Question

Five synchronous generators are connected in parallel as shown in below figure. The rating of each generator is 22 kV, 20 MVA, 0.5 pu reactance. If fault occurs at location 'P', then current carried by shown circuit breaker CB when fault occurs at point 'P' will be______ pu.

• A. -j12 pu
• B. -j10 pu
• C. -j8 pu
• D. -j2 pu

Answer 1

The correct option is C -j8 pu

For the given system when fault occurs at point P, fault current
$I_f=1/Z_{th}$


Per unit Thevenin impedance,
$Z_{th}=\frac{j0.5}{5}=j0.1$
$\therefore$ Fault current, $I_f=\frac{1}{j0.1}=-j10pu$

Drawing impedance diagram



By current division rule

Fault current, $I_f=-j10\times \frac{j0.5}{j0.5+\frac{j0.5}{4}}$

$=-j10\times \frac{0.5j}{j0.625}=-j8pu$