Question

A 60 Hz generator is rated at 500 MVA, 20 kV with $X_d^{{}^{\prime \prime }}=0.20p.u.$ it supplies a purely resistive load of 400 MW at 20 kV. The load is connected directly across the terminals of generator. If all the three phases of the load are short circuited simultaneously, the initial symmetrical rms current in the generator in per unit on the base of 500 MVA, 20 kV will be ________ p.u.

• A. 5.06

Answer 1

The correct option is A 5.06

At pre fault condition,

$I_{Lp.u.}=\frac{400}{500}=0.8p.u.$

$E_f^{{}^{\prime \prime }}=V_t+j{X}_d^{{}^{\prime \prime }}{I}_L$

$=1\angle 0{}^{\circ }+j\left(0.2\right)\left(0.8\right)$

$E_f^{{}^{\prime \prime }}=1\angle 0{}^{\circ }+0.16\angle 90{}^{\circ }$

$\left|E_f^{{}^{\prime \prime }}\right|=1.0127p.u.$
Post fault condition,

$I_f^{{}^{\prime \prime }}=\frac{E_f^{{}^{\prime \prime }}}{j{X}_d^{{}^{\prime \prime }}}$

$\left|I_f^{{}^{\prime \prime }}\right|=\frac{1.0127}{0.2}=5.0635p.u.$