MBC

I

240

Type MBCI Relay: Translay ‘S’ Differential Feeder and Transformer Feeder Protection

Features

MBCI Pilot wire differentialprotection relay

• High stability for through faults

• High speed operation for in-zone faults

• Simultaneous tripping of relays at each line end

• Low current transformer requirements

• Low earth fault settings

• Designed for the unit protection ofoverhead and underground feeders

• Suitable for pilots up to 1kΩ or 2.5kΩ withpilot isolation transformers

• Four electrically separate contacts

• Can be used as definite time overcurrentrelay in the event of pilot failure

Models Available• MBCI 01

• MBCI 02

Optional Extras

MRTP Supervision relay for acpilot circuits

• Alarm and indication of pilot failureand supervision supply failure

• Suitable for pilot circuits insulated for5kV or 15kV with pilot isolationtransformers

MVTW Destabilising andintertripping relay

• For use with pilot wire relays

• Destabilises the feeder protection sothat tripping occurs

• Intertripping: injects ac voltage intothe pilot circuit so that tripping occurs

MCRI Instantaneousovercurrent and start/checkrelay

• High speed operation

• Not slowed by dc transients

• Wide setting range

• Two phase and earth fault relay

MCTH Transformer inrushcurrent detector

• Allows MBCI to be applied totransformer feeders

• Blocks operation of the MBCI relayduring transformer inrush conditions

• Blocking does not occur for zero,normal load, or genuine fault current

Application

The Translay S differential schemes havebeen designed for the unit protection ofoverhead and underground feeders andtransformer feeders.

Plain Feeders

Differential protection

Differential feeder protection requires acomparison of the currents entering andleaving the protected zone. For faultsoccurring within the protected feeder it isdesirable to trip the circuit breakers at eachend to isolate the fault. Two MBCI relays aretherefore required, one for each end of thefeeder. A pair of pilot wires is used totransmit information between the tworelays so that each may be able tocompare the current flowing at itsrespective end with the current at theother.

The relays at both line ends operatesimultaneously, providing rapid faultclearance irrespective of whether thefault current is fed from both line ends oronly one line end.

When applying this protection tooverhead lines the limiting factor isgenerally the length of the pilot circuits:for cable feeders the limiting factors aremore likely to be the level of linecharging current and the method ofsystem earthing.

MBC

I

241

Pilot supervision

Correct interchange of information overthe pilot circuit is essential for the properfunctioning of any differential feederprotection. Pilots may be exposed tohazards and some risk of damage andfailure always exists. The most commonpilot failure is to the open circuit state,caused by the accidental excavation ofburied pilots or storm damage tooverhead pilots. With the pilots opencircuited the differential protection will beunstable and will trip the feeder ifsufficient through current is flowing.

For this reason the circulating currentsystem is often preferred as such schemeswill fail safe and trip so that attention isimmediately drawn to the fault.

The addition of pilot supervision will notprevent tripping for pilot faults but willindicate the cause. It will also detectshort circuit and cross-connected pilotconditions which would not otherwise bedetected. Indication is also provided forloss of the supervision supply.

Destabilising/intertripping

When the protected line is connected toa busbar system, a fault on the busbarswill in general be cleared by the busbarprotection by opening some or all of thelocal circuit breakers. Although suchfaults will usually appear to the feederprotection as through faults, withresultant stability of the feederprotection, it may be desirable to openthe remote line circuit breaker also, toclear the line completely. The remote unitof the differential feeder protection canbe caused to operate, provided sufficientline current is flowing, by open circuitingthe pilots. If line current is not flowing,the remote unit can be operated(intertripped) by injecting a current intothe pilots.

Overcurrent check/starting

Although the supervision scheme providesindication of pilot failure it does notprevent the protection operating if primarycurrent above setting is flowing. Wherethis hazard is unacceptable it is necessaryto add an overcurrent check feature.

The current transformer requirements arenot modified by the addition ofovercurrent elements since they present avery low burden.

When the starting feature is used theoverall operation time of the scheme isincreased by 3–5ms. However, there is noincrease in the overall operation time whenthe overcurrent protection performs a checkfunction only.

When overcurrent relays are used theprotection cannot be intertripped by acinjection into the pilots, and destabilisingthe protection will result in tripping only ifan overcurrent condition existssimultaneously.

Emergency use for overcurrentprotection

In the event of a pilot failure whichcannot quickly be rectified, the TranslayS scheme may be adapted for use as adefinite time overcurrent relay as follows:

If overcurrent relay is fitted

• disconnect pilot wires and leaveterminals open circuited

• set Kt to 3(300ms)

• check that overcurrent elements are onthe required setting above maximumanticipated load current.

If overcurrent relay is notfitted

• disconnect pilot wires and connect a1kΩ resistor across pilot terminals ofrelay

• set padding resistors Rpp to maximum(600Ω)

• set Kt to 3 (300ms)

• set Ks to required open circuit setting:Ks = 1 gives three phase equal torated load In.

Note: Other fault settings will dependupon the summation ratio.

A

B

C

Rs

Ts

Tt

RVD

Trip

øcT1

T2

To Pilot wires

Ro

Rpp

Tr

Ro

Rpp

To

Tr

RVD

øc

V

T2Trip

RsT1

ToTrTrRo

- Operating winding- Restraining winding- Tertiary winding- Linear resistor

Rppøc

- Pilots padding resistor- Phase comparator

T1T2RVDTs

- Summation transformer- Auxiliary transformer- Non linear resistor- Secondary winding

V

Figure 1:

Basic circuit arrangement

MBC

I

242

RL1–1

PilotWires

Trip/Alarm

Outputs

MBCI

IA

IB

ICIN

RS

SeeNote 5

SeeNote 2

VX–

+PowerSupplyCircuits

Enable

Enable

RL12

RL22

RL31

Protected Zone

RES

OP

LevelDetector

Squarer

Squarer

KS

&

&

IA

IB

IN

MBCI

PhaseRotation

RS

Protected Zone

SeeNote 4

ABC

RO

Case EarthSee Note 3

A

BC

Moduleterminal blockviewed fromrear

(a) C.T. shorting links make before (b) & (c) disconnect.

