Question

A generating station having n-section busbars each rated at 8 kVA with x% reactance is connected on the tie-bars system through busbar reactance of b%. If n is very large and a three phase fault takes place on one section then the short circuit kVA is

• A. $\frac{8}{x}$
• B. $\frac{8}{b}$
• C. $\frac{8}{x+b}$
• D. $8(\frac{1}{x}+\frac{1}{b})$

Answer 1

The correct option is D $8(\frac{1}{x}+\frac{1}{b})$


Equivalent impedance $Z_{eq}$ between the zero potential bus and the fault point is

$(\frac{b+x}{n-1}+b)\parallel x=\left(\frac{bn+x}{n-1}\right)\parallel x$

$\frac{1}{Z_{eq}}=\frac{1}{x}+\frac{n-1}{bn+x}$

$SCKVA=\frac{8}{Z_{eq}}\times 100$

$=8[\frac{1}{x}+\frac{n-1}{bn+x}]\times 100$

If n is very large,

Short circuit KVA $=8[\frac{1}{x}+\frac{1}{b}]$