EEE. 2mark and 16 Marks Quest-with Answer

8/17/2019 EEE. 2mark and 16 Marks Quest-with Answer

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UNIT- I

UNIT I INTRODUCTIONImportance of protective schemes for electrical apparatus and power system. Qualitativereview offaults and fault currents - relay terminolo y ! definitions - and essential "ualities of

protection.#rotection a ainst over volta es due to li htnin and switchin - arcin rounds -#eterson Coil - round wires - sur e a$sor$er and diverters#ower %ystem earthin ! neutral&arthin - $asic ideas of insulation coordination

PART- A

1. What are symmetrical components?

'n un$alanced system of N ! related vectors can $e resolved into Nsystems of $alanced vectors. The N sets of $alanced vectors are calledsymmetrical components.

2.Write the symmetrical components of three phase system.

(. #ositive se"uence components.). Ne ative se"uence components.*. +ero se"uence components.

3.What are positive sequence components?

The positive se"uence components of a *#hase un$alanced vectors consistsof three vectors of e"ual ma nitude displaced from each other $y (), , in phaseand havin the phase se"uence as the ori inal vectors.

4.What are negative sequence components?

The ne ative se"uence components of a *#hase un$alanced vectors

consists of three vectors of e"ual ma nitude displaced from each other $y (),,

in phase and havin the phase se"uence opposite to that of the ori inal vectors.


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5. What are zero sequence components?

The ero se"uence components of a *#hase un$alanced vectors consists of *-vectors of e"ual ma nitude with ero phase displacement from each other.

6.E press the symmetrical components ! a" #! a1#! a2 in terms of un$alance%vectors ! a# ! $# ! c

,

)(

))

(/ 0

*(

/ 0*

( / 0*

a a b c

a a b c

a a b c

V V V V

V V aV a V

V V a V aV

= + +

= + +

= + +

&. E press the un$alance% voltages ! a ! $ ! c in terms of symmetricalcomponents ! a1#! a2#! a".

, ( )

), ( )

), ( )

a a a a

b a a a

c a a a

V V V V

V V a V aV V V aV a V

= + +

= + += + +

'. What is arcing groun%?

Durin a line to round fault on an un rounded system till the fault iscleared1 there will $e intermitted dischar e to round throu h the capacitances

$etween the healthy phases and round this phenomenon is called arcinround.

(.What are sequence net)or*s?

The sin le phase e"uivalent circuit of a power system consistin of impedances to current of anyone se"uence only is called se"uence networ2.


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1".What is meant $y positive# negative + zero sequence reactance%iagram?

The reactance dia ram of a power system1 when formed usin positive1ne ative or ero se"uence reactance3s are called positive1 ne ative erose"uence reactance dia ram respectively.

11. What is meant $y positive# negative + zero sequence impe%ances?The impedance of a circuit element for positive1 ne ative of ero se"uence

component currents are called positive1 ne ative ero se"uence impedancesrespectively.

12. What are appro imations# in short circuit stu%ies?a. Representin each machine $y a constant volta e source $ehind proper

reactance which may $e 4 d(( 1 4d( 1 or 4 d. $. %ettin all the transformers to nominal taps. %ince we wor2 in per unit

system1 with this representation1 transformers will automatically $e out of circuit.

c. &"uatin the positive se"uence impedance e"ual to the ne ative se"uence

impedance also for * rotatin elements1 even thou h these se"uenceimpedances are e"ual only in the case of * stationary elements.

13. What are the unsymmetrical fault occur in po)er system?,-. .2""2/a. %in le line to round fault /560.

$. Dou$le line to round fault /5560.c. 5ine to line fault /550.


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14. 0tate the nee% for short circuit stu%y?

It helps in determinin the correct current ratin of C7 as well as the associatedcurrent transformers.

'nother purpose is radin /allocatin 0 the operatin time of various circuit $rea2ers.

15. Why faults occur in a po)er system?

The faults occur in a power system due to insulation failure of e"uipments1 flash over of lines initiated $y a li htnin stro2e.

16. What is symmetrical an% unsymmetrical faults?

If the fault current is e"ual in all phases then it is called symmetricalfault. If the fault current is not e"ual in all phases then it is called unsymmetricalfault.

1&. ive an application of su$transient reactance?

%u$ transient reactance of motors and enerators are used to determinethe initial current flowin on the occurrence of the short circuits.

