Question

A 25 MVA, 13.2 kV alternator with solidly grounded netural has a substransient reactance of 0.25 p.u. The negative and zero sequence reactance are 0.35 and 0.1 p.u. respectively. A single line to ground fault occurs at the terminals of an unloaded alternator, then the fault current is

• A. 3530 A
• B. 5025 A
• C. 4685 A
• D. 8025 A

Answer 1

The correct option is C 4685 A

For a line to ground fault,
Positive sequence current,

$I_{a1}=\frac{E_a}{Z_1+Z_2+Z_0}$

$I_{a1}=\frac{1}{j0.25+j0.35+j0.1}=-j1.428$

Base current, $I_b=\frac{25\times 1000\times {10}^3}{\sqrt{3}\times 13.2\times {10}^3}=1093.46A$

Fault current, $I_f=3I_{a1}\times I_b$

$=3\times 1.428\times 1093.46$

$=4684.38A$