Question

A 50 MVA, 10 kV, 50 Hz, star-connected, unloaded three-phase alternator has a synchronous reactance of 1 p.u. and sub-transient reactance of 0.2 p.u. If a 3-phase short circuit occurs close to the generator terminals, the ratio of initial and final values of the sinusoidal component of the short circuit current is

• A. 5

Answer 1

The correct option is A 5
$I^{\prime \prime }=\frac{E_g}{X_d^{\prime \prime }};I=\frac{E_g}{X}$

Fault current is inversely proportional to the reactance during the fault.

$\therefore \frac{I^{\prime \prime }}{I}=\frac{X}{X_d^{\prime \prime }}=\frac{1.0}{0.2}=5.0p.u.$