1'. 0tate the $oun%ary con%ition in single line to groun% fault

/i0 8 a 9 I a+ f /ii0 I $ 9 I c 9 ,

1(. What is neutral groun%ing?


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The process of connectin neutral point of *-phase system to earth /i.e.soil0 either directly or throu h source circuit element /e. . resistance1 reactanceetc.0 is called neutral roundin .

2". What are the a%vantages of eutral roun%ing?

The followin are the advanta es of neutral roundin :

i0 8olta es of the healthy phases do not e;ceed line to round volta ei.e. they remain nearly constant.

ii0 The hi h volta es due to arcin rounds are eliminated.iii0 The protective relays can $e used to provide protection a ainst earth

faults. In case earth fault occurs on any line1 the protective relay willoperate to isolate the faulty line.

iv0 The over volta es due to li htnin are dischar ed to earth.v0 It provides reater safety to personnel and e"uipment.vi0 It provides improved service relia$ility.vii0 Operatin and maintenance e;penditures are reduce.

21The methods commonly used for roundin the neutral point of a *-phase

systems are:i0 %olid or effective roundinii0 Resistance roundiniii0 Reactance roundiniv0 #eterson-coil roundin


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8 $ and 8 c respectively with phase se"uence a$c.

8 a, 8 $, and 8 c, 9 +ero se"uence components of 8 a8 $ 8 c respectively.

The positive ne ative and ero se"uence components are shown in fi .


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&ach of the ori inal un$alanced vector is the sum of its positive1 ne ativeand ero se"uence component. Therefore the ori inal un$alanced three phasevolta e vectors can $e e;pressed in terms of their components as shown $elow.

8 a 98 a( H 8 $( H 8 c( /(.FJ0

8 $ 98 $, H 8 $( H 8 $) /(.FF0

8 c 98 c, H 8 c( H 8 c) /(.FK0

8arious symmetrical components.

)( ( ) )

, , ( ( ) )

: : ......../(.K,0

: : ......../(.K(0bo bo b b b b

c c c c c c

V V V a V V V

V V V aV V V

= = == = =

Usin e"uations /(.K,0 and /(.K(01 the e"uations /(.FJ0 to /(.FK0 can $e writtenas shown $elow.

8 a 98 a, H 8 a( H 8 a) /(.K)0

8 $ 98 $, H a ) 8 $( H a8 $) /(.K*0

8 c 98 c, H a8 c( H a ) 8 c) /(.KA0

The e"uations /(.K)0 to /(.KA0 can $e arran ed in the matri; form as shown ine"u/(.KE0.

,)

(

))

( ( (

( a a ......./(.KE0

( a a

a a

b a

c a

V V

V V

V V

=

The e"uation /(.KE0 can $e used to compute the un$alanced vectors from theLnowled e of symmetrical components.

======================================

==========================

3 .Write short notes on symmetrical components of un$alance% currentvectors.


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The symmetrical components of un$alanced current vectors can $eo$tained $y an analysis similar to that of volta e vectors. 'll the e"uationsdeveloped for volta e can $e used for current if we replace 8 $y I.

5et1

Ia I $ Ic 9 Un$alanced current vectors with phase se"uence a$c.

Ia( I $( Ic( 9 positive se"uence components of I a I $ Ic respectively with phasese"uence a$c.

Ia) I $) Ic) 9 Ne ative se"uence components of I a I $and I c respectively with pulsese"uence ac$.

Ia* I $* Ic*9+ero se"uence components of I a I $and I c respectively.

The followin e"uations are used to compute the un$alanced current vectorsfrom the 2nowled e of their symmetrical components ?Refer e"uations /(.K)0 to/(.KE0@.

, ( )

), ( )

), ( )

..../(.(,G0

...../(.(,J0

...../(.(,F0

a a a a

b a a a

c a a a

I I I I

I I a I aI

I I aI a I

= + +

= + +

= + +

,)

(

))

( ( ((

( a a ......./(.(,K0*

( a a

a a

b a

c a

I I

I I

I I

=

The followin e"uations are used to compute the symmetrical components of un$alanced current vectors ?Refer e"uations /(.(,)0to /(.(,E0@.