(b) Short terminals break before (c)(c) Long terminal.

RPP

Kt

3. Earthing connections are typical only.

4. C.T. connections are typical only.

5. For overcurrent start schemes, terminal 12 must be connected directly to D.C. +VE to provide a supply for the L.E.D. and reset circuits.

1.

2. Link terminals 11 and 13 except when used with overcurrent check replay type MCRI.

23

RL1–2

RL2–1

RL2–2

IC

Notes

19

18

1710

8

97

642

531

11

14

13

12

282726252423

P2 P1S2 S1

2425

2627

28

P1 P2S1 S2

1 23 45 67 89 1011 1213 1415 16

17 18

19 20

21 22

23 24

25 26

27 28

Case earth

RL4–11

3

5

2

Output Contacts Change state for pilot fail1 2

3 45 67 89 1011 1213 1415 1617 18

19 20

21 22

23 2425 2627 28

Case earth

Module terminal blockviewed from rear

Note 1. (a) C.T. shorting links make before (b) & (c) disconnect.(b) Short terminals break before (c).(c) Long terminal.

RL42

RL4–2 4

6

7

RL5–1 9

11

8

RL5–2 10

12

Output Contacts Change state for supply fail

RL52

RL71

RL61

RL81

SupplyFail

PilotS/C

PilotO/C

RL22

RL32

OPReset

OPReset

OPReset

tRL3–1

RL2–1

RL1–1

+VE

StartRL1–2

RL2–2

RL3–2

RL12

A.C.AuxiliarySupply

A.C.PowerSupply

D.C.PowerSupply

Case Earth

20

19

27

28

13

14Vx

18

17

17 1818

17

17 18

MBCI MBCIPilots

MRTP02Case Earth

19

20

MRTP01

Figure 3 Application diagram: pilot supervision relay and injection filter type MRTP 01

Figure 2 Application diagram: differential feeder protection relay type MBCI

MBC

I

243

RL4–11

3

5Output Contacts Change state for pilot fail

See Note 7

Module terminal blockviewed from rear

Note 1.

(a) C.T. shorting links make before (b) & (c) disconnect.(b) Short terminals break before (c).(c) Long terminal.

RL42

RL4–2 4

6

7

RL5–1 9

11

8

RL5–2 10

12

Output Contacts Change state for supply fail

RL52

RL71

RL61

RL81

SupplyFail

PilotS/C

PilotO/C

RL22

RL32

OPReset

OPReset

OPReset

tRL3–1

RL2–1

RL1–1

+VE

StartRL1–2

RL2–2

RL3–2

RL12

A.C.AuxiliarySupply

A.C.PowerSupply

D.C.PowerSupply

Case Earth

20

19

27

28

13

14Vx

18

17

18

17

MBCI MBCIPilots

MRTP 03

S1

P1

S2

P2

P6P5P4P3P2

P1

S2

X2

X1S1

Supervisionisolation transformer

P6P5P4P3P2

P1

Pilot isolationtransformer

S1

X1

X2

S2

1 23 45 67 89 1011 1213 1415 1617 18

19 20

21 2223 24

25 26

27 28

Case earth

2

Pilot isolationtransformer

Figure 4 Application diagram: pilot supervision relay 15kV isolation type MRTP 03

Powersupplycircuits

PhaseRotationRs

P2P1S2

SeeNote 4

ABC

Case earthSee Note 3

A

BC

Module terminal blockviewed from rear

(a) C.T. shorting links make before (b) & (c) disconnect.(b) Short terminals break before (c).(c) Long terminals.

S1

1.

MCRI

MBCI

Case earthSee Note 3

D.C. +_

23

24

IS

RL12

14

25

26

( )

27

28

IS

IS

2.

3.4.

Connection for overcurrent startConnection for overcurrent checkEarthing connections are typical onlyC.T. connections are typical only

Notes

RL1–2624

RL1–1513

13232425262728

11

1314

Case earth1 2

3 4

5 6

7 8

9 10

11 12

13 14

15 16

17 18

19 20

21 22

23 24

25 26

27 28

(C)

(A)

(A)(B)

See Note 2(B)

See Note2(A)

Figure 5 Application diagram: overcurrent relay type MCRI 01

MBC

I

244

Transformer Feeders (useof transformer inrushcurrent detector)

In the case of transformer feeders wherethere is no circuit breaker separating thetransformer from the feeder, thephenomenon of transformer magnetisinginrush must be considered. This is atransient condition which may occur atthe instant of transformer energisation, orimmediately following a system voltagedrop due to a nearby heavy faultcondition. Magnetising inrush current isnot a fault condition and therefore doesnot necessitate the operation ofprotection equipment which, on thecontrary, must remain stable during theinrush transient. The inclusion of a typeMCTH relay, designed to provide ablocking signal in the presence oftransformer inrush currents, enables apilot wire differential protection schemeto be applied to a transformer feeder.Where line and therefore transformerenergisation can occur at one end onlyof the transformer feeder, then a MCTHunit would be required on that side only.

When the feeder transformer isenergised any resulting inrush currentwill be detected by the MCTH relay, theoutput blocking unit of which will pick-upcausing the pilot wires of the Translay Sto be short-circuited. This will stabilisethe differential relay and prevent it fromresponding to what would otherwiseappear to be an in-zone fault. Theimmunity to operation due to inrushcurrent is coupled with fast faultclearance times and the built-inovercurrent detectors of the MCTH relayensure that the blocking feature isoverridden if a fault is detected in onephase whilst inrush is present in another.

