Switchgear and Protection MCQ

81. The types of faults that occur in electric motors are

(a) stator.
(b) rotor.
(c) overload.
(d) undervoltage.
(e) unbalanced supply voltages and reverse or open phase starting.
(f) all of the above.

82. Negative-sequence currents flow in the stator windings of an ac motor due to

(a) unbalance in the supply voltage.
(b) unbalanced load.
(c) phase reversal.
(d) either (a) or (b).

83. A negative sequence relay is commonly used to protect

(a) an alternator.
(b) a transformer.
(c) a transmission line.
(d) a bus bar.

84. Earth fault protection for an electric motor is provided by means of

(a) instantaneous overcurrent relay.
(b) instantaneous relay having a setting of approximately 30% of motor rated current in the residual circuits of two CTs.
(c) ground wire.
(d) both (b) and (c).

85. Any winding of a power transformer develops short-circuit owing to

(a) loose connection.
(b) insulation failure.
(c) impulse voltage.
(d) mechanical vibrations.

86. Which one of the following relays has the capability of anticipating the possible major fault in a transformer ?

(a) Overcurrent relay
(b) Differential relay
(c) Buchholz relay
(d) Overfluxing relay

87. Which relay is used to detect and protect internal faults of a transformer ?

(a) Buchholz
(b) Directional relay
(c) Thermal relay
(d) Distance relay

88. To protect the power transformer (Y – Y with neutral grounded) against fault, what type of connection do the CTs have ?

(a) Δ – Y connection
(b) Δ – Δ connection
(c) Y – Y connection
(d) Y – Δ connection

89. In Merz-Price percentage differential protection of a Δ-Y transformer, the CT secondaries connection in the primary and secondary winding of the transformer would be in the form of

(a) Δ – Y.
(b) Y – Δ.
(c) Δ – Δ.
(d) Δ – Δ.

89. A 10 kVA, 400 V/200 V single phase transformer with 10% impedance draws a steady short-circuit line current of

(a) 50 A.
(b) 150 A.
(c) 250 A.
(d) 350 A.

90. To prevent mal operation of differentially connected relay while energizing a transformer, the relay restraining coil is biased with

(a) second harmonic current.
(b) third harmonic current.
(c) fifth harmonic current.
(d) seventh harmonic current.

91. The inrush current of a transformer at no load is maximum if the supply voltage is switched on

(a) at peak voltage value.
(b) at zero voltage value.
(c) at half voltage value.
(d) at 0.866 time voltage value

92. Magnetizing inrush phenomena occur in

(a) large transformer primary only, when the transformer is switched on.
(b) large transformer primary when a 3-phase fault occurs in the secondary.
(c) large transformer secondary only when the transformer is switched on in the primary.
(d) none of the above.

95. In case of power transformer :

(a) positive, negative and zero-sequence impedances are all equal.
(b) positive and zero-sequence impedances are equal but less than negative-sequence impedance.
(c) positive- sequence impedance is greater than both negative-and zero-sequence impedances.
(d) positive- sequence impedance is less than negative-sequence impedance but equal to zero-sequence impedance.

96. For the protection of transformers, harmonic restraint is used to guard against

(a) magnetizing inrush current.
(b) unbalanced operation.
(c) lightning.
(d) switching overvoltage.

97. The connections of CTs are opposite to that of transformer windings so that current in the pilot wires of two are/have

(a) same phase.
(b) opposite in phase.
(c) a phase difference of 60°.
(d) a phase difference of 90°.

98. For preventing the mal operation of Merz-Price protection scheme on inrush of magnetizing current

(a) the relay restraining coil is biased with second harmonic current.
(b) time lag is provided in the relay.
(c) relay sensitivity is reduced by employing a shunt.
(d) all of the above.

99. Problems associated with differential protection is/are

(a) magnetizing inrush current.
(b) mismatching characteristics of CTs.
(c) charge of ratio as a result of tapping.
(d) all of the above.

100. A bus-bar is rated by

(a) current only.
(b) voltage only.
(c) current, voltage and frequency.
(d) current, voltage, frequency and short-circuit current.