[ ],

)(

))

( ......./(.((,0*(

....../(.(((0*(

......../(.(()0*

a a b c

a a b c

a a b c

I I I I

I I aI a I

I I a I aI

= + +

= + +

= + +


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,)

(

))

( ( ((

( a a ......./(.((*0*

( a a

a a

a b

a c

I I

I I

I I

=

======================================

=========================

4 . he symmetrical components of phase7a fault current in a 37phase

un$alance% system are ; a" < 35" (" " - ;a 1

The current I a1I $ and I c are iven $y the followin matri; e"uations.

,)

(

))

( ( (

( a a

( a a

a a

b a

c a

I I

I I

I I

=

, ( )

), ( )

), ( )

a a a a

b a a a

c a a a

I I I I

I I a I aI

I I aI a I

∴ = + +

= + +

= + +

6iven that

,,

,(

,)

*E, K, , *E,G,, K, , G,,

)E, K, , )E,

a

a

a

I j I j

I j

= < = += < − = +

= < = +

, , ,(

) , , ,(

, , ,)

) , , ,

)

( (), G,, K, G,, *, E(K.G) *,,

( )A, G,, K, G,, (E, E(K.G) *,,

( (), )E, K, )E, )(, )(G.E( ()E

( )A, )E, K, )E, **, )(G.E( ()E

a

a

a

a

aI j

a I j

aI j

a I j

∴ = < × < − = < = +

= < × < − = < = − +

= < × < = < = − +

= < × < = < = − +


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, ( )

), ( )

,

), ( )

*E, G,, )E, ,

*E, E(K.G) *,, )(G.E( ()E

9-J*G.(*H>E)E9K,A.(GM(AE

*E, E(K.G) *,, )(G.E( ()E

a a a a

b a a a

b a a a

I I I I j j j

I I a I aI j j j j

A

I I aI a I j j j j

= + + = − + =

= + + = − + − −

= + + = − + − −, 9-J*G.(*H>E)E9K,A.(GM*E A


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capacitive currents flowin in the healthy phase R and P are I R and I Prespectively. The resultant capacitive current I C is the phasor sum of I R and I y. In addition to these capacitive currents1 the power source alsosupplies the fault current I The followin are the disadvanta es of solid roundin :

i0 %ince most of the faults on an overhead system are phase to earthfaults1 the system has to $ear a lar e num$er of severe shoc2s. Thiscauses the system to $ecome unsta$le.

ii0 The solid roundin results in heavy earth fault currents. %ince thefault has to $e cleared $y the circuit $rea2ers1 the heavy earth faultcurrents may cause the $urnin of circuit $rea2er contacts.

iii0 The increased earth fault current results in reater interference in thenei h$orin communication lines.

-pplications.

%olid roundin is usually employed where the circuit impedance issufficiently hi h so as to 2eep the earth fault current within safe limit. Thissystem of roundin is used for volta es upto ** 28 with total power capacity

not e;ceedin E,,, 28'.

9iscuss a$out esistance roun%ing?

In order to limit the ma nitude of earth fault current1 it is a common practice to connect the neutral point of a *-phase system to earth throu h aresistor. This is called resistance roundin .

hen the neutral point of a *-phase system /e. . *-phase enerator1 *- phase transformer etc.0 is connected to earth /i.e. soil0 throu h a resistor1 it iscalled resistance roundin .


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ustin the value of R1 the arcin rounds can $e minimised.%uppose earth fault occurs in phase 7 as shown in


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ii0 The earth fault current is small due to the presence for earthinresistance. Therefore1 interference with communication circuits isreduced.

iii0 It improves the sta$ility of the system.

9isa%vantages.

The followin are the disadvanta es of resistance roundin :

i0 %ince the system neutral is displaced durin earth faults1 thee"uipment has to $e insulated for hi her volta es.

ii0 This system is costlier than the solidly rounded system.iii0 ' lar e amount of ener y is produced in the earthin resistance durin

earth faults. %ome times it $ecomes difficult to dissipate this ener yto atmosphere.1

-pplications.

It is used on a system operatin at volta es $etween ).) 28 and ** 28with power source capacity more than E,,, 28'.

