Question

The neutral of a 10 MVA, 11 kV alternator is earthed through a resistance of 5 $\Omega$. The earth fault relay is set to operate at 0.75 A. The CT's have a ratio of 1000 : 5. The percentage of alternator winding unprotected is _______%.

• A. 11.8

Answer 1

The correct option is A 11.8
Pick up value of relay
$=\frac{1000\times 0.75}{5}=150A$
Let the percentage of winding unprotected is $xpercent.$
Voltage induced in this section,
$=(\frac{11\times {10}^3}{\sqrt{3}}\times \frac{x}{100})$

$150=\frac{\frac{11\times {10}^3}{\sqrt{3}}\times \frac{x}{100}}{5}$

$x=11.80\%$