# Multiple Choice Question (MCQ) of Power Systems page-6

71. The percentage reactance can be converted into ohmic value which the following formula

a) Xohms = (X(%) ×100 (kV)2)/kVA
b) Xohms = (X(%) ×1000 (kV)2)/kVA
c) Xohms = (X(%)×(KV)2)/(1000 kVA)
d) Xohms = (X(%)×10(kV)2)/kVA

Ans: (d)

72. When an alternator is short circuited on the three phases, it settles down to the steady short circuit value and limited by

a) Sub-transient reactance
b) Transient reactance
c) Synchronous reactance
d) Either (a) or (c)

Ans: (c)

73. The short circuit current of an alternator when short circuited on three phases will be

a) Zero as time goes to infinity
b) Maximum when time goes to infinity
c) Infinity when time goes to infinity
d) Small as time goes to infinity but not zero
e) None of the above

Ans: (a)

74. The reactors used to limit the short circuit current in the alternators have very small resistance in comparison to reactance

a) Because the high resistance will not help to limit the short circuit current
b) To avoid the energy waste
c) Because the high resistance will raise the temperature and insulation of reactor will be spoiled
d) To improve power factor

Ans: (b)

75. ‘a’ is an operator used to symmetrical component analysis

a) Which rotates the vector, it operates upon, through an angle 90° in the clockwise direction
b) Which rotates the vector, it operates upon, through an angle 120°, in the counter-clockwise direction
c) Which rotates the vector, it operates upon, through an angle120°, in the clockwise direction
d) Which rotates the vector, it operates upon, through an angle 240°, in the clockwise direction

Ans: (b)

76. Which of the following statement is true?

a) a = 0.5 – j 0.866
b) a = 0.5 + j 0.866
c) a4 = - a
d) a2 = - (1 + a)

Ans: (d)

77. The arc voltage produced in the circuit breaker is always

a) In the phase with the are current
b) Lagging the arc current by 90°
c) Leading the arc current by 90°
d) Leading the are current by 180°

Ans: (a)

78. The fusing factor of the fuse always greater than one.

a) true
b) false

Ans: (a)

79. The material used for fuse must have

a) Low melting point and low specific resistance
b) Low melting point and high specific resistance
c) High melting point and low specific resistance
d) Low melting point with any specific resistance

Ans: (b)

80. The Buchholz relay is used to protect the

a) Alternators against all internal faults
b) Oil immersed transformers against all internal faults
c) Synchronous motors against all internal faults
d) Transmission lines against all short circuit faults

Ans: (b)

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