Symbols:

15kV Isolating transformer

15kV Isolating transformerwith injection filter

Scheme Pilot Supervision O/CInsulation Start/Check Arrangement of EquipmentLevel (kV) (Viewed from front)

A 5kV — —

B 15kV — —

C 5kV • —

D 15kV • —

E 5kV — •

F 15kV — •

G 5kV • •

H 15kV • •

Table 1. Typical scheme arragements for plain feeders. See key below.

Scheme Pilot Supervision TransformerInsulation Arrangement Arrangement of EquipmentLevel (kV) (Viewed from front)

I 5kV —

J 15kV —

K 5kV •

L 15kV •

M 5kV —

N 15kV —

O 5kV •

P 15kV •

Table 2. Typical scheme arragements for transformer feeders. See key below.

No. Type of relay

1 MBCI 01/02 Differential2 MRTP 01 Pilot supervision and injection filter3 MRTP 02 Injection filter4 MRTP 03 Pilot supervision5 MCRI 01 Overcurrent start/check6 MVTW 01 Destabilising7 MVTW 03 Destabilising and Intertripping

Schemes A to D can be fitted with relay types 6 or 7.Schemes E to H can be fitted with type 6 which will providedestabilising if the overcurrent start/check relays (MCRI 01) haveoperated. Schemes I to L must use type 7 or 8.

8 MCTH 01 Transformer inrush current detector9 MFAC 14 High impedance earth fault relay10 MMLG Test plug/block

It is advisable on all schemes to include the test unit to facilitatecommissioning and routine testing. The unit will be situated on theright–hand side of the scheme.

1 1

1 1

2 1 3 1

4 1

1 5 1 5

1 5 1 5

4 1 5 1 5

1

2 1 5 3 51

1 8 9 7 1 8

1 8 9 7 1 8

1 8 2 9 7 1 8 3

1 8 4 9 7 1 8

1 8 1 8

1 8 1 8

1 8 2 1 8 3

1 8 4 1 8

Pilots

MBC

I

245

Description

Differential protection

The differential feeder protection circuitis derived from the well known Merz-Price circulating current system butemploys phase comparators as themeasuring elements. This novelcombination provides high stabilityperformance for external faults with theminimum of bias (restraint) quantitythereby ensuring that the low earth faultsettings are effectively retained evenwhen load current is flowing. Figure 1shows the basic circuit arrangement. Asummation current transformer T1 ateach line end produces a single phasecurrent proportional to the summatedthree phase currents in the protectedline. The neutral section of thesummation winding is tapped to providealternative sensitivities for earth faults.

The secondary winding supplies currentto the relay and the pilot circuit inparallel with a non-linear resistor (RVD).The non-linear resistor can beconsidered to be nonconducting at loadcurrent levels. Under heavy faultconditions it conducts an increasingcurrent and thereby limits the maximumsecondary voltage. At normal currentlevels the secondary current flowsthrough the operating winding To ontransformer T2 and then divides into twoseparate paths, one through Ro and theother through the restraint winding Tr ofT2, the pilot circuit and resistor Ro of theremote relay.

The resultant of the currents flowing in Trand To is delivered by the third windingon T2 to the phase comparator and iscompared with the voltage across Tt ofTransformer T1. The emf developedacross Tt is in phase with that across thesecondary winding Ts which is in turnsubstantially the voltage across Ro.

Taking into account the relative values ofwinding ratios and circuit resistancevalues, it can be shown that thequantities delivered for comparison inphase are:

(IX + 2IY) and (2IX + IY)

where IX and IY are the currents fed intothe line at each end (for through faultsIX=IY). The expressions are of oppositesign for values of IY which are negativerelative to IX and are between 0.5IX and2IX in value. The system is stable with thisrelative polarity and operates for allvalues of IY outside the limits.

The phase comparator has angular limitsof ± 90° giving a circular biascharacteristic in the complex plane. If thepilots are open circuited, current input willtend to operate the relay. Conversely,short-circuited pilots will cause the relay torestrain, holding its contacts open.

Transformers T1 and T2 also provide thenecessary insulation barriers for staticcircuitry. The input circuits of the phasecomparator are tuned to the powerfrequency so that the threshold ofoperation increases with frequency. Thisde-sensitises the relay to the transienthigh frequency charging current thatflows into the line when it is energised.A further advantage provided by thetuned input is that the waveform of thederived signal, which may be severelydistorted by current transformersaturation, is improved, ensuring highspeed operation under adverseconditions. In order to maintain the biascharacteristic at the designed value it isnecessary to pad the pilot loopresistance to 1kΩ.

A padding resistor Rpp is provided inthe relay for this purpose.

Pilot isolation transformers

When pilot isolation transformers areused, the range of primary taps enablespilots of loop resistance up to 2.5kΩ tobe matched to the relay. The pilotinsulation level is also raised to 15kV bythese transformers.

Telephone type pilots

When the pilots to be used are of thetelephone type, an alternative limiterbased on a zener diode is available toensure that the maximum voltage whichcan appear on the pilot system is withinprescribed limits. Pilot isolatingtransformers can be used in thisarrangement also, both to provideinsulation to 15kV and also indirectly toenable pilots of relatively high resistanceto be used.

Pilot supervision

Figure 3 shows the arrangement for pilotsupervision in a pilot circuit insulated for5kV. In this instance the injection filtersand the supervision unit are assembledwith the relay case. (Types MRTP01/02).

Figure 4 shows the similar arrangementfor pilot circuits insulated for 15kV (TypeMRTP 03).

The injection filters are then assembledas part of the isolation transformer andhave to be isolated from the supervisionrelay. The supervision isolationtransformer provides the necessary 15kVisolation barrier. For further technicalinformation see Publication R6026.

Destabilise and IntertripFacilities

MVTW01

Refer to Figure 6.

