GATE EE 2014 Set 1 — Question 25
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Power Systems → Fault Analysis → Symmetrical Faults
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Question
Three-phase to ground fault takes place at locations and in the system shown in the figure
If the fault takes place at location , then the voltage and the current at bus A are and respectively. If the fault takes place at location , then the voltage and the current at bus A are and respectively. The correct statement about voltages and currents during faults at and is
Correct answer
(C) V_(F1) lags I_(F1) and V_(F2) leads I_(F2)
Solution
Take the reference direction of the bus-A current as shown by the arrow (from A towards B), and treat the line as purely inductive (reactance per unit length).Fault at (between and bus A). Bus A is between the fault and source , so fault current flows from B through bus A towards , which is opposite to the reference arrow. The voltage at bus A equals the drop across the line section from A to the fault ( is at zero potential):So is behind : lags .Fault at (between bus A and bus B). Now feeds the fault through bus A, so the current flows along the reference arrow:So is ahead of : leads .This is the principle used by directional relays: the voltage–current phase relation at the bus reverses with the side of the fault.Answer: (C) lags and leads
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