ta in tha t threason for the fa ilure has been found and c leared .

Somet imes the inves t iga t ion may be lengthy and involved. In th is case keep chronologica l note of the i tems or par ts of c i rcui t tes ted and the condi t ions applying a t the t im e those tes ts were do ne. This wi l l he lp to avoid the confus ion of th ink

ing tha t a d i fferent result was obta ine d an ho ur earl ier wh en the ' sam e ' tes ts werdone. In fac t , an auxi l iary swi tch , se lec tor swi tch , or indica t ing lamp may now be ia d i fferent po s i t ion f rom wh at i t was befo re , thus a l ter ing the condi t ionscomple t e ly.

1 8 . 6 . 3 F a u l t s o n s o l id - s t a te e q u i p m e n t

The procedures previously descr ibed for commiss ioning and rout ine maintenancof convent ional equipment are , in the main , a lso appl icable to sol id-s ta te protec t ion




a )

(b)

Insulat ion resistance tests shou ld no t b e carried ou t at a voltage in excess o500 V.During such tests diodes and transistors which might sustain damage shoulbe shor ted out .

Failures in solid-state equ ipm ent can usually be traced to a part icular p rinted circucard or m odule wi th the aid of the m anufac turer ' s tes t procedures . Replacement oa faul ty card or m odu le , which usually has plug con nectors , enables the equ ipm ento be res tored to service wi th a minimum of delay. Tracing and replacement ofaulty components on the printed circuit card requires special is t knowledge anequipm ent and is bes t carr ied o ut a t a central depo t or a t the m anu facturer ' s work

Owing to diff icult ies in proving the card after repair unless a complete equipmenidentical to the one from which the faulted card originated is available, the lat tecourse is to b e p referred. A furth er poin t in its favour is tha t replacem ent compon ents used by man ufacturers wi ll usually have been se lected by a process of soaand heat tests before use. These tests el iminate most early fai lures and hencimprove the subsequent re liabil ity of the equip m ent . I f in an emergen ccomponents f rom local suppl iers are used they should be replaced by componensuppl ied by the equipment manufacturer a t the ear l ies t oppor tuni ty.

In order to achieve quick res tora t ion to service of faul ted equ ipm ent , a sui tabstock of spare cards and m odu les should be available. H olding such spares is epensive and this prohibits their provision at each equipment location. Aeconomical arrangement is to hold spares at a central location to serve a part iculaarea. As a general principle an overal l spares holding, based on populat ion, of 10or one of each i tem whichever i s the greater should prove adequate to meet mosituations. This wil l obviously be subject to some variat ion in the knowledge

exper ience wi th par t icular equipments . Cer ta in cards and modules are f requencconscious and in some cases i t wo uld be very di fficul t to jus t i fy ho lding one of eaci tem. In such c i rcumstances , considera t ion should be given to holding s tocks ocom pon ents to permit the adap ta t ion of cards for a var iety of f requencies .

18.7 The avoidance of errors wh en test ing

18.7.1 General

The greatest care is necessary when making tests on si te to ensure safety to l i fe ansecur i ty o f supply. Every effor t m ust be m ade to avoid tes t ing errors , bu t the l is t oprecaut ions can never be com plete .

(a) The correct equ ipme nt:M ake sure that the correct equipm ent is approa ched.This soun ds very easy , bu t on sui tes of panels or on panels wh ich are badly la id o

i t is quite possible to become confused. A common cause of error is for the correcpanel to be identif ied o n the fro nt , and th e wro ng one on th e bac k; or vice-versa.



T e s t in g , c o m m i s s i o n i n g a n d m a n a g e m e n t o f p r o t e c t i o n 4 4 9

PROTECTIVE & CONTROL GEAR TESTS ON SITE

R E M I N D E R S H E E T

Station : ............ ............. .......... Equ ipment'............................................................. Da te'. .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . .Brief Descriptim of Work ................. .......... ........... ........... .......... ........... ........... .......... ........... ........... .......... ........... ...........Reason for test' . ............ ............. ............. ............. ............. ..... Engineer n charge of test: ............. ............. ............. ......

A tic k sEould beplaced agains t the i temsdisfu bed

EQUIPMENT Distu,bed

Trip Link or Fuse

Intertrip Test Links

c . P. o .P,'O~L~nksBattery Biasini] Link

C.T. r~ Terminal Board Links

VoitaEe Fuses

Te st Switches

Wir inl' (a) Leads off

( b ) Te s t l e ~ so n

(C) Temporary Straps

Relays: (a) Settings

(b) Wedges

(c) C oils Shorted

(d) S tabilisinz Resistances(e) M etrosils ....

/~uto Rec=o3e

Automatic Voltaie Control

Local/Remote Selectors

Standby/Remote Se lectusP, sbar Pwotection a) Discr imina tinz Zones

(b) C heck Zone (s)

Adjacent Circuits ?

Remote Substations?

I d e n t i f y c o r r e c t e q u i p m e n t

I n s t r u c t i o n s - c l e a r a n d c o n c i s e

Diag rams up to do te ?

l

NOTES

I IL

l

Restored..

I I

G E N E R A L T E S T I N G H I NT S

C h e c k s e p a r a t e l y a n d i n d e p e n d e n t l y

Te s t l eads ( a ) Co n t inuous (b ) We l l - i n su l a t ed

Care wi th mu l t i - r ange in s t rumen t s

S p e c i a l v i g i l a n c e i f p r og r am m e i s c h a n g e d D a n g e r o f " u n . p r o g ro m m e d e x t r a "

T H I N K b e f o r e t a k i n g a c t i o n

F i n a l c h e c k - u p a f t e r t e s t s

F U RT H E R D E TA I L S O F A B N O R M A L I T I E S M AY B E W R I T T E N O V E R L E A F

F i g , 1 8 . 7 , 1 A R e m i n d e r s h e e t

the application of warning labels to be panels (back and fron t) , on ei ther side of thpanel to be worked on, and the locking of those relay cubicles not being workeO i l .

(b ) L i s t o f abnorm a l i ti es: A list should be m ade of all things disturbed duringtesting, such as wiring disconnected, test leads applied, relay sett ings altered, paper



45 0 Te sting omm issioning and management o f protection

helps res tora t ion to normal to be made quickly and ensures tha t nothing is ovelook ed. A s tandard form can be used for th is purpose , a typ ica l form being showin Fig. 18.7.1 A.

(c) P rimary injec tion tests:During p r imary in jec t ion tes ts :

( i ~ i )( i v )( v )

(~)

( i) I t m us t be ensured tha t the p r imary equ ipm ent is dead and sa fe to work upo( i i) The d iagrams should be s tudied and c . t .s thro ug h w hich pr im ary curren t wi

be passed should be noted . The necessary precaut ions should be taken tsafeguard the secur i ty of adjacent c i rcui ts (see sec t ion re la t ing to pr imarinject ion tests) .Lis t any abn orm al i t ies as in (b) abov e.Ensure th a t no c . t. s a re open -c i rcui ted .Check tha t the tes t ammeters are on the correc t ranges .After tes ts are co m plete d , check tha t a l l tes t leads are removed and th aeverything is back to norm al .

