MCQs on Power Systems

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01․ A three phase, 33 kV oil circuit breaker is rated 1200 A, 2000 MVA, 3 secs. The symmetrical breaking current is
35 kA.
45 kA.
50 kA.
55 kA.


02․ 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
5000 MVA.
5100 MVA.
5200 MVA.
5500 MVA.

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

03․ Current chopping mainly occurs in
air blast circuit breaker.
vacuum circuit breaker.
SF6 circuit breaker.
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.

04․ Where air circuit breakers are used?
Up to 11 kV.
Up to 12 kV.
Up to 33 kV.
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.

05․ On what factor does the performance of a circuit breaker depend?
Breaking speed.
Both A and C.
Critical length of breaking.
None of these.

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

06․ Keeping in view the cost and overall effectiveness, the following circuit breaker is suited for capacitor bank switching
air blast.

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.

07․ 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)?
3 × 106 kV / sec.
1 × 106 kV / sec.
2 × 106 kV / sec.
3.5 × 106 kV / sec.


08․ A CT is connected in ___________________ with the line.
not connected.
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.

09․ In relay coil which is used?
Current transformer.
Potential transformer.
Power transformer.
Instrument transformer.

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

10․ The pickup value of relay is 6.5 Amp and fault current in relay coil is 39 Amp. Its plug setting multiplier is
6 A.
7 A.
8 A.
10 A.