Question

A cylindrical rotor synchronous 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 in p.u (rounded off to 2 decimal places) is

• A. 1.02

Answer 1

The correct option is A 1.02
Prefault current,

$I_0=\frac{E_f-V_t}{j{X}_d}=\frac{1+j0.7-1}{j0.7}=1$

Subtransient induced emf,

E${}_f^{\prime \prime }$ = V${}_0$ + jX${}_d^{\prime \prime }$ I${}_0$

= 1 + j0.2 x 1 = 1 + j0.2

$\left|E_f^{\prime \prime }\right|=\sqrt{1^2+{0.2}^2}$ = 1.0198