GATE 2020 EE – Question 59
A cylindrical rotor generator has steady state synchronous reactance of 0.7 pu and subtransient reactance of 0.2 pu. It is operating at $(1+j0)$ pu terminal voltage with an internal emf of $(1+j0.7)$ pu. Following a three-phase solid short circuit fault at the terminal of the generator, the magnitude of the subtransient internal emf (rounded off to 2 decimal places) is ________ pu.
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Correct answer: 1.01 to 1.03
Explanation
The pre-fault current is $I=\frac{E-V}{jX_s}=\frac{j0.7}{j0.7}=1\angle0^\circ$ pu. The subtransient internal emf is $E''=V+jX_d''I=1+j0.2$, whose magnitude is $\sqrt{1+0.04}=1.02$ pu.