GATE 2018 EE – Question 31
The positive, negative and zero sequence impedances of a 125 MVA, three-phase, 15.5 kV, star-grounded generator are $j0.1$ pu, $j0.05$ pu and $j0.01$ pu respectively on the machine rating base. The machine is unloaded and working at the rated terminal voltage. If the grounding impedance of the generator is $j0.01$ pu, then the magnitude of fault current for a $b$-phase to ground fault is ________ kA (up to 2 decimal places).
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Correct answer: 73.13 to 73.87
Explanation
The fault current is $I_f=\frac{3E}{Z_1+Z_2+Z_0+3Z_n}=\frac{3}{0.1+0.05+0.01+0.03}=15.79$ pu. The base current is $\frac{125\times10^6}{\sqrt3\times15.5\times10^3}=4656$ A, so $I_f=73.5$ kA.