Question

A synchronous generator is rated 25 MVA, 11 kV. It is star connected with neutral point solidly grounded. The generator is operating at no-load rated voltage. Positive, negative and zero sequence reactance are 0.2 p.u., 0.2 p.u. and 0.08 p.u. respectively. A line to line fault occurs on b and c phase of generator. Then the magnitude of fault current will be

• A. 1.312 kA
• B. 5.68 kA
• C. 2.2724 kA
• D. 0.7578 KA

Answer 1

The correct option is B 5.68 kA

Line to line fault occurs on b and c phase of generator,




$I_f=I_b=-I_c$

and $I_a=0$

The sequecnce network of line to line fault is


$I_1=\frac{V_f}{z_1+z_2}$

$\Rightarrow I_f=I_b=({\alpha }^2-\alpha )I_1=-j\sqrt{3}{I}_1=\frac{-j\sqrt{3}{V}_f}{z_1+z_2}$

and $I_{fp.u}=\frac{-j\sqrt{3}\times 1}{j0.2+j0.2}$

$\left|I_{fp.u}\right|=\frac{\sqrt{3}}{0.4}=4.33p.u.$

Base current = $\frac{25\times {10}^3}{\sqrt{3}\times 11}=1312.16A$

Fault current, $I_f=4.33\times 1312.16=5.68kA$