# Power Systems Questions and Answers – Short Circuit of a Synchronous Machine (No Load)

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This set of Power Systems Multiple Choice Questions & Answers (MCQs) focuses on “Short Circuit of a Synchronous Machine (No Load)”.

1. Transient in synchronous generator is similar to which of the following circuit?
a) Parallel RLC circuit
b) Series RLC circuit
c) Series RL circuit
d) Parallel RL circuit

Explanation: The current flowing in armature of synchronous generator when its terminals are short circuited is similar to that flowing when sinusoidal voltage is suddenly applied to an RL series circuit.

2. When all three phases of a synchronous generator on no load are suddenly short circuited then symmetry of the short circuit current depends on which of the following?
a) Position of fault
b) Symmetry of fault
c) Instantaneous Voltage at which fault occurs
d) Resistance of armature winding

Explanation: Symmetry of the short circuit current about time axis and DC offset depends upon the value of phase voltage at which short circuit occurs.

3. In a synchronous generator for how much time subtransient period of symmetrical short circuit current lasts?
a) For 200 Cycles
b) For 500 Cycles
c) For 30 Cycles
d) For 2 Cycles

Explanation: Subtransient period lasts for only about 2 cycles. During this period the subtransient current decay is very rapidly.
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4. In a synchronous generator for how much time transient period of symmetrical short circuit current lasts?
a) For 200 Cycles
b) For 500 Cycles
c) For 30 Cycles
d) For 2 Cycles

Explanation: In a synchronous generator transient period of symmetrical short circuit current lasts for about 30 cycles. During this period current decreases somewhat slower than the current in subtransient period.

5. After how many cycles in a synchronous generator symmetrical short circuit current reaches to its steady state value?
a) After 200 Cycles
b) After 500 Cycles
c) After 30 Cycles
d) After 2 Cycles

Explanation: In a synchronous generator subtransient period of short circuit current lasts for up to 2 cycles the transient period of that lies for about 30 cycles. After that the current reaches to its steady state value.

6. What is the phase current in phase Y and in phase B out of R-Y-B when sudden short circuit to all three phases occurs at no load? It is given that after fault current in phase R is equal (Vm Sin ωt)
a) Y = Vm Sin(ωt +60°) & B = VmSin(ωt +120°)
b) Y = Vm Sin(ωt +120°) & B = VmSin(ωt+240°)
c) Y = Vm Sin(ωt +240°) & B = Vm Sin(ωt +120°)
d) Y = Vm Sin(ωt +90°) & B = Vm Sin(ωt -90°)

Explanation: The given phase sequence is R-Y-B . When all three phases are short circuited then the fault is symmetrical that means fault current flowing through each phases will be 120° apart from each other.

7. In case of synchronous generator the reactants is constant.
a) True
b) False

Explanation: In case of synchronous generator the reactance is not a constant one but is a function of time. The reactance of synchronous generator is different for subtransient state, transient state and steady state.

8. If Xl is leakage reactance, Xf is reactance due to field winding, Xdw is reactance due to damper winding and Xa is armature winding reactance then reactance of synchronous generator in subtransient state is due to which of the following?
a) Xl ,Xdw and Xa
b) Xl and Xa
c) Xl , Xf ,Xdw and Xa
d) Xl and Xdw

Explanation: During the initial part of short circuit or during subtransient period equivalent circuit of alternator contains the field winding reactance damper winding reactance armature winding reactance in parallel and leakage reactance in series.

9. Which of the following reactance is eliminated first in synchronous generator just after the symmetrical fault?
a) Leakage reactance
b) Damper winding reactance
c) Armature winding reactance
d) Field winding reactance

Explanation: As the damper winding currents are first to die out, damper winding reactance effectively becomes open circuited. After that Approximate circuit model of synchronous generator contains only leakage reactance, field winding reactance and armature winding reactance.

10. Steady state direct axis reactance is greater than subtransient direct axis reactance and transient direct axis reactance.
a) True
b) False

Explanation: During transient and subtransient period the armature winding reactance contains other reactances like damper winding reactance and field winding reactances in parallel with it. So, the net reactance during subtransient and transient period are less than the net reactance during steady state period.

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