Operation of the destabilising relay (UN)results in the summation currenttransformer in the differential relaybeing short circuited and the local relayprevented from tripping. The remoterelay then sees a single end feedcondition and trips, provided the throughcurrent exceeds the no-load fault settingof the protection (see Table 5 page 18).Typical operating times are shown inFigure 7.

Terminals 17 and 20 should normally belinked together on the destabilising relay.However, the operating level of theremote equipment can be reduced toone half of the normal fault setting(under destabilising operation only) ifthis link is omitted.

It should be noted that, with this linkomitted, if the destabilising relay UN isoperated for longer than the supervisiontime delay (6-10 seconds) an indicationof pilot failure will be given. This doesnot apply if pilot isolating transformersare used.

When overcurrent elements are used toprovide a starting or check function thereis no advantage in removing this linksince, for operation, the through currentmust exceed the overcurrent setting.

MVTW 03

A circuit diagram for the MVTW 03 typerelay which depicts the destabilising,intertrip and inverter function is shown inFigure 8. On energising the relay, agreen LED illuminates and the normallyclosed contacts of RL1 open to indicatethe supply is healthy and the inverter isoperating.

MBC

I

246

The MVTW 03 incorporates a full bridgeinverter, which receives complementarysquare wave signals from the oscillatorcircuit at a frequency of 80Hz. Thisfrequency was chosen because it liessufficiently far from the pilot frequency of50 or 60Hz and cancellation of theintertripping signal will not be caused bythe beat frequency that may be produced.

The inverter is continually energised andsupplies a transformer which isolates thepilots to 5kV from the input circuits. Thetransformer supplies the intertrippingcurrent and the power supply for theoutput relay (RL2).

Intertripping is initiated by applying atrip signal to terminal 11 whichenergises the output relay.

The signal is isolated from the pilots to5kV by the opto isolator. When theoutput relay (RL2) is operated, the localMBCI is made stable by shortingterminal 18 to terminal 19. This actiondestabilises the remote MBCI. If the loadcurrent level is greater than thedifferential current setting the remoteMBCI trips; however if the current level islower than the setting the remote MBCIdoes not trip. To ensure intertrippingoccurs the output relay (RL2) injects a20mA intertrip current into the pilots; theremote MBCI sees the intertrip current asa differential current which causes it totrip.

For further technical information seePublication R6027.

Transformer Inrush CurrentBlocking

Transformer inrush currentdetector feature

Refer to Figure 9.

The principle of operation of thetransformer inrush current detector(MCTH) is based upon a unique featureof substantially zero for significantperiods in each cycle. During load orfault conditions, the current waveformremains at zero for negligible periods ineach cycle. The relay detects these zeroperiods in the inrush waveform andinitiates a blocking relay, which causesthe pilot wires of the Translay S relay tobe short-circuited, preventing tripping fortransformer inrush current.

A typical scheme for a delta–star powertransformer is shown in Figure 9. Theline current transformers are connectedin star on the delta side of thetransformer. Appropriate choice of CTratios ensures that for normal load andthrough fault conditions, equal currentsflow into the differential tripping units(MBCI) at each end.

A high impedance differential relay(type MFAC 14) is included in theneutral lead of the star-connected linetransformers to give lower earth faultsettings on the delta side of the powertransformer. The MFAC 14 highimpedance differential relay may beused to initiate an intertrip unit (type MVTW 02).

1 2 3 4 5 6 7 8 9 10 20 30 40 50 800

40

80

120.

160

200

Current - Multiple of setting

Milli

seco

nds

Kt = 6

Kt = 14Kt = 20Kt = 40

Figure 7 Time characteristic for destabilised operation

1 23 45 67 89 1011 1213 1415 1617 18

19 2021 2223 2425 2627 28

Pilot isolation transformer

Pilots

P6 S2

P1 S1MBCI

17

18

19

Case earth

MVTW 01

UN-1

UN-2

UN-3

UN3

17

18

19

20

11

12

14

Vx(1)+Vx(2)+Vx(3)+ 13

Case earthSee note 2

Module terminal blockviewed from rear

(a)

(b)(c)

CT shorting links makebefore (b) & (c) disconnectShort terminals break before (c)Long terminals

2. Earthing connections are typical only

Notes1.

Figure 6 Application diagram: destabilising relay type MVTW 01

MBC

I

247

1 23 45 67 89 1011 1213 1415 1617 18

19 2021 2223 2425 2627 28

Module terminal blockviewed from rear

(a)

(b)(c)

CT shorting links makebefore (b) & (c) disconnectShort terminals break before (c)Long terminals

Earthing connections as shown is typical only1. 2.

Case earth

Pilot isolation transformer

Pilots

S2

P1MBCI

17

18

19

MVTW 03

17

RL11

Case earth See note 2

S1

P6

Trip send

Vx Supply fail12

RL1-1RL2-1

RL2-2

Supplyhealthy

Powersupply

RL22

181911+VE

13+VE

Notes

14–VE

Figure 8 Destabilising and intertripping relay type MVTW 03

A

B

C

S1 S2

P2I

II

III

i

ii

iiiyn

A

B

C

P2 P1

S2 S1

N.E.R.

MCTH MCTH

MBCI MBCI

Pilots

MFAC14

RVD3

232425262728

232425262728

232425262728

232425262728

17

18

17

1819

1819

17

1

3

27

28

17

19

17

19

Note1: It is essential that the current transformer connections are earthed at one point only.