(d) Relay w edge s:During tes ts i t i s of ten necessary to wedge re lays in theopera ted o r no n~ pe ra te d pos i t ion . In induc t ion d isc re lays such wedges shou ld bplaced und er the d isc (so as no t to ap ply excess ive pressure to the b o t to m bear ingThey should be conspicuous so tha t the i r presence cannot be over looked.

(e) T est leads and clips:Test leads should be per iodica l ly checked for insula t ionand co n t inu i ty so tha t no m ishaps occur due to fau l ty leads.

Tes t c lips of the typ e w hich have a re la t ive ly large amo un t o f bare m eta l exposedshould no t be used in cong es ted locat ions wh ere one c lip could shor t across two

relay studs.

(f) Instructions:Where i t i s necessary dur ing tes ts to pass ins t ruct ions f rom oneto the o the r, e i the r d i rec t ly o r by t e lephone , ca re shou ld be t aken to ensure tha tsuch messages are thorou gh ly u nde rs tood . A te leph one d ins t ruct ion in par t iculashould be repeated by the rec ip ient to make sure tha t the message has beenproper ly rece ived and u nders too d .

(g) Re spon sibility fo r tests:Where i t is necessary for two or more engineers towork together on a ser ies of tes ts , i t must be agreed beforehand who is in chargeNo ac t ion shou ld be t aken w i thou t the know ledge o f tha t pe r son .

(h) M ultipu rpose instrume nts:I f a mul t ipurpose ins t rument i s be ing used, grea tcare should be ta ken to ensure th a t i t is on the correc t range for the m easure m entsbeing m ade. Fai lure to do th is m ay resul t in a c . t . be ing effec t ive ly open -c i rcui ted

or in an in ternal f lashover which may wreck the ins t rument and in jure the user.




controls the only source of supply to consumers .

(1 ) F inal che ck: Before leaving the si te get into the habit of having a carefullook round . T hink ba ck over the wo rk tha t has been done to make sure tha t everything has been res tored to normal . Al though advice can be proffered and var ioudevices used o n si te to reduce the chances of e rror, i t is the con tinua l practising oself-discipl ine that wil l do most to keep test ing errors to a minimum.

18.8 The test equip m ent

Provided one is prepared to operate relays manually on a tr ipping test , no equip

ment is required for such a test . Only a megger is required for insulat ion resistanctests . Secondary inject ion tests , however, are in a different category, and opiniondiffer as to the best select ion of equipment required for such tests .

Much can be done wi th a s l ider type res is tor, an ammeter, a t ransformer, aswitch and a s top-watch. The assembly of these , however, poss ibly several t imes w eek, is tediou s and inefficient and i t is desirable to have such portable eq uip m enalready assembled in boxe s to which external conn ect ions on ly need be made.

The des ign of such a tes t se t depends on the type of protect ion for which i t ineed ed, bu t considerable f lexibili ty should be bu il t into i t to cater for all typ es oprotect ion and current ranges .

One principle should be kept f irmly in mind; one is not t rying to set up as tandardis ing labo ratory o n s i te , nor t ry ing to reprodu ce the type approval tes ts.

The purpose of por table secondary in ject ion equipment i s to conf i rm as far asis practicable tha t the pro tect io n is in order and tha t i t keeps in cal ibrat ion w ithinl imi ts dur ing the years wh ich fol low com miss ioning.

A supply is needed an d a means of connec t ing that s upply to the tes t equipm entA sup ply m ay also be required for a hand lam p, soldering iron or an oscil loscope, ssome thought should be given to the provis ion of a tes t supply box to which onesupp ly can be fed but f rom w hich a num ber o f supplies can be taken.

In the tes t equipm ent some current contro l is necessary, the ranges usual ly beingfrom 0 to 3 A on 'mains di rect ' or f rom 0 to 20 A through a t ransformer. Someinst rum entat ion is needed, to gether wi th swi tches for controll ing the supply; and t iming device to measure speed of operat ion. F or d istance protect ion or d i rect ionarelays a voltage sup ply is also necessary, again con trollable. Th e sec ond ary inject iontest e qu ipm ent is then com pleted by sun dry test leads, al l igator cl ips, spl it plugand a pear switch. Each assembly should be portable, s tackable and free fromexcrescences, and the requirements are discussed in more detai l under the appropr ia tehead ings be low.

(a) Te st sup ply:For ful l f lexibil i ty this should be three-phase, four-wire, 415

vol ts o f a s inusoidal waveshape and ade quate ly protecte d. The greatest care must btaken when connect ing to that supply to ensure that i t i s not accidenta l ly ear thed



45 2 Te sting omm issioning and management o f protection

, ° .

. J . .

Fig. 18.8A Te s t g e a r - t e s t s u p p l y b o x

A three-pole and neutral switch specially and permanently allocated for testsupplies should be provided in each relay room. Leads pushed into sockets andwedged in by match sticks are strongly deprecated.

(b) Te st sup ply lead.This should preferably be four-core flexible t.r.s or p.v.c.cable of ample length to connect from the supply to the test supply box located atthe point of test.

The connection of the cable on the test supply box should be designed toprevent strain on the terminations.



Testing com missioning and management o f protection 453

F~ ' L R Y B N

- I : , ( -

Fig. 18.8B Te s t g e a r - t e s t s u p p l y b o x

(c) Te s t supp ly box : The415 V, three-phase, four-wire supply entering the testsupp ly b ox is fused on entry and con nects a four-pole main swi tch to four insula teoutput terminals . The box a lso conta ins a 240 V pi lo t lamp (connected betweeone phase and neu t ra l ) , a 240 V 5 A socke t , a 240 /110/25 /3 V t rans former, ansockets for a hand lamp and a soldering iron. A two-pin socket is provided for connect ing a 3 V lamp used for re lay inspect ion. A ph oto grap h o f one such bo x is g ivin Fig. 18.8A, and Fig. 18.8B shows i ts connections.

(d) C urrent box :The current box der ives i t s supply f rom the tes t supply boxthis is achieved by an insulated bar containing four slot ted brass 'spades ' feedinglength of four-core cable running to the current box. The slot ted brass spades easis lip und er the insula ted terminals on the ou tpu t s ide of the tes t supply bo x.

