GATE 2017 EE – Question 64
The positive, negative, and zero sequence reactances of a wye-connected synchronous generator are 0.2 pu, 0.2 pu, and 0.1 pu, respectively. The generator is on open circuit with a terminal voltage of 1 pu. The minimum value of the inductive reactance, in pu, required to be connected between neutral and ground so that the fault current does not exceed 3.75 pu if a single line to ground fault occurs at the terminals is ________ (assume fault impedance to be zero). (Give the answer up to one decimal place.)
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Correct answer: 0.1 to 0.1
Explanation
For a single line to ground fault the fault current is $I_f = \frac{3V}{X_1 + X_2 + X_0 + 3X_n}$. Setting $I_f = 3.75$ gives $X_1 + X_2 + X_0 + 3X_n = \frac{3}{3.75} = 0.8$. So $0.2 + 0.2 + 0.1 + 3X_n = 0.8$, which gives $X_n = \frac{0.3}{3} = 0.1$ pu.