11

MVT

W03

Figure 9 Typical application diagram: overall protection of transformer feeders

MBC

I

248

Figure 13

Minimum earth fault current foroperation with through load

%In

Through Load (xIn)

100

80

60

40

20

Earth Fault SettingKs = 2.0

N = 3 N = 6

0 0.5 1.0 1.5 2.0

%In

Through Load (xIn)

50

40

30

20

10

Earth Fault SettingKs = 1.0

N = 3 N = 6

0 0.5 1.0 1.5 2.0

60

70

A-N

C-NB-N

C-NB-N

A-N

C-N

A-N

B-N

C-NB-N

A-N

Figure 12 Effect of pilot capacitance and pilot isolating transformers on setting

Facto

r by w

hich

settin

g is

incre

ased

2.0

Pilot intercore capacitance -mF

Without 15 kV pilot isolating trans.With 15 kV pilot isolating trans. (Ks = 1.0 or 2.0)With 15 kV pilot isolating trans. (Ks = 0.5)(Note: Pilot isolating transformer ratio = 1:1)

1.8

1.6

1.4

1.2

1.00 1 2 3 4 5

Pilot

Volta

ge (V

Peak

)

200

160

120

80

40

0 1 0 20 30

A-N fault current (x In)

MetrosilLimiter

ZenerLimiter

Figure 10: Current circuit burden

15

10

5

0

Volta

ge a

cross

summ

atio

n cu

rrent

trans

form

er p

rimar

y – vo

lts 1

A re

lay

Current in summation current transformer primary – Amps

N = 6 A–N

N = 6 C–N

N = 3 A–N

N = 3 B–N

A–C

A–B

Volta

ge a

cross

summ

atio

n cu

rrent

trans

form

er p

rimar

y – vo

lts

3

2

1

210

1A relay5A relay

15

Figure 11

Pilot voltage characteristics

MBC

I

249

Figure 16

Response to spurious induced loop voltage in pilots

60

40

20

0 0.5 1.0 1.5 2.0

Setting multiplier – Ks

Induc

ed p

ilot lo

op vo

ltage

to o

pera

te

Figure 15 Time characteristics for internal faults

240

200

160

120

80

40

01 2 3 4 5 6 10 20 30 40 50 60 80 100

Kt = 6Kt = 14Kt = 20Kt = 40

Current (multiples of setting)

Ope

ratio

n tim

e (M

illise

cond

s)

Figure 14 Mesh type switchgear arrangements

CTs

CTs

RL RL RL

CTs

PilotsEnd ARelay

End BRelay

MFAC

MBCI

MCTHinrush

detector

23 25 27 17

24 26 28 19

REFA B C

232527

1.251

6242628

Feeder

MCTHinrush

detector

17 23 25 27

19 24 26 28

Delta StarN.E.R.

MBCI

232527

242628

1.251

6

17

18

17

18Pilots

Transformer (Dy 11)

A B C

Figure 17

Overall protection of transformer feedersshowing connections to MBCI relay

MBC

I

250

Technical Data (MBCI Relay)

Current rating (IIn)

1A, 2A or 5A

Frequency rating

50Hz or 60Hz

Current withstand ratings

Duration (s) DifferentialContinuous 2In3 45In2 55In1 80In 0.5 100In Table 2

Current circuit burden

Highest phase burden(with three phase rated current)6VA N = 6 3.5VA N = 3 At setting current 0.5VA

Auxiliary supply

Rated Operative Current drain (mA)voltage (Vx) range (V) Quiescent Operated24/27 19.2–32.4 30 17.530/34 24–37.5 15 17548/54 37.6–72 15 175110/125 87.5–150 15 90

Contacts

Contact arrangements2 make and 2 change-over (See Figure 2)

Contact ratings

• Make and carry for 0.2s 7500VAsubject to maxima of 30A and 300Vac or dc

• Carry continuously5A ac or dc

• Breakac 1250VA dc 50W resistive

25W inductive L/R = 0.045s

subject to maxima of 5A and 300V

Durability

• Loaded contact10,000 operations minimum

• Unloaded contact100,000 operations minimum

Reset time

Less than 100ms

Indication

A non-volatile LED trip indicator is used.If the auxiliary supply is lost the LED willreturn to the same state when the supplyis restored.

Stability level

The stability of the protection for throughfaults is greater than 50In

High voltage withstand

• Dielectric withstand

IEC 60255-5: 19772.0kV rms for 1 minute between allterminals and case earth.

2.0kV rms for 1 minute between allterminals of independent circuits, withterminals on each independent circuitconnected together.

5.0kV rms for 1 minute between pilotterminals and all other terminals andcase earth.

1.0kV rms for 1 minute across normallyopen contacts.

• High voltage impulse

IEC 60255-5: 1977Three positive and three negativeimpulses of 5.0kV peak, 1.2/50µs, 0.5Jbetween all terminals and all terminalsand case earth.

Electrical environment

• DC supply interruption

IEC 60255-11: 1979The unit will withstand a 10msinterruption in the auxiliary supply,under normal operating conditions,without de-energising.

• AC ripple on dc supply

IEC 60255-11: 1979The unit will withstand 12% ac ripple onthe dc supply.

• High frequency disturbance

IEC 60255-22-1: 1988 Class III2.5kV peak between independentcircuits and case.

1.0kV peak across terminals of the samecircuit.

• Fast transient disturbance

IEC 60255-22-4: 1992 Class IV4.0kV, 2.5kHz applied directly toauxiliary supply.

IEC 60801-4: 1988 Level 4 4.0kV,2.5kHz applied to all inputs.

• Surge immunity

IEC 61000-4-5: 1995 Level 32.0kV peak, 1.2/50ms between allgroups and case earth.

2.0kV peak, 1.2/5-ms betweenterminals of each group.

• EMC compliance

89/336/EECCompliance to the EuropeanCommission Directive on EMC is claimedvia the Technical Construction File route.

EN 50082-2: 1994EN 50082-2: 1995Generic Standards were used to establish conformity.

• Product safety

73/23/EECCompliance with the EuropeanCommission Low Voltage Directive.

EN 61010-1: 1993/A2: 1995EN 60950: 1992/A11: 1997Compliance is demonstrated by reference to generic safety standards.