Only one phase is used in the current box, but al l three phases are brought to a

arrangement of three sockets to provide select ion of the phase required. The voltabox, descr ibed la ter, obta ins i t s supply f rom the three phases of the current boThe selected single-phase supply is fed through a switch or contactor to a resistorpotent iometer network and thence through a ' f ine control ' res is tor to the outputerminals for the 0-3A range. A two-w ay switch on the ou tpu t s ide of the 'f incontrol ' resistor enables the supply to be switched to 'mains direct ' or al ternativeto the pr imary of an external 240 /30 V 20 A t ransformer. A pear swi tch on the eno f a long lead enables the cu rrent to be sw itched whilst a relay high up o n a panelobserved o r t im ed. The curren t bo x is show n in Fig. 18.8C and a diagram of iconn ect ions in Fig 18 8D




. , :~ : , • , • . . . . . . . . . .. . ,, ,; : . , .~

. . . . . . . . . . . . . . . . . . . . - ~ . ~ , , • . . . .. . , , , . i . - . ~ , i

~I[.C~9., 1F.~A.R°¢W~'[~TIt~N [ ~ ' [ r , ~ f , . , r .~ I , , I ~ I I I A T ~ D I A ~ ' : ~ ¢ J@ I ~ L :. : , . - . . . : . ~ , , , , ,~ . . . . . . . : ~ ~ . : . : . . : . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . , a ~ , l l : . t l I

F i g . 1 8 . 8 C T e s t g e a r - c u r r e n t b o x

if too much resistance is introduced in series with the primary of the transformersome distortion of output wave form will result. On the other hand, the resistor inseries with the relay on the 'mains direct' side will give a good waveshape - pro-

vided, of course, that the voltage supply is sinusoidal to begin with.A small 2401110 V transformer is included in the current box to give a supply



Testing comm issioning and management o f protec tion 45 5

The external 2 40 /30 V 2 0 A t ransformer draws i ts supply f rom two pins of four-pin socket mounted in the s ide of the current box, and feeds the 30 V outpuback in to the o ther two pins on the same sock et , from w here i t runs to ou tputerminals for test ing 5 A relays.

(e) Voltage b ox :This p lugs in to the current box on a mul t ipoin t p lug conta iningthe three-phase four-wire 415 V supply and various contactor leads. Two selectoswitches al low any pairs of phases or any phase-to-neutral to be applied separatel

. . . . . . . . . . . . ; ;7 . . . . . . . . . . . . . . . . . . . . . . 7 : . . . . . . . . . . . . . - . . , , , , - ; , . . . . . . . . . . . . . . . : : ~ : " : : '' ' ...

. . . . . . . . . ~ < . ~ " " ' " . . . . . ~ . . , . x . . . . : . , ~ ~ ' i :

i . . . . . . . . . . , ~ ; . : ' . . .. . . .. . 1 . . . . . . ' , 7 . i ~ , .? . . . . ~ ,

i .: :: :: + . > F ' .: .; ;: ;. ii ' ~ ' - " - " . . . . . . . . . . { . ; ; " , . . ; . ~ . . . . . . . ) . . . / L : ; : ~ . . . , J l . . . . .

. . . . ~ , '- - ~ ' ~ ~ ~ . i: ~ "" - : .................................; . i . . . . . . . . : . ' ;~; : . . . . . . . . . . . . . . . . . . . . . . . . . " "

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . :> . . . . . . . . . . . . . .



6 T e s t i n g , o m m i s s io n i n g a n d m a n a g e m e n t o f p r o t e c t io n

T E S TS U P P LY

B O X

O.c.b. c o n t r o l

O O" I ' "

II f " - - "I I

= = = -_ _ J J I Local[ t e s t

I

L . . . . .

i "~1 :~ I I I im in l~ trlak e

Clocksupplyswitch

4 1 5 VS u p p l y V O L r A ( ; I I U N I Ts w i t c h

R e s t r a i n t' c i r c u i t

4 1 5 I I 0

~ 3Q240 ~ l0

f -- 0

I off

o

[ T, . . ., o ,~~ , 1m " ' . .. . " I , o o ~ ^ , , , , o ~

' ~ I L c i r c u i t 4 1 5 _ I I 0 _

I~, [ 'Z . 30

240 Variac- - J r- 0o f f 11 5 V / I 3 5 V

0



T es t in g , c o m m is s i o n i n g a n d m a n a g e m e n t o f p r o t e c t i o n

~ O h m s S u b s t i t u te m e i e rO - / 4 8 0 0 O .S Ao- ~ _ 2 00 c/o o s ^ ~ . ,, ,~k 9 s . c ke . Q o . s ^

~- - ~ SWi tch po in ch . . s~ . . . . . . ~~ , , o o ~ _~_ ~o - I ~ ooo ~ ~ . _ ~ o , ~ ~ 3 , , ~° ' l . o 3o o I L iv e O - , , A ~ ' k ~ l ~ r ~O,= in

~ . i t u t e22 e ~r

S A34 ,)hms SA check 9 suckers 9 S A

I . . 0 2 2 - - 2 A I point 1 1,~. , . . . .

22 ~ " " - k;

22

I ~ 2 I . ~ 1 ¢ : o l ~ *

F:x te rna la~S v~i t ch I I L _ socke t s• I I

N ~ u t r a l l i n k

it

I -

6 0 0 l~

Res t ra in1a d j u s t m e n t 6 0 0 I Z

V I Ranlte0 - 3 0O - l S O V

Setr e s t r a in t

V2 Range

0 - 1 00 - - 5 0 V

R e l a yres t r a in tc i r c u i t s

" 1 ) ~ R . . . . . d O

a ~ O ~ RelayN o r m d i : : : : i s h a l

,, r ~ i i . . . . . o




to in ternal t ransform ers and V ar iacs for feeding:

( i) relay direct io nal circui ts ,( i i) relay restrain t circui ts (col lapsible to s imu late faul t con dit ion s) .

Fig . 18 .8 E shows the vol tage bo x, and Fig . 18 .8 D shows i ts co nnec t ions .

( f ) Pn 'mary injec tion eq uip me nt :This equipment is designed to pass a heavycurrent a t low vol tage through the pr imar ies of current t ransformers .

One in c om m on use i s o f 5 kV A in te rm i t t en t r a t ing ; i t has bo th the p r im ary ansecond ary w indings arranged in sec t ions in such a way tha t the pr im ary can be fef rom a 240 V or 415 V sup p ly, and the secondary can give an ou tpu t vo lt age o f 7 o r 14 V a t a cur ren t o f abo u t 500 A. The cur ren t d rawn f rom the mains m ay, foshor t per iods , be as h igh as 30 A . Co ntrol of the pr im ary c i rcuit can be provided ba 5 kVA V ar iac on the p r ima ry s ide o f the t r ans former.

Heavy f lexible tes t leads w i th c lamp s for fastening to the pr im ary con nect ionare necessary, and a measur ing c . t . and ammeter are needed to determine thou tpu t curren t . Because o f th e low vol tage and heav y curren t , i t is essentia l tha t aconnect ions are c lean and t ight .

The equ ipm ent ( f ) above enab les p r imary cur ren t s to be s imula ted b y which

can be proved whether the c . t . s a re correc t ly connected to the protec t ion . Thre lays themselves can be in jec ted w i th sm aller currents f ro m the sec ond ary in jec t iotes t eq uip m en t and proved to be in ca l ibra t ion . Thu s , by the use of th is tes t equiment the whole cha in can be p roved f rom the in i t i a l p r imary cur ren t to the f inc los ing of the pro tec t ive re lay con tac t .

(g) Instrum en ts and othe r devices:In con junc t ion wi th the above t es t equ ipment ,a var ie ty of ins t rum ents and othe r devices m ay be used. In considering ins t rum enone shou ld cons ide r wha t quan t i ty they a rerequi redto m easure and wha t they a recalibrated to m easure . Ins t rume nts o f the m oving i ron , dy nam om ete r and ho t -wirtype wi l l read correc t ly on d .c . ; they wi l l a lso read correc t ly on a .c . because theread the r.m.s , value of the curren t o r vol tage .

