GATE 2019 EE – Question 60
A 30 kV, 50 Hz, 50 MVA generator has the positive, negative, and zero sequence reactances of 0.25 pu, 0.15 pu, and 0.05 pu, respectively. The neutral of the generator is grounded with a reactance so that the fault current for a bolted LG fault and that of a bolted three-phase fault at the generator terminal are equal. The value of grounding reactance in ohms (round off to one decimal place) is ________.
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Show answer and explanation
Correct answer: 1.79 to 1.81
Explanation
The three-phase fault current is $\frac{E}{0.25}$ and the LG fault current is $\frac{3E}{0.25+0.15+0.05+3X_n}$. Equating them: $0.45+3X_n=0.75$, so $X_n=0.1$ pu. The base impedance is $\frac{30^2}{50}=18\ \Omega$, so $X_n=1.8\ \Omega$.