Electrical Protection

1. A three phase, 33 kV oil circuit breaker is rated 1200 A, the symmetrical breaking capacity is 2000 MVA,

3 secs. Then the making capacity is

1. 5000 MVA.

2. 5100 MVA.

3. 5200 MVA.

4. 5500 MVA.

Making capacity = 2.55 × breaking capacity (for all circuit breakers) = 2.55 × 2000 = 5100 MVA.

2. Current chopping mainly occurs in

1. air blast circuit breaker.

2. vacuum circuit breaker.

3. SF6 circuit breaker.

4. oil circuit breaker.

Air blast circuit breakers retain the same distinguish power irrespective of the magnitude of the current to

be interrupted. When breaking low current with such breakers, powerful de-ionising effect of air blast

causes. The current to fall abruptly to zero before the natural current zero is reached. This phenomenon is

known as current chopping and result in the production of high voltage transient across the contact of the

circuit breaker.

3. Where air circuit breakers are used?

1. Up to 11 kV.

2. Up to 12 kV.

3. Up to 33 kV.

4. Up to 66 kV.

Air circuit breaker are used in DC circuits and AC circuits up to 12 kV. AC air circuit breaker are widely

used in indoor type medium and low voltage switch gear.

4. On what factor does the performance of a circuit breaker depend?

1. Breaking speed.

2. Both A and C.

3. Critical length of breaking.

4. None of these.

Circuit breaker performance depends upon breaking speed, critical length of breaking and also some other

factors.
5. Keeping in view the cost and overall effectiveness, the following circuit breaker is suited for capacitor

bank switching

1. vacuum.

2. oil.

3. air blast.

4. SF6.

The arc time constant is the least (a few microsec) in vacuum CB as compared to other breaker types. The

rapid building up of dielectric strength after final arc extinction ( 20 kV / microseconds ) is unique feature

of VCBs. These arc, therefore ideally suited for capacitor switching.

6. In a short circuit test circuit breaker, time to reach the peak re-striking voltage 60 microsecond & the peak

re-striking voltage 120 kV. Determine average RRRV (Rate of rise re-striking voltage)?

1. 3 × 106 kV / sec.

2. 1 × 106 kV / sec.

3. 2 × 106 kV / sec.

4. 3.5 × 106 kV / sec.

7. A CT is connected in ___________________ with the line.

1. series.

2. across.

3. not connected.

4. both A and B.

A CT is connected in series with the line. PT is connected across the line. In CT number of turns is

inversely proportional to current and in PT number of turns is inversely proportional to voltage.

8. In relay coil which is used?

1. Current transformer.

2. Potential transformer.

3. Power transformer.

4. Instrument transformer.

CT used in the relay coil for reducing heavy current of a power circuit to a suitable value of operate the

relay.

9. The pickup value of relay is 6.5 Amp and fault current in relay coil is 39 Amp. Its plug setting multiplier is

1. 6 A.

2. 7 A.

3. 8 A.
 4. 10 A.

10. Buchholz relay is used for protection of

1. transformer.

2. alternator.

3. bus bar.

4. none of these.

Buchholz relay is a gas actuated relay used in oil immersed transformer for protection of all kinds of

internal faults of transformer.

11. Mho relay is used for protection of

1. short lines.

2. long lines.

3. medium lines.

4. none of these.

Mho relay is used for protection of long distance transmission lines.

12. What will be the ratio of the C.T. on the H.V side of a 3 - Φ star / delta 33 / 11 kV transformer if the

protecting CT on the LV side has a ratio of 300 : 5 ?

1. 200 : 5 / √3.

2. 100 : 5 / √3.

3. 150 : 4 / √3.

4. 120 : 6 / √3.

If 300 amps is the line current on the LV side the CT secondary current is 5 Amp. The line current on HV

(star) side = 300 × 11 / 33 = 100 Amps. The CTs on the star side are delta connected and the current

required on the relay side of the CT is 5 Amps. Therefore the current in the CT secondary (phase current)

is 5 / √3. Therefore the CT ratio on the H.T side will be 100 : 5 / √3.

13. The shape of the disc of an induction disc relay is

1. circular.

2. elliptical.

3. spiral.

4. none of these.

The shape of the disc of an induction relay is spiral.

14. If the time of operation of a relay for unity TMS is 10 sec. the time of operation of 0.5 TMS will be

_______________ secs.

1. 3.
 2. 5.

3. 7.

4. 20.

Time of operation = Unity TMS × TMS = 10 × 0.5 = 5 secs.

15. The rate of rise of restricting voltage depends upon the

1. Inductance and capacitance of the system.

2. Capacitance of the system.

3. Inductance of the system.

4. Type of CB.

The restricting voltage depends upon the power factor and factors like inductance and capacitance and it is

expressed as

16. In an induction relay for a particular current setting the operating time 4 sec. When the time multiplier

setting is 0.8 if the time multiplier setting is rest to 1 the operating time is

1. 1.

2. 5.

3. 10.

4. un changed.

Operating time of the relay = current setting × time multiplier = 4 × 1.0 = 4 secs.

17. In a biased differential relay the bias is defined as the ratio of

1. fault current and operating coil current.

2. operating coil current and restraining coil current.

3. number of turns of restraining and operating coil.

4. fault current and restraining coil current.

The relay operate when magnitude of the difference in the currents at two ends of a protected element

exceeds a certain percentage Nr / No of half the magnitude of their sum.Hence the name is percentage

differential relay. Where No = number of turns of the operating coil and N r = number of the restraining

coil.

18. What type of back up protection is used for alternators?

1. IDMT relay.

2. Buchholz relay.

3. Mho relay.

4. Impedance relay.
 IDMT stands for inverse definite minimum time lag (IDMT) relay. IDMT relay is used for over current

and earth fault protection of alternators.

19. Main bus bar protection scheme is /are

1. differential protection.

2. both fault bus and differential protection.

3. fault bus protection.

4. none of these.

Differential protection is most sensible and reliable method and fault bus protection gives earth fault

protection of bus bar. Busbars in the generating stations and sub-stations form important link between the

incoming and outgoing circuits. If a fault-current on a busbar, considerable damage and disruption of

supply will occur unless some form of quick-acting automatic protection is provided to isolate the faulty

busbar. The standard of construction for busbars has been very high, with the result that bus-faults are

extremely rare. However, the possibility of damage and service interruption from even a rare bus fault is

so great that more attention is now given to this form of protection. Improved relaying methods have been

developed, reducing the possibility of incorrect operation.

20. Merze price current scheme protection is used in

1. transformer.

2. alternator.

3. both A and B.

4. bus bars.

Merze price current scheme protection is a differential protection used in transformer and alternators.