A permanen t -magne t moving-co i l ins t rument , however, wi l l incorpora te rect if ier i f i t is to be used on a.c . and is consequently very sensi t ive to wave shapeSuch an ins t rument responds to theaveragevalue of a wave, but the scale is cal i-

bra ted to indica te the r.m.s , va lue . Reference to Fig . 18 .8F shows tha t a s ine wavhas a rat io of r.m.s , value to average value of 1.11 ( the ' form factor ' ) . Providedtherefore , the ins t rument i s used on a s ine wave i t wi l lmeasure the average but isca l ibra ted to indicate (correc t ly) the r.m.s . I f , however, the wave shape i s non-s inusoidal because i t conta ins harmonics , the form fac tor wi l l be d i fferent f rom1.11; the ins t rument wi l l thus g ive a wrong reading. This emphasises the need tensure t ha t the r ight ins tru m en t is used for an y given app licat ion. I t is advisable thave the ca l ibra t ion o f a ll tes t ins t rum ents per iodica lly checke d.



Testing comm issioning and managem ent o f prote ction 45 9

M.S. va lue

I ~ e r a g e v a lu e

. , | ,

0 _E. n n 5rr 3rr2 2

I '{ } rm f a c t o r = I . i

=

t_ j L .... I I/

0 ~ rr~rr -- .- - 2rr2

3' {} rm fac to r = 1 .3

R e c t i fi e r i n s t r u m e n t e r r o r

on R M S - : ~_1.34 - 1 . 11 )x 100 '1 .34

= 17.1s%

R . M . S . v a l u e- = . . . .

- " i A i e r a g e v a lu e

5_._.~n2n

F i g . 1 8 . 8 F T he e f f e c t o f w a v e f o r m o n m e a s u r e m e n t s

which are in c i rcui t when on the a .c . ranges , so cons idera t ion should be g iven twaveshape as a l readydiscussed.

On these ins t rum ents the re is o f t en a 'd iv ide by tw o ' b u t t on wh ich doub les threading i f th is fac i l i ty is needed for any reason. Care should be take n to ensure thathe bu t to n does no t s ti ck dow n and so g ive too h igh a r ead ing .

The in s t rum ent has d i fferent im pedanc es dep ending on the range in use . I t ia lways des i rable when us ing the ins t rument on the vol tage ranges to have i timpedance h igh com pared w i th any o the r r e si st ance in the c i rcu it , o the rwise thsame vo lt age m ay be read d i ffe ren t ly on d i ffe ren t ranges o f the mete r. The num beof ohms-per-vo l t i s a m easure o f the cu r ren t consum ed by the ins t rum ent , and iusual ly m arked on th e back o f the ins t rum ent or is g iven in the l ite ra ture .

An easy w ay of ca lcula t ing the cu rrent dr aw n b y a vo l tm eter a t fu ll -sca le deflect ion is to a pp ly the fo rmula :

1 0 0 0mA for ful l -scale deflect ion =

ohms per vol t



4 6 0 Te stin g c o m m i s s i o n in g a n d m a n a g e m e n t o f p r o t e c t i o n

r J ~

F i g . 1 8 . 8 G

C . . . . . . . . . _ _ _

Load Z connected to supply

F i g . 1 8 . 8 H

~ V A

, ,

]

• _ . . . . . . . . . . . _ - . _ _

C u r r e n t m e a s u r e d is c o r r e c t .Load vo l t age i s l e s s (by VA) t han

t h e v o l t a g e m e ~ u r e d .

Vo ltmete r on supply-side o f amm eter

I Z + i V Q IZ

i V

Volts measured are correct .Load c urren t is less (by IV ) tha nthe current measured.



Testing com missioning and management o f pro tection 461

On the current ranges i t should again be real ised that the meter represents animpedance in the circui t ; this impedance is greatest on the lowest current range. Ahigh-burden ammeter can upset the d is t r ibut ion of current in c . t . and re lasecondary circui ts and may give misleading resul ts when measuring spi l l currents .

(ii) Ammeters and voltmeters: The poin ts discussed in the previous paragraph sreally cover most of wh at need be sa id abo ut the ins t rum ents . How ever, i f anammeter and vol tmeter are being used together to compute accura te ly the impedance of some c i rcuit i t should be remem bered tha t the a m m eter has some resistancand th at the v ol tme ter takes som e curren t . C onsider Fig . 18 .8G. I f it is des ired tmeasure the impedance o f the load c ircu i t deno ted b y Z , i t cou ld be done by

applying a supply and measur ing the vol tage and the current taken, then div id inthe vol ts by the amp s. But where should the vol tme ter be co nnected i f a real laccurate answer is needed? If i t is connected as in Fig. 18.8H the ammeter wilread th e t rue cur rent , bu t the voltage across the load wo uld be slightly less thanthat read because of the vol t -drop of the load current pass ing through theimpedan ce of the am m eter. I f , how ever, the vol tmeter is conn ected as in Fig . 18 .8i t wil l read the t rue voltage across the load, but the ammeter wil l read the loadcurrent p lus the smal l current taken by the vol tmeter.

In most circui ts i t makes negligible different which method is used. If thecurrent is heavy th en the m etho d of Fig . 18 .8I is be t ter because the vo l tmetercurrent i s inf in i tes imal compared wi th the load current . I f , however, the loadcurrent i s smal l , F ig . 18 .8H would be the bes t method because the vol t -drop acrosthe am m eter is negligible comp ared wi th the supp ly vol tage .

These examples are quoted as a reminder tha t accura te res is tance measurementis not a lways as easy as i t fi rs t appears . The way to con nect the ins t ru m ent i s, how

over, la rgely a mat ter of common sense , and the ins t rument pos i t ion can i fnecessary be al lowed for in the calculat ions.

Final ly, br ief m ent io n might be ma de o f va lve-voltmeters wh ich have v i r tua l ly aninfini te imp edan ce and the refore give a t rue reading even in high-im pedan cecircuits.

(iii) Wattmeters: Wattmeters have a current coi l and a voltage coi l . Often thevoltage coi l has a resis tor in series with i t , and if accurate m easu rem ents arerequired i t is im po r tan t to see tha t the m eter i s con nected up correc t ly in the wa yalready discussed for amm eters and vol tmeters . I f ext rem ely accura te m easurem entsare required i t may be necessary to a l low for the /~R losses in the current andvoltage coils.

An othe r po int t o be w atched is the poten t ia l of the current an d voltage coi ls . InFig. 18.8J the c urre nt and v oltage coils are at the same po tential . I f , how eve r, thecon nec tion s w ere reversed, as in Fig. 1 8.8K , there is phase -to-neutral voltage

betw een the tw o coils. I f accura te m easurem ents are required , Fig . 18 .8K should beavoided because the e lec t ros ta t ic a t t rac t ion betw een the tw o coi ls can in t rodu ce



462 Testing commissioning and m anagement o f pro tection

C _ , AL yVo l t a g eco i l

R e s i s t a n c e

NC . . . . w

C u r r e n t c o i l. . . . . .

