GATE EE 2023 Set 1 — Question 60
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Power Systems → Fault Analysis → Unsymmetrical Faults (LG, LL, LLG)
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Question
The two-bus power system shown in figure (i) has one alternator supplying a synchronous motor load through a Y- transformer. The positive, negative and zero-sequence diagrams of the system are shown in figures (ii), (iii) and (iv), respectively. All reactances in the sequence diagrams are in p.u. For a bolted line-to-line fault (fault impedance = zero) between phases ‘b’ and ‘c’ at bus 1, neglecting all pre-fault currents, the magnitude of the fault current (from phase ‘b’ to ‘c’) in p.u. is _____________ (Round off to 2 decimal places).
📷 Figure: Single-line diagram and sequence networks of a power system
Correct answer
7.1 to 7.3
Solution
For a bolted line-to-line fault at bus 1, the magnitude of the fault current is given by:where is the pre-fault voltage (assumed 1.0 p.u.), is the Thevenin positive sequence impedance at the fault point, and is the negative sequence impedance.From Figure (ii) (Positive-sequence network) at Bus 1:
- Left branch:
- Right branch:
- Left branch:
- Right branch:
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