Atmospheric environment

• Temperature

IEC 60255-6: 1988Storage and transit –25°C to +70°COperating –25°C to +55°C

IEC 60068-2-1: 1990 Cold

IEC 60068-2-2: 1974 Dry heat

MBC

I

251

• Humidity

IEC 60068-2-3: 196956 days at 93% RH and 40°C

• Enclosure protection

IEC 60529: 1989IP50 (dust protected)

• Mechanical environment

Vibration IEC 60255-21-1: 1988Response Class 1

Pilots

Pilots isolation

Pilot isolation transformers are requiredwhen any longitudinally induced voltagein the pilot circuit is likely to exceed 5kV:in effect this means when protectingfeeders operating at voltages in excessof 33kV, unless these are short in length.

The use of pilot isolation transformersalso extends the acceptable range ofpilots. This is achieved by the matchingratios available as shown in Table 4.

Pilots

Pilot voltage

The voltage applied across the pilotsvaries with fault current as shown inFigure 12. For normal through loadconditions the peak pilot voltage will bein the order of 50V rising to a maximumof: 200V for MBCI 01, 80V for MBCI 02under fault conditions.

When pilot isolation transformers areused this value of voltage is multiplied byKM .

Note: Types MBCI 01 and 02 are notcompatible. Relays should be ofthe same type at either end.

Pilot current

The pilot current is typically 30mA fornormal through load conditions andrises to a maximum of 300mA underthrough fault conditions.

Line charging current

In applications pertaining to cables, withor without in zone shunt reactors, andoverhead lines, it is necessary for themost sensitive fault setting to beincreased to:

• 1.1 times the steady state linecharging current for solidly earthedsystems

• 3.2 times the steady state linecharging current for resistance earthedsystems

• 1.9 times the steady state linecharging current for resistance earthedsystems with one relay per phase

In all cases, allowance should be madefor some system overvoltage. This requirement ensures stability duringexternal ground faults which will causethe three phase capacitance currents tobe unequal, resulting in an increasedoutput from the summation transformer.

The high frequency line in-rush currentscan be neglected because the setting ofthe relay automatically increases at highfrequencies by a factor:

Inrush frequency Rated system frequency

KM 0.8 1.0 1.2 1.5 2.5 Matching ratio

Loop resistance 800 1000 1200 1500 2500 Ω

Capacitance 6.25 5 4.2 3.3 2 µF

Terminals P1-P6 P1-P5 P1-P4 P1-P3 P1-P2

Where KM = (turns ratio)2 for respective tap of pilot isolation transformers.

When pilot isolation transformers are not used KM = 1.

The optimum value for KM is the nearest value Rp/1000 in Table 4, where Rp is themeasured pilot loop resistance.

There are two types of pilot isolation transformers: ZC0244-002 for schemes withoutpilot supervision: HN0068-001 for schemes with pilot supervision. The latter includesthe injection filter for the pilot supervision circuit.

The pilot padding resistor (Rpp) at each end should be set to:

1/2(1000 – Rp/KM)

Table 4

MBC

I

252

Overcurrent starter/checkelement settings

The maximum resetting value of theovercurrent elements is not less than 90%of the operating value. Thus, to ensurethat they will reset when the current isrestored to the full load level, the settingshould be at least 1.2 times themaximum anticipated through loadcurrent.

The setting of the earth fault elementsshould be at least 1.2 times anystanding zero sequence current and nothigher than 80% of the minimum earthfault current.

The value chosen as an assessment ofminimum earth fault current must beconservative, due allowance being madefor all aspects of minimum systemoperating conditions and faultimpedance; alternatively, a largertolerance below the nominal minimumfault current value is advisable.

Unit Protection of PlainFeeders

Fault settings for plain feeders

The input transformer has a summationratio of 1.25:1:N where N = 3 fornormal use. N = 6 is used where lowearth fault settings are needed. Theminimum operating current will thereforebe dependent on the phase or phasesinvolved in the fault. The minimum earthfault current (If) should be greater thantwice the least sensitive earth fault settingto ensure rapid fault clearance.

The range setting of fault settings isshown in Table 5.

Minimum operation for earthfaults with through load

Bias being a direct function of throughcurrent, increases minimum operatingcurrent with through load. Figure 14shows the minimum earth fault currentrequired for various levels of throughload.

The curves shown are for the first relay totrip. The second relay will trip sequentiallyprovided the resulting in-feed at that endis above the setting value. To ensure thatsimultaneous tripping will occur theminimum fault current should be greaterthan twice the minimum operating currentgiven in Figure 14.

Line current transformerrequirements for plain feeders

Class X BS 3938

• VK ≥ 0.5N Kt In (RCT + XRL)

Where

VK = knee point voltage of currenttransformers for through faultstability

RCT = resistance of current transformersecondary circuit (Ω)

RL = lead resistance of single leadfrom relay to currenttransformers (Ω)

X = 1 for 4 wire connectionsbetween the main currenttransformers and the relay: 2for 6 wire connections

N = relative neutral turns onsummation transformer winding(3 or 6, as shown underheading Current Settings)

Kt = the selected time-dependentconstant (40, 20, 14, 6 or 3)

*Note; For all applications at or above220kV where X/R ratios arelarge use:

VK ≥ N Kt In (RCT + XRL)

Differential Fault Settings(Summation ratio = 1.25/1/N) N=3 N=6

Ks is a setting multiplier A – N 0.19Ks.In 0.12Ks.In and may be varied B – N 0.25Ks.In 0.14Ks.In from 0.5-2.0 C – N 0.33Ks.In 0.17Ks.InIn is the rated relay current A – B 0.8Ks.In

B – C 1.0Ks.In C – A 0.44Ks.In A – B – C 0.5Ks.In

Table 5

The minimum operating current of therelay will be increased by any shuntimpedance connected across the pilotwires, for example, pilot capacitanceand the magnetising path of the pilotisolation transformers. The effect of thepilot capacitance is shown in Figure 12.