T h e s e t w o coilsa r e a t t h e s a m e I Ip o t e n t i a l - - c o r r e c t

. . . . . . - - , L

Fig . 18 .8J Correct wattmeter connections for very accurate measurement

C u r r e n t c o i l

R e s i s t a n c e T h e s e t w o c o il s h a v e

a p o t e n t i a l b e t w e e nt h e m - - i n c o r r e c t

Vo l t a g ec o i l

NO " v - - ,

F i g . 1 8 . 8 K Incorrect wattmeter connections for very accurate measurements

(iv) Signal generator: T hefunction of a signal generator is to produce a range offrequencies. Such an instrument should be available when test ing carrier equipmenThese ins t ruments can be o bta ined w i th di fferent f requency ranges, and a mo deshould be selected which wil l be most suitable for the intended applicat ion. Fotests on carder protect ion a f requency range between 70 kHz and 700 kHz irequired; the ins t rument should have low harmonic-dis tor t ion and good f requencystabil i ty.

The m axim um ou tpu t available f rom freque ncy generators is norm al ly limi ted ta few volts . Such outputs are adequate for test ing carrier equipment at the protect ion f requen cy, bu t to tes t the performan ce o f fi lters at f requencies outs ide thepass-band and to check the l ine-coupl ing equipment , much higher outputs arrequired. For these tests i t is necessary to pass the output from the signal generatothrough an ampl if ie r wi th an ou tp u t o f approx im ate ly 10 W.

(v) Oscilloscopes: Anoscil loscope is required when carrying out work on carrier

or v. f . equipment , and i t can a lso be used to advantage when the wave form of 50 Hz test supply is in doubt. I ts usefulness arises from the fact that i t provide



Testing com missioning and management o f protection 463

speed of the e lect ron beam.The def lect ion of the beam is propor t ional to the vol tage appl ied to the def lec

t ion plates, the signal under test being applied to the Y plates (vert ical deflect ionand an in ternal t ime base to the X pla tes (hor izonta l def lect ion) . In ternal ampl i

f iers are n orm ally buil t into the instru m ent to im prove sensi t ivi ty. These amplif ierare cal ibrated, and the ins trum en t can thu s be used to ob tain an est imate of s ignampli tude. In this respect i t is essential to confirm from the label on the selectoswitch th at the ban d-w idth o f the am plifiers covers the range o f frequencies of thsignals under test. In general, the instrument should be suitable for use with signaof f requencies up to 700 kHz, th is being the upper l imi t of f requencies on carr iesystems.

Current wave forms may a lso be examined by pass ing the current through a

non indu ctive resistor and observing the voltage dro p across the resistor in thnorm al w ay. The value o f the resis tor should of course be smal l com pared wi th thimpedance of the circuit under test , to minimise i ts effect on the circuit .

The oscil loscope can also be used for frequency comparisons. An approximates t imate of f requency can be obta ined f rom the t ime-base f requency i f a s ta t ionart race is obta ined and the n um ber of cycles on the screen coun ted.

A more accurate method of f requency-measurement i s poss ible by making us

of a signal g enerator. This is connected to the X plates in place of the in ternal t imbase , and the u nk no w n frequency is appl ied to the Y pla tes . When the two f requenc ies are identical , a s tat ion ary trace , kn ow n as the 'Lissajous f igure ' is obta inein the form of a straight l ine, el l ipse or circle. The shape of the f igure depends othe phase relat ionship between the two signals .

The other points worth ment ioning are that the osci l loscope can a lso be used tmeasure d.c. , and tha t the co m m on termin al for a .c. and d.c. is usually earthed tthe frame of the osci l loscope, which in turn is earthed through the supply lead

Great care should therefore be taken in connecting up a.c. or d.c. supplies to thterminals when this earth is present .

18.9 Records

For the proper management of protect ive gear, good records are essential as theenable the performance of protect ive gear to be assessed for long or short- term

periods. Typical records are discussed below under their appropriate headings.

18.9.1 Relay settings

For the proper ca lcula t ion of re lay se t t ings , many facts have to be known. Aparfrom the mathe mat ical abi l ity ( including familiari ty wi th sym metr ical com pon entsrequired to do the actual ca lcula tions , much inform at ion is needed before thcalculat ions can even be started. This information includes:(a) C urre nt transformers:Ratio, errors, internal resistance, excitat ion charac-

t i ti i l di k i t l t



464 Testing comm issioning and management o f protec tion

b )c )

a )

e )

t )

V oltage transformers:Ratio and errors .Protection: Ty pes of relays, relay rat ings and burden s, set t ing ranges, operat ingtimes, bias characteris tics ( i f any ).Primary equipment:Lengths of l ines and cables; impedances of generators ,t ransform ers, reactors , l ines and cables; types of line con struct io n (as somtypes of l ine have different impedance characteris t ics) .Diagrams:Diagrams of the local system are required so that the requirementfor discrimination can be assessed.F au lt levels

( i) M axim um fault levels need to be kno wn to determ ine the degree ostabil i ty the prote ct ion has to have on externa l faults , and to calculatrelay operat ing t imes.

(ii) M inimu m fau lt levels are assessed to ensure tha t the re is alwa ys eno ugcurrent available to operate the protect ion, and to calculate relayoperat ing t imes.

( i ii ) Data on the infeed from othe r points on the system are necessary atimes.

Relay settings, once calculated, have to be recorded and readily available at aappropr ia te points - for exam ple , not only a t the locat ion where they were ca lculated but also on si te and in the Grid or Area Control Room.

One such sys tem of recording employs typed sheets f rom which ext ra pr in tcan be taken. The sheets l is t the stat ion, circuit , type of protect ion, c . t . rat ioserial number and ratings of all relays, range of settings available, settings in usand the date on which the set t ings were applied.

I t i s v i ta l ly important tha t theserecords are a lways up to date , and the f low osuch informat ion should be careful ly m oni tored . Excep t in an em ergency, thpassing of relay set t ing information by telephone is deprecated.

18 .9 .2 Tes t r e su l t s

The recording of commissioning tests and routine maintenance tests has alreadbeen described in earl ier chapters . I t is , however, worth l is t ing the main pointagain:

(a) C om missioning log :For recording al l test results on commissioning andkeeping a running record of progress.

(b) Block and circuit diagrams:Block diagrams showing the single-line primaryconn ections of a substat ion toge ther with c. t. posi tions, the protect io n those c. topera te an d the c ircuit breakers the prote ct ion tr ips are a ready means of enablinan overall picture of the substat ion to be readily assimilated.

Circuit diagrams show c. t . , d .c . and other circuits in more detai l in a s imple nonh i l f h i h b dil d d S h di



Testing commissioning and management o f pro tection 46 5

are invaluable on commissioning tests , routing maintenance tests and faul t invesgation.

(c) T est record schedules:Schedules are desirable for recording the resul ts of

secondary in jec t ion , t r ipping, insula t ion res is tance and other tes ts . These schedulwi ll vary in des ign depen ding on the type of tes t and w heth er i t is a comm issionites t or a rou t ine m aintenan ce tes t . A com m iss ioning tes t is usual ly only done o nand usual ly in grea t de ta i l , whereas a rout ine maintenance tes t wi l l be done mantim es in less deta il .

The schedules can be convenient ly conta ined in b inders for a par t icular s ta t ioCommiss ioning logs are used for tes ts on new equipment , and rout ine maintenanlogs for tests done subsequently. These lat ter schedules were discussed more ful ly Sect ion 18.5 .3 .

