GATE EE 2025 Set 1 — Question 49
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Power Systems → Power System Stability → Equal Area Criterion
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Question
In the system shown below, the generator was initially supplying power to the grid. A temporary LLLG bolted fault occurs at very close to circuit breaker 1. The circuit breakers open to isolate the line. The fault self-clears. The circuit breakers reclose and restore the line. Which one of the following diagrams best indicates the rotor accelerating and decelerating areas?
Correct answer
(B) Fig. (ii)
Solution
System states. The generator feeds the grid through two parallel lines.
- Pre-fault (both lines in): (the upper curve). The rotor runs at , where this curve meets the mechanical input line .
- During the fault: a bolted three-phase fault at , very close to CB1, short-circuits the line right at the generator-side bus. The bus voltage collapses and no power reaches the grid, so .
- Breakers open (one line in): with the faulted line isolated, power flows over the remaining line only, so with (the lower curve).
- Reclosure (both lines in again): after the fault self-clears and the breakers reclose, the operating point jumps back up to the pre-fault (upper) curve.
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