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GATE 2020 EE – Question 18

Power Systems · Principles of over-current, differential, directional and distance protection; Circuit breakers · 1 mark · Multiple choice

A lossless transmission line with 0.2 pu reactance per phase uniformly distributed along the length of the line, connecting a generator bus to a load bus, is protected up to 80% of its length by a distance relay placed at the generator bus. The generator terminal voltage is 1 pu. There is no generation at the load bus. The threshold pu current for operation of the distance relay for a solid three phase-to-ground fault on the transmission line is closest to:

  1. 1.00
  2. 3.61
  3. 5.00
  4. 6.25

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Show answer and explanation

Correct answer: (D) 6.25

Explanation

The relay must operate for a fault at 80% of the line, i.e. at a reactance of $0.8\times0.2=0.16$ pu from the relay. With 1 pu voltage and no other source, the fault current is $\frac{1}{0.16}=6.25$ pu.