Values of Ks from 0.5 to 1.0 areprovided to achieve effective faultsetting equal to the nominal valueindicated in Table 5. This is achieved bysingle end injection tests duringcommissioning. Values of Ks from 1.0 to2.0 are used to increase the settingwhen the application demands. Refer tosection: Line charging current.

MBC

I

253

It is not necessary for the line currenttransformers at the two ends of theprotected system to have the same kneepoint voltage. Differences of up to 4:1can be tolerated provided both areabove the minimum value. However, thethree line current transformers at anyone end should have similar magnetisingcharacteristics.

• Magnetising current:

In order that the minimum effectiveoperating current of the relay remainslow, it is necessary to apply a limit to thevalue of the magnetising currentdemanded by the line currenttransformers:

IM ≤ 0.05 In at 10 VIn

• Mesh type switchgear arrangements

The relay may be fed by parallelconnected current transformers as shownin Figure 15. It is essential to balancethe lead resistance in the circulatingsecondary current path to ensurestability for a through busbar fault.Connecting the current transformers asshown in Figure 15 will result in therequired balance being obtained. It isessential that the current transformers atthe same end should have similarmagnetising characteristics. The value ofRCT to be used in calculations should bethe resistance of one current transformerplus the resistance of one lead betweenthe two parallel connected currenttransformers. The value of RL should bethe resistance of a single lead from thecommon connection of the currenttransformers to the relay.

• Methods of reducing the currenttransformer requirements:

In general the larger the currenttransformer the better the overallperformance. However, when currenttransformer size is critical the followingnotes should prove helpful.

• The operation time varies with faultcurrent as shown in Figure 16. Stabilityis maintained with smaller currenttransformers if the value of Kt is reduced.

This will of course result in the operationtime increasing, typical operation timesbeing:

Kt 40 20 14 6 3 Time at 5x setting 30 50 65 90 300 (ms)

• Kt= 20 will suit most currenttransformers on distribution systemsKt = 40 is the preferred setting for EHVsystems where high speed operation isrequired

• It should be noted that the knee pointrequirement increases with the nominalcurrent rating In. Advantage is thereforeobtained by using a low value of ratedcurrent eg. 1A or even 0.5A.

• Wires may be connected in parallel toreduce the lead resistant (RL) and hencethe current transformer requirements.

• If the relay is fed from delta connectedline current transformer then N = 1.

• If one relay is used per phase thenassume N = 1

Stabilising resistance

Rs = VK Ω but not greater than 12 Ω40In In2

Note: This resistor is not required forsingle phase protection or whenTranslay S is fed from deltaconnected current transformers.

Additional requirements:

• It is a stability requirement that therelays at both ends have the same valueof N & Kt selected.

• It is preferred, although not absolutelyessential, that the equipment at the twoline ends have the same rated current In.

Small in-zone teed loads

Small three phase loads may beconnected to the feeder within theprotected zone; normally these will besupplied by a delta/star transformerconnected to the line through HRC fuses.Substantial faults on this circuit will causethe fuses to blow very quickly before thedifferential relay can operate.

The limiting condition is a value of faultcurrent which will just produce an A–Cphase setting current in the relay: thiscurrent must cause the fuse to operatequickly enough to discriminate. Mostfusing time curves show pre-arcing timeand some allowance must be made for thearcing period.

To accommodate the largest teed load, usemay be made of the Ks (setting multiplier)adjustment, and/or selection of a value ofKt corresponding to a lower operatingspeed.

The particulars tabulated below for teedloads connected to a fused 11kV feedermay be helpful as a general example.

Feeder 11kV 300A rating

Tee Transformer Fuse Ks Ktrating(kVA) rating (A) 300 20 1 40400 25 1.7 40400 25 1.5 6500 30 1.75 3

Table 6

The table above refers to individual teedloads. When smaller loads are connectedat separate locations, on the basis thatonly one will be subject to a fault at anyinstant, the aggregate load may begreater. For example twelve 100kVAtransformers each protected by 10A fusescould be connected to the above line, withmain protection settings Ks = 1 and Kt =40. Similarly for ten 300kVA transformerseach protected by 20A fuses, relaysettings Ks = 2.0 and Kt = 20 would besuitable. In general the aggregate tee-offload should not exceed 0.25Ks x currenttransformer rating.

MBC

I

254

Maximum induced pilot loopvoltage

Ideally the pilot cores should be wormed(twisted together) so that the inducedloop voltage is kept to a minimum. Therequired level of this voltage to causeoperation varies with the settingmultiplier Ks as shown in Figure 17.

Unit Protection ofTransformer feeders

Fault Setting

The relay internal summation is identicalto that used for plain feeders but theturns ratio used is 2.25:6. This isconnected as shown in Figure 18 andwill result in secondary settings as givenin the table below:

Relay setting in amps = Ks x In constantin table below A to N 0.44B to N ∝C to N 0.17A to B 0.44B to C 0.17C to A 0.123 Phase 0.14

Table 7

Where Ks = setting multiplier which may be

adjusted between 0.5 and 2.0

In = relay rated current.

NB. The Figures quoted in this table arethose to be expected underconditions of secondary injectiontesting

Note 1: As shown in Figure 18 there is arestricted earth fault relay in theneutral of the star connected CTs onthe delta side of the powertransformer. This provides protectionagainst earth faults on the delta sideof the power transformer when theinfeed is into the delta. It will providesettings lower than any of the phaseto neutral settings given above.

Note 2: The MBCI relay, when used in thetransformer feed application, doesnot require a stabilising resistor.

