Power Systems Important Questions
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Power Systems Important Questions
1. The most common type of fault is
a) Single-phase to ground.
b) Phase to phase
c) Two-phase-to-ground.
d) Three-phase to ground.
Ans: a
2. The maximum short-circuit current occurs in the case of
a) a three-phase bolted fault.
b) a double line-to ground fault.
c) a line-to-line fault.
d) a single line-to-ground fault.
Ans: a
3. The value of the expression 1+a+a2 is
a) 0.
b) 1.
c) -1.
d) 2.
Ans: a
4. Negative-sequences reactance of a transformer is
a) Equal to the positive-sequence reactance.
b) Larger than the positive-sequence reactance.
c) Smaller than positive-sequence reactance.
Ans: a
5. Zero-sequence currents flow in a transmission line when there is
a) Double line to ground fault.
b) An overvoltage in the line caused by charged loads.
c) A line to line fault.
d) A fault across all the three lines.
Ans: a
6. The Zero-sequence fault currents are absent when the fault is
a) Single line to ground.
b) Line to line
c) Double line to ground.
Ans: b
7. A fuse in a motor circuit provides protection against
a) Short-circuit.
b) Over-load
c) Open circuit.
d) Short- circuit and over-load.
Ans: a
8. Fuse material must have
a) High melting point and high specific resistance.
b) Low melting point and high specific resistance.
c) Low melting point and low specific resistance.
d) High melting point and low specific resistance.
Ans: a
9. Fuses normally protect a system against
a) Over-circuit.
b) Under-current.
c) Resonance
d) Lightning surge.
Ans: a
10. HRC fuses provide best protection against
a) Radiation
b) Convection.
c) Conduction.
d) All of these.
Ans: a
11. When a fault occurs in a high voltage transmission line, first the
a) Circuit breaker operates then the relay.
b) Relay operates, then successively the isolator and the circuit breaker
c) Relay operates then the circuit breaker.
d) Isolator operates, then successively the relay and the circuit breaker.
Ans: b
12. The initiation of electric arc at the instant of contact separation is caused by
a) Thermionic emission of electrons.
b) Field emission of electrons.
c) (a) and (b) both
d) None of the above
Ans: c
13. In a CB contact wipe is necessary
a) To decrease the speed of opening of the CB.
b) To increase the speed of opening and reduce the arcing of contacts.
c) To increase the dielectric strength across the CB contacts.
d) None of the above.
Ans: c
14. The arc voltage in a circuit breaker is
a) In the phase with the arc current
b) Lagging the arc current by 900
c) Leading the arc current by 900
d) Lagging the arc current by 1800
Ans: a
15. The maximum value of restriking voltage in a circuit breaker in terms of system voltage will always be
a) 2 times the normal peak.
b) 2.5 times the normal peak.
c) Two times the voltage available at the time of current zero.
d) 1.5 times the normal peak.
Ans: a
16. It is difficult to interrupt a capacitive circuit because
a) The current has a leading power factor.
b) The restriking voltage can be high.
c) Current magnitude is very small.
d) Stored energy in the capacitor is very high.
Ans: b
17. Main purpose of oil in OCB is to
a) Provide insulation.
b) Provide cooling of contacts.
c) Quenching arc.
d) None of these.
Ans: c
18. Arc interruption in an oil circuit breaker takes place when
a) Contacts apart.
b) Voltage goes through zero.
c) Current goes through zero.
Ans: c
19. In a minimum oil circuit breaker the oil is used
a) To act as circuit breaking medium only.
b) For circuit breaking and providing insulation
c) For providing insulation only.
d) For none of the above purposes.
Ans: a
20. Minimum oil circuit breakers use oil for
a) Arc quenching only.
b) Insulation only.
c) Cooling only.
d) Lubrication only.
Ans: a
21. The air-blast circuit breaker is usually employed for... duty.
a) Instantaneous
b) Intermittent
c) Repeated
d) Short
Ans: c
22. The air-blast circuit breakers for 400 kV system are designed to operate in
a) 0.1 s.
b) 0.5 s
c) 50 ms.
d) 100 µs
Ans: c
23. In air-blast circuit breaker resistance switching is used to
a) Reduce the magnitude of fault current.
b) Control the CB operating time
c) Damp out the fast transient.
d) Change the fault current power factor.
Ans: c
24. The single most important property that makes SF6 a very efficient medium for circuit breaking is
a) It is non-toxic and non-inflammable.
b) It has dielectric constant
c) It has high breakdown strength
d) It is highly electro-negative in character.
