Question

There are two 10 MVA generators of subtransient reactance 15% each and one 5 MVA generator of subtransient reactance 12%. The generators are connected to the station busbars from which load is taken through three 5 MVA, step up transformers each having reactance of 8%. The maximum fault MVA at LV side is _____ MVA.

• A. 175.4

Answer 1

The correct option is A 175.4

$X_{eqp.u.}=\frac{(\frac{j0.15}{2}\times j0.24)}{\frac{j0.15}{2}+j0.24}=j0.057p.u.$

Per unit fault MVA is

${\left(MVA\right)}_{fp.u.}=\frac{1}{X_{eqp.u.}}$

and fault MVA is ${\left(MVA\right)}_f=\frac{{\left(MVA\right)}_b}{X_{eqp.u.}}=\frac{10}{0.057}=175.4MVA$