GATE 2016 EE (EE2) – Question 63

Power Systems · Symmetrical components, Symmetrical and unsymmetrical fault analysis · 2 marks · Numerical answer

Two identical unloaded generators are paralleled as shown. Each has positive-, negative- and zero-sequence impedances j0.4,j0.3,j0.15 pu. Prefault voltage is 1 pu. Find line-to-ground terminal fault current in pu.

Diagram for GATE 2016 EE (EE2) question 63

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Correct answer: 5.5 to 6.5

Explanation

Positive-sequence sources are paralleled, giving Z1=j0.4/2=j0.2. Negative-sequence impedances likewise give Z2=j0.15.

Only the generator with grounded neutral supplies zero-sequence current; the ungrounded star blocks it. Thus Z0=j0.15, rather than half that value.

For a solid single-line-ground fault, $|I_f|=|3V/(Z_1+Z_2+Z_0)|=3/0.5=$ **6 pu**.

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