(d) R ou tine maintenance test records:As mentioned earl ier, opinions differ as tothe f requen cy of rout ine m aintenance tes ting . How ever, some scheme should bdevised of ensuring th at the engineer responsible is rem inde d, well in adv ance , thates ts are due or overdue on a par ticular p iece of equ ipm ent . This rem inder can takthe form of cards, schedu les, or even notes on calendars or in diaries . Regular prgram m ing is des i rable , no m at ter h ow recorded.

(e) F au lt record sheets:To form an assessment of the need for rout ine main-tenance tes ts, a ' faul t h is tory ' of a ll equ ipm ent is a good guide . I f engineers recoron s imple forms, everything found wrong wi th a par t icular scheme of protec t ion , otype of re lay, auxi l iary swi tch , p i lo t cable and the l ike , t rends in performance cabe w a tched . In an am bi t ious scheme the in fo rmat ion cou ld be coded on to punched-card sys tem so tha t the informat ion, fu l ly cross- referenced, could b

reviewed whenever necessary. Such records enable the f requency of rout ine maintenance tests to be varied to l ine up with the faul t l iabi l i ty of a part icular equipm e n t .

The sheets record br ief de ta i l s of the t rouble and of the correc t ive measureappl ied . I f these two c lass if ica t ions are entered und er headings on di fferent par ts othe same sheet they enable outs tanding faul ts to be seen a t a g lance in those casewh ere i t was no t poss ib le to c lear the faul t a t the t im e i t was fou nd.

Or) D ata storage using com p u ters:The C entra l Elec t r ic i ty Gen era t ing Board haveevolved a cent ra l s torage sys tem for technica l da ta . A nat ional ca ta logue o f pow esys tem da ta has been p rodu ced , and a p ro tec t ion da ta scheme has been s ta r ted .

Each p iece o f p r im ary equ ipm ent is iden ti fi ed by a code and a ll kn ow n da tappears in coded fo rm. Ques t ions re fe r r ing to any type o f equ ipment can banswered qu ick ly by the compute r.

The same codes a re used in t h e p ro tec t ion da ta scheme, in which sys tem fau ltare recorded in code, opera t ions of a l l re lays are tabula ted , and informat ion onincorrec t perfo rm ance i~ g iven in deta i l . The inform at ion fed in to the c om pu ter cabe used to de te rmine the per fo rmance of any type of pro tec t ion equ ipment in use




(g) P rotective gear ope ration reports:The just if icat ion for the careful test ing o fprotect ive gear is i ts high overal l performance. There must be some method oassessing whether i t has behaved correct ly; to this end a detai led record should bkept of every faul t and every protect ive gear operat ion, together wi th the c i rcu

breakers which tr ip. A careful note should also be made of any fai lure to tr ip.An annual s tudy of these f igures enables the protect ive gear performance to b

assessed. The system fault performance can be expressed as the rat io of the numbeof sys tem faul ts correct ly c leared to the to ta l n um ber of sys tem faults .

I t somet imes happens that dur ing work on protect ive gear or control ci rcui ta mishap occurs and circuit breakers are tr ipped. This could be due, for examplto secondary wiring or vibrat ion. In assessing the overal l performance of protect ivgear, these unnecessary tr ips should also be recorded. Here the performance can bexpressed as rat io between the number of circuit breakers unnecessari ly tr ipped anthe to ta l number of c i rcui t breakers a t r i sk . On the UK elect r ic i ty sys tem, sucfaul ts are kn ow n as no n-sy stem faul ts , because the tr ips occu r at a t ime w hen theris no faul t on the pow er sys tem .

The sys tem fault performance thus gives the technical performance of thgear, the non -system fault perform ance gives the p erform ance influenced by thhum an e lement , and the com binat ion o f the two gives the overal performance o

the p rotect ive gear and the staff.

(h) Lines o f commu nicat ion:When dealing with protect ive gear, at tention todetai l is of p aram ou nt im portan ce. I t is therefore essential tha t all relevant inform atio n is ful ly and clearly recorde d. This is not easy w ith a m ult ipl ici ty of geand voltages and a staff which m ay be dispersed over several tho usa nd square m iles

Da y-to-day con tact can be mad e via teleph one cal ls or visits to si te , bu t w ri t teinformation needs further considerat ion. I t is not always real ised that the f low oinform at ion needs to be two w ay - f rom headquar ters to s i te and f rom s ite theadquarters .

Relay set t ings, plan t data , circuit diagrams, fault levels , com missioning logroutin e ma intenance records, fault record sheets and protect ive gear perform ancstat is t ics require a two -way f low o f informa t ion, and to avoid chaos so m e systemby which th is informa t ion is channel led to and f ro should be devised. The meth oused will vary to suit different con dit ion s.

For informat ion which usual ly f lows in one di rect ion only, some form om em oran du m or c i rcular may be des irable . For comm unicat ion s on protective geaprotec t ion m em orand a b ear ing serial nu mb ers m ay be issued. In th is w ay, Ides m abe devoted exclus ively to these memoranda. Methods of tes t could form the basof a n um bered ser ies of tes t c irculars to cover d i fferent typ es of prote ct ion , tes t inhints , warnings and the l ike.

The volume o f inform at ion so handled can be qui te formidable bu t i f severafi les are used for different subjects , reference to a part icular topic can quickly bmade .



ndex

Abnormalities, during testing, 4 4 8 - 4 5 0Air-insulated reactors, 182, 183Alarms

testing, 433Alston series capacitor, 230Alum inium shield of reactor), 185Arc-suppression coil

cross country fault, 278operation, 278tuning, 280

Armature, motorcurrent, 149 -152reaction, 151winding, 149-152

Attenuationcarrier signal, 427

Automatic switchingapplication of equ ipment,344, 382, 391

commissioning, 356,357delayed automatic reclosing,345 - 3 48design of equipment and programming,

3 4 4,3 4 8 - 3 5 6electromechanical relay equipm ents,

348-353microprocessor-based eq uipm ent,

353 -356high-speed automatic reclosing,345

Auto-transformer protection,57, 58differential, 58tertiary winding,58

Auxiliary switches for busbar protec tion,131on busbar selectors, 127 - 1 30on section disconnectors, 124

Back tripping, 134 - 1 3 8 , 441maintenance testing, 4 41 -4 4 3

Back-up protection

principles, 328, 33 0,3 31,3 5 8reactors, 192, 193standby earth fault, 236, 241, 333, 334

Bank, capacitor, 2 2 1 - 2 2 3,2 3 0 - 2 3 5BICC

electronic prote ction device,229sealed spark gap, 225

Bonding bar, switchgear, 87Boosters, 199-205

function, 200induction regulator, 210 -212intertripping, 214moving-coil regulator, 205 -210protection, 213 -215transformer tap-changing,200

Booster transformers,2 0 0 - 205 213, 214bucking, 199, 201,208in-phase, 203oil-immersed, 214quadrature, 204

Breaker fail protection,50, 51British Standards

a.c. circuit breakers BS 5311,432air-break circuit breakers BS 3659,432capacitors BS 1650, 245reactor rating BS 171,186 ,245

