Switchgear and Protection MCQ

61. Over conventional electromechanical relays, the transistor relays have the limitations of

(a) characteristics varying with temperature and ageing.
(b) low short-time overload capacity.
(c) reliability dependent upon the large number of small components and their electrical connections.
(d) all of the above.

62. In a static overcurrent elay, inverse time characteristics are obtained by

(a) a transistor amplifier.
(b) an integrating circuit.
(c) a transistor switch.
(d) a differentiating circuit.

63. Two-input phase comparator in a static relay is made up of a

(a) transformer amplifier.
(b) transistor logic circuit.
(c) rectifier bridge.
(d) thyristor bridge.

64. The phase comparators in case of static relays and electro-mechanical relays normally are

(a) cosine and sine comparators respectively.
(b) sine and cosine comparators respectively.
(c) both are sine comparators.
(d) both are cosine comparators.

65. Merz-Price protection is employed for protection of

(a) alternators.
(b) transformers.
(c) transmission lines.
(d) both (a) and (b).

66. Negative-sequence voltages in the stator induces

(a) high frequency currents in the rotor.
(b) high frequency voltage in the rotor.
(c) low frequency currents in the rotor.
(d) high frequency currents in the stator.

67. The short-circuit current of an alternator, in case of line to line fault, depends on its

(a) short-circuit resistance.
(b) transient reactance.
(c) synchronous reactance.
(d) none of the above.

68. We do not require any protection against prime mover failure in case of

(a) turbo-generator sets.
(b) hydro-generator sets.
(c) diesel-engine driven alternators.
(d) back pressure turbo-generators.

69. A large ac generator supplying power to an infinite bus, has a sudden short circuit occurring at its terminals. Assuming the prime mover input and the voltage behind the transient reactance to remain constant immediately after the fault, the acceleration of the generator rotor is

(a) inversely proportional to the moment of inertia of the machine.
(b) inversely proportional to the square of voltage.
(c) directly proportional to the square of the short-circuit current.
(d) directly proportional to the short-circuit power.

70. For the protection of stator winding of an alternator against internal fault involving ground, the relay used is a

(a) biased differential relay.
(b) directional overcurrent relay.
(c) plain impedance relay.
(d) Buchholz relay.

71. The bias factor S in unit protection synchronous generators

(a) lies between 0.05 and 0.1 pu.
(b) is less than 0.05 pu.
(c) lies between 0.1 to 0.25 pu.
(d) is greater than 0.025 pu.

72. Field failure in an alternator occurs due to

(a) failure of exciter.
(b) faulty field circuit breaker.
(c) inrush of large currents.
(d) both (a) and (b).

73. As soon as a fault develops in a generator stator, it is essential to suppress field excitation, otherwise

(a) terminal voltage will drop.
(b) it may lead to loss of synchronism.
(c) it will continue to supply power to the stator winding fault.
(d) all of the above.

74: If there is an open circuit in the field circuit of a generator

(a) the generator will continue to operate as an induction generator supplying load at a very low leading power factor.
(b) the supply voltage will drop, and the system stability will be affected.
(c) the circulating current at slip frequency may overheat the generator.
(d) all of the above.

75. The earth fault in stator causes

(a) arcing to core.
(b) severe heating in conductors and thereby damaging the insulation.
(c) open-circuit in the stator.
(d) both (a) and (b).

76. Failure of insulation on stator winding of generator results in

(a) short circuit between turns.
(b) short circuit between one or more phases and earth.
(c) short circuit between phases.
(d) all of the above.

77. The magnitude of earth-fault current for a given fault position within a winding depends upon

(a) the winding connections.
(b) the method of neutral grounding.
(c) unmatched characteristics of CTs.
(d) both (a) and (b).

78. For the protection of a large squirrel-cage induction motor against single phasing normally

(a) an overcurrent relay is used.
(b) a differential relay is used.
(c) a directional relay is used.
(d) negative sequence current sensitive relay is used.

79. The main function of undervoltage protective device generally employed with a motor starter is to

(a) open the supply circuit on failure of power supply.
(b) control the motor voltage.
(c) prevent the opening of supply circuit.
(d) none of the above.

80. While adding extra stop buttons to an existing control system they must be in

(a) parallel to the load.
(b) series with the contactor coil.
(c) series with the motor.
(d) parallel to the start button.