GATE EE 2016 Set 2 — Question 27
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Electrical Machines → Fault Analysis → Symmetrical Faults
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Question
A 50 MVA, 10 kV, 50 Hz, star-connected, unloaded three-phase alternator has a synchronous reactance of 1 p.u. and a sub-transient reactance of 0.2 p.u. If a 3-phase short circuit occurs close to the generator terminals, the ratio of initial and final values of the sinusoidal component of the short circuit current is ________.
Correct answer
4.9 to 5.1
Solution
1.The initial (sub-transient) short circuit current is given by , where is the pre-fault voltage and is the sub-transient reactance.
2.The final (steady-state) short circuit current is given by , where is the synchronous reactance.
3.Given p.u. and p.u.
4.Assuming pre-fault voltage p.u. (unloaded alternator).
5. p.u.
6. p.u.
7.The ratio of initial to final values of the sinusoidal component is .
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