Bucholz relays, 5 4,1 9 3 - 1 9 5booster, 214earthing transformer, 245reactors, 196

Busbar fault clearanceback-up overcurrent and earth-fault relays,

83distance protection, 84, 85

Busbar protection, 8 1,3 8 5 - 3 8 7 , 3 9 7basic principles, 81, 82biased schemes, 94combined phase and earth fault,83


















































































































































































468 Index

frame earth systems, 8 7 - 9 3influence of c.t. performance on through

fault stability, 103maintenance, 4 4 1 - 4 4 3stability limits, 142

supervision relay , 1 31,1 32,1 4 2test facilities, 139, 140tripping circuits, 88-93, 96-100, 130typical schemes, 113-118unit systems, 86, 93zones, 82, 9 6 - 9 9

multiple check, 123Bushings, transform er, 414, 416, 419Buswire shorting, 132By-pass circuit break er, 228,229

Cable gland, insulated,89Capacitors, 21 5- 245

delta connected, 231external protection, 224-230 , 235-237function and use, 215,219, 220internal protection, 2 2 3,2 2 4,2 3 0 - 2 3 5series connected , 215-217,219, 220,223-

223-230

shunt connected 2 1 5 - 2 2 0,2 3 0 - 2 3 7split star connected,232star connected, 230-235tank type, 221unit type, 221,222

Carrier prote ction , 3 2 7,3 2 8,3 5 9,3 6 0 , 3 9 2commissioning tests,4 2 3 - 4 3 3

CEGBreactive compensation programme,220synchronous compensators,215

Charging currents, effect of,364Circuit-breaker, fail pro tect ion ,142-147

tests, 4 4 1 - 4 4 3 Circuit breakers

contro l diagram, 403tripping and closing tests, 4 31,432

Circulating current principle, 94, 95for busbar protection,9 5 - 1 0 3high-impedance schemes,103-127

fimiting overvoltages, 108, 109operation, 108stability, 103-108stability voltage, 106

Circulating-current prote ctio n see al soM erz-Price principle)capacitors, 235,238reactors, 191

Coil assembly of rea ctor, 184Commissioning

log, 4 0 5,4 1 4,4 3 2 ,4 6 4 , 4 6 5tests, 4 0 0 - 4 3 7 , 44 3,444

planning, 401-405procedure, 405,406schedule, 404

types of, 407-437Com mutator motors,159Compound m otors, 149-155Contactors

in motor con trol, 163in stalling relays, 168principle, 163undervoltage release,180, 181

Copp er shield of reactor, 185Core and coil assembly of moving-coil

regulator, 209, 210Current balance tests,418, 419

test box, 4 5 3,4 5 4,4 5 6 , 4 5 7Current-limiting rea ctor, 186Current transformers

balance tests, 418, 419circuit supervision relay, 131,132, 142connection and polarity tests,4 1 3 - 4 2 3effect of magnetising currents, 374

equivalent circuit, 105, 110knee-point voltage, 108, 412, 413location, effect of, 334-337 , 377, 381,

382magnetisation curve,412, 413 overlap, 411position of, for busbar protection,

118-127ratio tests, 417, 418 records, 463saturation of, 325test links for busbar protectio n,130

Data storage on co mputers, 465Diagrams, types o f

block, 401,402 , 464circuit, 4 0 1,4 0 3 ,4 5 6 , 4 5 7 , 4 6 4

Differential protectionbiased, 419

capacitors, 235,238generators, 7 -11generator, transformer units,63reactors, 190-192, 195spill current, 435synchronous compensators,237, 241,242transformers, 3 9 - 4 5

Directional overcurrent protection,4 8 - 5 0 , 79,436

Disconnecting points for relay testing,440












































































































































































































































































































Distance protectionadvantages and disadvantages, 361,362applied to complex primary circuit,391applied to long feeders, 360-362applied to short feeders,363applied to teed feeders,379-382applied to transformer-feeders, 383, 394,

397applied to underground cables,364reduction in reach, 3 3 1 - 3 3 3 ,3 7 9 - 3 8 1

Earth-fault protectionboosters, 213capacitors, 235,236reactors, 192, 195sensitive, 275,276tests, 4 1 0 , 4 11,4 1 5 - 4 1 8 ,4 3 7

Economic considerations for selection ofprotection, 357,392

Electronic overload relay, 170, 171

Fault investigation,4 4 6 - 4 4 8primary, 446,447protective equipment,447solid-state e quipm ent,447 ,448

Fault indicators for overhead lines, 265=267Fault settings for busbar protectio n,

140-142Fault-throwing switches, 76, 199Flick test for c.t.s, 413 ,414Fo rm factor in testing, 458Fuses

application, 250group fusing, 250

capacitor protection, 223,230characteristics

slow blow ing, 255 ,264cooling, 262deterioration, 262 ,264discrimination, 251-255expulsion, 223high breaking capacity, 223insertion, 431,432

inspection, 406pre-loading, 262repeater, 257short-circuit protectio n, 168types, 250

Gas-turbine-driven generators, 3 4 - 3 8Ge nerator faults,2 - 4

external, 3, 4mechanical, 3

rotor, 2, 3

I n d e x 4 6 9

stator, 2Generator protection systems, 7 - 3 8

back-up overcurrent and earth-fault,12 -17

biased differential, 9 - 11

interturn, 21loss of excitatio n field failure), 22 -24low forward power, 29, 30mechanical and hydraulic,3 1 - 3 3negative phase sequen ce, 17-21overspeed, 30, 31pole slipping 2 4 - 2 7rotor earth fault, 2 7 - 2 9sensitive power, 29tripping supplies, 33, 34unbiased differential, 7 - 9underexcitation, 31

Generator transformer units,6 0 - 6 6biased differential protection,63overfluxing protectio n,65, 66stator earth-fault protection,64tripping arrangements,33, 34, 65

Harmonic currents, 234,237

Harm onic restrain,43-45High-impedance circulating current busbar

protection, 103-127H.V. systems, 297-302

parallel operation, 2 9 9 - 3 0 2back-up pro tect ion to pilot-wire schemes,

300-302pilot-wire protection, 299, 300

radial operation, 297-299ring main system, 297,298

earth-fault-passage indicators,299overcurrent and earth-fault protection,

297-299

Ignitron, 230Inductance, series, effect on power network,

215Induction motors

polyphase, 1S 5 - 157single phase, 157-159split phase, 158squirrel cage, 155-1S 7starting cha racteristic, 165wo und roto r slip ring), 155,157

Induction regulator, 210, 212Inspection tests, 4 0 5,4 0 6 ,4 3 9Insulation-resistance tests, 439, 440, 442,

448, 451Interconn ected l.v. systems,303-316

distributed block h.v. network, protective

























































































































































































































































47 Index

discrimination requ irements, 306, 309-313

fringe fuses, 306,309intertripping, 306reverse power protectio n,306-309

h.v. netwo rk, 305,306distributed block h.v. network,305 ,306interleaved h.v. network, 305

l.v. network, 304self clearance of faults,304

substations, 304,305transformer protection,304,305

Interlocking low forward power relays, 65Intertripping

boosters, 214choice of method, 338-340 , 383-385 ,

397commissioning tests,44 3,444d.c. signalling, 3 3 8,3 4 0 - 3 4 2fault throwing, 199, 339, 343 ,344maintenance, 4 4 4 - 4 4 6power-line, carrier, 339 ,343reactors, 197voice-frequency signalling, 338,343