99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999

6. Write a $rief note on -rc 0uppression :oil roun%ing ,or esonantroun%ing/?

e have seen that capacitive currents are responsi$le for producin arcinrounds. These capacitive currents flow $ecause capacitance e;ists $etween

each line and earth. If inductance 5 of appropriate value is connected in parallelwith the capacitance of the system1 the fault current I < flowin throu h 5 will $ein phase opposition to the capacitive current I C of the system. If 5 is so ad>usted

that I 5 9 I C1 then resultant current in the fault will $e ero. This condition is2nown as resonant roundin .

hen the value of 5 of arc suppression coil is such that the fault currentI< e;actly $alances the capacitive current I O it is called resonant roundin .

Circuit details. 'n arc suppression coil /also called #eterson coil0 is aniron-cored coil connected $etween the neutral and earth as shown in $elow


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The reactor is provided with tappin s to chan e the inductance of the coil. 7yad>ustin the tappin s on the coil1 the coil can $e tuned with the capacitance of the system i.e. resonant roundin can $e achieved.


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current in the fault can $e reduced. If inductance of the coil is so ad>usted thatI5 9 I C1 than resultant current in the fault will $e ero.

!alue of @ for resonant groun%ing.


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&;p. /i0 ives the value of inductance 5 of the arc suppression coil for resonantroundin .

-%vantages> The #eterson coil roundin has the followin advanta es:

i0 The #eterson coil is completely effective in preventin and dama e $yan archin round.

ii0 The #eterson coil has the advanta es of un rounded neutral system.

9isa%vantages> The #eterson coil roundin has the followindisadvanta es:

i0 Due to varyin operational conditions1 the capacitance of the networ2

chan es form time to time. Therefore1 inductance 5 of #eterson coilre"uires read>ustment.

ii0 The lines should $e transposed.


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' well desi ned and efficient relay system should $e selective i.e itshould $e a$le to detect the point at which the fault occurs and cause theopenin of the circuit $rea2ers closest to the fault with minimum or no dama eto the system.

In order to provide selectivity to the system1 it is usual practice to dividethe entire system into several protection ones. hen a fault occurs in a iven

one1 then only the circuit $rea2ers within the one will $e opened. This willisolate only the faulty circuit or apparatus1 leavin the healthy circuits intact.The system can $e divided into the followin protection ones:

/a0 enerators

/$0 low-tension switch ear

/c0 transformers

/d0 hi h-tension switch ear

/e0 transmission lines

There is certain amount of overlap $etween the ad>acent protection onesin transmission lines. acent onesoverlap1 more $rea2ers will $e opened than the minimum necessary todisconnect that faulty section. 7ut if there were no overlap1 a failure in thereli ion $etween ones would not lie in either re ion and1 therefore1 no $rea2er would $e opened. acent ones.

/ii0 0pee%> The relay system should disconnect the faulty section as fast as possi$le for the followin reasons:

/a0 &lectrical apparatus may $e dama ed if they are made to carry the faultcurrents for a lon time

/$0 ' failure on the system leads to a reat reduction in the system volta e. If the faulty section is not disconnected "uic2ly1 then the low volta ecreated $y the fault may shut down consumers motors and enerators onthe system may $ecome unsta$le.


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/c0 The hi h speed relay system decreases the possi$ility of development of one type of fault into the other more severe type.

,iii/ 0ensitivity> It is the ability of the relay system to operate with low valueof actuating quantity.

%ensitivity of a relay is a function of the volt-ampere input to the coil of the relay necessary to cause its operation. The smaller the volt-ampere inputre"uired to cause relay operation1 the more sensitive is the relay. Thus1 a (8'relay is more sensitive than a *8' relay. It is desira$le that relay system should

$e sensitive so that it operates with low values of volt-amp here input.

,iv/ elia$ility>

It is a$ility of the relay system to operate under the pre-determinedconditions. ithout relia$ility1 the protection would $e rendered lar ely

ineffective and could even $ecome a lia$ility.

,v/ 0implicity>

The relayin system should $e simple so that it can $e easily maintained.Relia$ility is closely related to simplicity. The simpler the protection scheme1

the reater will $e its relia$ility.

,vi/ Economy>

The most important factor in the choice of a particular protection scheme isthe economic aspect. %ometimes it is economically un>ustified to used an idealscheme of protection and a compromise method has to $e adopted. 's a rule1 the

protective ear should not cost more than E of total cost. =owever1 when theapparatus to $e protected is of utmost importance /e. . enerator1 main

transmission line etc01 economic considerations are often su$ordinated torelia$ility


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EEE. 2mark and 16 Marks Quest-with Answer