Question

A 3-$\varphi$ 20 MVA, 10 kV alternator has internal reactance of 5% and negligible resistance. An external reactance per phase to be connected in series with alternator so that steady state current at shortcircuit does not exceed 6 times the full load current is_______.

• A. 0.375 $\Omega$
• B. 0.584 $\Omega$
• C. 0.284 $\Omega$
• D. 0.675 $\Omega$

Answer 1

The correct option is B 0.584 $\Omega$

The short-circuit current, $I_{sc}=6$ pu

$\frac{E}{X+0.05}=6pu$
$\frac{1}{X+0.05}=6pu$

$X=0.1167pu$
$X_{base}=\frac{(kV{)}^2}{MVA}$

$X_{base}=\frac{(10{)}^2}{20}$

$X_{base}=5\Omega$

$X_{actual}=X_{base}\times X_{pu}$

$X_{actual}=5\times 0.1167=0.5835\Omega$