Ans: d
25. For long EHV/UHV transmission lines, we normally use
a) Air circuit breaker
b) Air-blast circuit breaker
c) Oil circuit breaker
d) Isolating switch
Ans: b
1. The most common type of fault is
a) Single-phase to ground.
b) Phase to phase
c) Two-phase-to-ground.
d) Three-phase to ground.
Ans: a
2. The maximum short-circuit current occurs in the case of
a) a three-phase bolted fault.
b) a double line-to ground fault.
c) a line-to-line fault.
d) a single line-to-ground fault.
Ans: a
3. The value of the expression 1+a+a2 is
a) 0.
b) 1.
c) -1.
d) 2.
Ans: a
4. Negative-sequences reactance of a transformer is
a) Equal to the positive-sequence reactance.
b) Larger than the positive-sequence reactance.
c) Smaller than positive-sequence reactance.
Ans: a
5. Zero-sequence currents flow in a transmission line when there is
a) Double line to ground fault.
b) An overvoltage in the line caused by charged loads.
c) A line to line fault.
d) A fault across all the three lines.
Ans: a
6. The Zero-sequence fault currents are absent when the fault is
a) Single line to ground.
b) Line to line
c) Double line to ground.
Ans: b
7. A fuse in a motor circuit provides protection against
a) Short-circuit.
b) Over-load
c) Open circuit.
d) Short- circuit and over-load.
Ans: a
8. Fuse material must have
a) High melting point and high specific resistance.
b) Low melting point and high specific resistance.
c) Low melting point and low specific resistance.
d) High melting point and low specific resistance.
Ans: a
9. Fuses normally protect a system against
a) Over-circuit.
b) Under-current.
c) Resonance
d) Lightning surge.
Ans: a
10. HRC fuses provide best protection against
a) Radiation
b) Convection.
c) Conduction.
d) All of these.
Ans: a
11. When a fault occurs in a high voltage transmission line, first the
a) Circuit breaker operates then the relay.
b) Relay operates, then successively the isolator and the circuit breaker
c) Relay operates then the circuit breaker.
d) Isolator operates, then successively the relay and the circuit breaker.
Ans: b
12. The initiation of electric arc at the instant of contact separation is caused by
a) Thermionic emission of electrons.
b) Field emission of electrons.
c) (a) and (b) both
d) None of the above
Ans: c
13. In a CB contact wipe is necessary
a) To decrease the speed of opening of the CB.
b) To increase the speed of opening and reduce the arcing of contacts.
c) To increase the dielectric strength across the CB contacts.
d) None of the above.
Ans: c
14. The arc voltage in a circuit breaker is
a) In the phase with the arc current
b) Lagging the arc current by 900
c) Leading the arc current by 900
d) Lagging the arc current by 1800
Ans: a
15. The maximum value of restriking voltage in a circuit breaker in terms of system voltage will always be
a) 2 times the normal peak.
b) 2.5 times the normal peak.
c) Two times the voltage available at the time of current zero.
d) 1.5 times the normal peak.
Ans: a
16. It is difficult to interrupt a capacitive circuit because
a) The current has a leading power factor.
b) The restriking voltage can be high.
c) Current magnitude is very small.
d) Stored energy in the capacitor is very high.
Ans: b
17. Main purpose of oil in OCB is to
a) Provide insulation.
b) Provide cooling of contacts.
c) Quenching arc.
d) None of these.
Ans: c
18. Arc interruption in an oil circuit breaker takes place when
a) Contacts apart.
b) Voltage goes through zero.
c) Current goes through zero.
Ans: c
19. In a minimum oil circuit breaker the oil is used
a) To act as circuit breaking medium only.
b) For circuit breaking and providing insulation
c) For providing insulation only.
d) For none of the above purposes.
Ans: a
20. Minimum oil circuit breakers use oil for
a) Arc quenching only.
b) Insulation only.
c) Cooling only.
d) Lubrication only.
Ans: a
21. The air-blast circuit breaker is usually employed for... duty.
a) Instantaneous
b) Intermittent
c) Repeated
d) Short
Ans: c
22. The air-blast circuit breakers for 400 kV system are designed to operate in
a) 0.1 s.
b) 0.5 s
c) 50 ms.
d) 100 µs
Ans: c
23. In air-blast circuit breaker resistance switching is used to
a) Reduce the magnitude of fault current.
b) Control the CB operating time
c) Damp out the fast transient.
d) Change the fault current power factor.
Ans: c
24. The single most important property that makes SF6 a very efficient medium for circuit breaking is
a) It is non-toxic and non-inflammable.
b) It has dielectric constant
c) It has high breakdown strength
d) It is highly electro-negative in character.
Ans: d
25. For long EHV/UHV transmission lines, we normally use
a) Air circuit breaker
b) Air-blast circuit breaker
c) Oil circuit breaker
d) Isolating switch
Ans: b