Line coupling equipm ent,423 ,426 , testing, 4 2 4,4 2 6 , 4 2 7

Line traps, 424 ,425testing, 424,426

Links for testinginsertion, 4 31,432inspection, 406

Lissajous figure,463

M agnetic shield of reac tor, 183M ain protection , application principles,

328-330Maintenance

frequency schedules,438, 439records, 465routine tests, 4 3 7 - 4 4 6

M ercury sw itches,1 9 4,1 9 5 , 1 9 7 ,2 1 4Merz-Price principle, 191M icroprocessors, use o f , for autom atic

switching, 3 5 3 - 3 5 6 ,3 6 0Motors, 149-181

a c

characteristics, 155-162types, 155-162

applications, 162control, 163d c

characteristics, 149-155

excitation, 150

fault types, 164protection, 164-181

Moving-coil regulator,2 0 5 - 2 0 9

Negative potential biasing, rotor earth-fault detection, 27, 28

Oil-immersed reactor, 185, 190protection, 190-198

On-load tests, 43 5 -4 37Overcurrent protection

directional, 48 -50 , 79e.i.d.m.t.l., 17high set, 3 6 8,3 9 4 - 3 9 7i.d.m.t.l., 12 -17 , 47, 48, 55, 325-331instantaneous, 7 0 - 7 3interlocked, 50 reactors, 1 9 2,1 9 3 , 1 9 6 , 198two-stage, 337, 338 ,368

O verload protectio n, thermal,1 65,166 ,234 , 237

Overvoltage protection for capacitors,236

Phase-sequence com ponen ts, 421Phase shift during testing,409 ,428Phasing tests, 4 3 3 - 4 3 5Pole-slipping protection,2 4 - 2 7Power directional comparison protection,

3 7 7 - 3 7 9Primary injection tests, 4 1 4 - 4 2 3

equipment, 458Primary networks, rural

auto reclosing, 286protection

duplicate feeder transformers,2 8 4 - 2 8 6ring main systems, 283single-feeder transfo rmer s, 283 ,284

single-circuit-breaker su bstation ,289Primary substations in urban distribution,

302 ,303Private generation in urban networks,

316-319Protection applications in a major transmis-

sion systembanked transformers, 3 7 3 - 3 7 6complex primary circuits,387-392double busbar station, 385-387long overhead feeder, 358-363

carrier protectio n, 359,360distance protection, 360-363

mesh station, 387short overhead feeder, 362,363tank earth-fault, 376-376

teed feeder, 3 7 6 - 3 8 2



























































































































































































































































transformer, 365-373transformer-feeder, 383-385

Pro tection applications in min or transmissionand major distribution systemscomplex primary circuit,397

double busbar or m esh station,397feeder, 392teed feeder, 393transformer-feeder, 3 93 -3 97transformers, 393

Protec t ion in urban ne tworks- fu turetrends, 319

Protection operating times, 324, 325 ,361 , 393, 394

Pro tection signallingchoice of m ethod, 3 3 8 - 3 4 0d.c. signalling, 3 3 8,3 4 0 - 3 4 2 , 362-365power-line carrier, 3 3 9,3 4 3 ,3 6 2voice-frequency signalling,338, 342, 362,

363Protection signalling equipm ent

commissioning tests,443 ,444maintenance, 4 4 4 - 4 4 6

Q ualitrol device, 54Radial l.v. systems, 294-299

l.v. cables, 294short-term ratings, 294

services, 294substation transformers,295-297

parallel operation of transformers,297protection by circuit breakers,296protection by fuses, 295 ,296

Reactors, 181 - 199application, 186-190busbar, 187feeder, 189generator, 186group feeder, 190

current limiting, 186discharge, 229function, 181, 215

series, 155, 181shunt, 181

protection, 190-199overall differential, 190-192, 195overcurrent and earth fault,192, 196

rating, 185types, 181 - 185

Recloser, pole mounted, automatic,257alternative op erating cycles,264,265application, 261

characteristics, 259,260

I n d e x 4 7

cold-weather performance,260construction and operation, 258-261co-ordination with fuses,261-264grading

reclosers in series,269

with fuses, 2 6 1 - 2 6 4 , 268with relays, 268Reclosing, automatic

dead time, 256 ,272multishot, 256 ,272principle, 255 272schemes, 272single shot, 256 ,272

Records, 4 6 3 - 4 6 6relay settings, 463test results, 4 4 0 , 4 6 4 , 4 6 5

Regulatorsinduction, 210, 212moving coil, 205 ,209

RelaysBuchholz, 193-195current check, 1 2 2,1 4 3 - 1 4 5for c.t. circuit supervision, 131 ,132, 142gas and oil operated, 193-195

high-impedance, 112duplication of, 117per circuit, 113per zone, 113rating, 132, 142

records, 464settings, 463single pole, 180, 191stalling, 168-170tests, 408 -411

attracted armature, 408biased, 409directional, 4 1 0,4 1 1,4 3 5 - 4 3 7distance, 410inverse definite minim um time,408

thermal, 165-167thermal trip, 170-173

Relays, reclosingperiodic, 269 ,270solid state, 272-274

R em ote circuit breaker, tripping of, 121, 122Resistors, use in busbar protection

non-linear, to lim it overvoltages,108, 109rating, 132, 142series, 155shunt setting, 110stabilising, 106,107

Reverse power pro tection, 241,242

Reyrolle




























































































































































































































































472 Index

spinner gap, 227Ro tating field, starting metho d,158Rotor heating, 4 2 0 - 4 2 3Rural system protection

performance/cost comparison,280

arc-suppression coils,281automatic pole-mounted reclosers, 282fuses, 281reclosing substation circuit breakers,282

Rural systemsconstruction, 247design, 247faul t levels, 247 ,248

Sanction-for-test card,409, 414

Schrage moto r, 159-161Secondary injection tests, 4 0 7 - 4 11 ,4 4 0

biased relays,409busbar protection, 44 2directional relays, 410, 411distance relays, 410equipment, 4 5 1 - 4 5 8instantaneous relays, 408inverse definite m i n i m u m time, 408

Sectionalisersco-ordinationwith reclosers 267

non-extinguishing, non-triggered, 224non-extinguishing, triggered, 227

Spill curren t, 103Spinner gap, 227Squirrel-cage motor, 155-157

Standby earth-fault protection,333 ,334capacitors, 236 ,238synchronous compensators, 241 ,242

Star connectio n busbar reactors), 188Star-delta interposing c.t.s.

generator h.v. overcurrent protection,1 3 - 1 7

transformer-feeder instantaneous over-current/earth-fault protection, 7 1 - 7 3

Starting characteristic, 165induction motor, 165shunt-wound motor, 166

Station transformer protection,57Substation circuit breakers

reclosing, 269control schemes, 272operating sequence, 271

requireme nts for reclosing duties,274vacuum 275

Substationsmodification to, 437
































































































Documents

Power System Protection [Vol 3 - Application] 2nd Ed (IEEE, 1995) WW