Line Current TransformerRequirements for TransformerFeeders:

Operating times less than 80ms will beachieved and through fault stabilityassured provided the following CTrequirements are satisfied (Kt = 14):

For star connected CTs

Vk ≥ 50 In (2.2 + RCT + RL)In2

For delta connected CTs

Vk > 50In (9.7 + RCT + RL)√3 In2

If longer operating times can be toleratedCT requirements to the following formulaewill give operating times less than 160msand assured through fault stability (Kt =14):

For star connected CTs

Vk ≥ 35 In (2.2 + RCT + RL)In2

For delta connected CTs

Vk > 35In (9.7 + RCT + RL)√3 In2

Where Vk = kneepoint voltage (V) In = rated current of relay (A)

RCT = resistance of CT secondarywinding (Ω)

RL = resistance of a single lead fromthe CTs to the relay (Ω)

Note 1: Operating times are quoted at 5xrated current.

Note 2: The above equations for through faultstability are applicable for up to 20%CT mismatch.

Note 3: In normal applications, to ensure thefast operation of the MFAC, the kneepoint voltage Vk must be greater thantwice the voltage setting Vs of theMFAC relay. However, when usedwith the MBCI/MCTH relaycombination, lower knee pointvoltage, down to Vs, may be usedprovided operating times up to thescheme operating time of 80ms areaccepted.

Vs = If ( 3 + RCT + RL)In2

Where

Vs = setting of MFAC (V)

If = maximum through fault currentreferred to CT secondary forwhich stability is required (A-rms)

In = rated currency of relay (A) RCT= resistance of secondary

winding (Ω)

RL = resistance of a single lead fromthe CTs to the relay (Ω)

The effective primary operating circuit(Iop) of the MFAC 14 is given by:

Iop = n(IR + NIIu)

Where

IR = relay operating current andmetrosil current at settingvoltage (see MFAC publication)

Iu = current transformer magnetisingcurrent at setting voltage (A)

Nl = number of connected currenttransformers

n = current transformer turns ratio

The following notes on this applicationare also important:

• A setting of 14 is recommended for Ktto ensure sufficient time for inrushblocking. Tripping for internal faults willthen occur (typically) within 60 – 80 ms.

• The N = 6 setting on the MBCI relaymust be used to achieve increasedsensitivity.

• Where the CT lead resistance is apredominant part of the CT burden atone, or both, line ends then the use of1A line CTs is recommended. Theselected rating of current transformersmust be the same as the relays (MBCIand MCTH) which they supply.

MBC

I

255

• Additional conductors may beconnected in parallel in order to reducethe lead resistance (RL) and, hence, thecurrent transformer requirements.

• The pilot resistance should not exceed700Ω.

• With 15kV pilots, the MCTH outputcontacts should be connected on therelay side of the isolating transformer toterminate numbers 17 and 18 of theMBCI relay.

• The MCTH overcurrent settings foreach phase, set by 3 front-mountedpotentiometers (one per phase), shouldbe set at least 50% above the maximumpossible load current.

• The steady state magnetising currentmust not exceed the three phase faultsetting of the MCBI relay. For a Ks = 1setting, the three phase fault setting is14% of rated current. If the transformeris likely to be subjected to overfluxing,with the corresponding increase insteady state magnetising current, thenthe three phase setting must bepermanently set above this highermagnetising current by increasing Ks.

Cases

Relay type MBCI is provided in case size6 as shown in Figure 18.

Auxiliary Equipment

For outline drawings of pilot isolationtransformers, and stabilising resistor, seeFigures 20, 21, 22 and 23.

Associated Publications

R6001 Midos system

R6026 MRTP supervision for ac pilotcircuits

R6027 MVTW destabilising andintertripping relay

R6028 MCRI instantaneous overcurrentrelay

R6004 MMLG/B test block

R6006 MSTZ power supply

R6007 MFAC high impedancedifferential relay

R6066 MCTH transformer inrush currentdetector relay

Information Required withOrder

Basic scheme reference (refer to Table 1)

Type(s) of relay

Type of pilots (private or telephone)

Pilot loop resistance and intercorecapacitance values.(This information isrequired to determine whether pilotisolating transformers are required formatching purposes.)

Pilot insulation level (5kV or 15kV). Is pilotsupervision equipment required?

Is the overcurrent relay required?

Is the destabilising facility ordestabilising/intertrip facility required?

Pilot voltage: Metrosil (MBCI 01) or Zenerlimiting (MBCI 02).See Figure 11.

Current rating

Frequency rating

Auxiliary dc supply rating

Auxiliary ac supervision supply rating

AC intertrip supply rating

Push buttonprojection 10 max

All dimensions in mm

Panel cut-out:Flush mounting fixing details

4 holesø 4.4

Flush mounting

149

155

21232 25 min.

11

Reset

157 max.

151

159168

103.624

177

Figure 18

Case outline size 6

MBC

I

256

116 mm

64.5 mm

9 mm

4 Fixing holes, M6 clearance

30 mm

171.5 mm

244 mm

190 mm

45 mm

8-M6 Terminals

176 mm

P1 2.5 1.51.2 1.0 0.9 S1 S2

X1 X2

InjectionsInputs

172 mm

340 mm

6 fixing holes, M6 clearance

64.5 mm

154 mm

9 mm116 mm176 mm

30 mm

Figure 20

Pilot isolation transformer without filter.With insulation for 15kV

Figure 19 Pilot isolation transformer with filter.With insulation for 15kV

MBC

I

257

48 mm

48mm

2BA connectionscrews

354 mm

310 mm

342 mm

2 holes 6.5 mm

S2

S2

69 mm

98 mm

19 mm

2 off M5 studs

2 off fixing holes M5

134 mm

2 off M5 studs

19 mm

52 mm50 mm

69 mm

8.5 mm

52 mm

Figure 21 Pilot supervision isolation transformer.With insulation for 15kV

Figure 22 Stabilising resistor