Q) Find out level calculation at 11 KV bus . ( % Z = 9.88 )

Fault level at 110 KV bus = 1517 MVA ( from utility's fault study )

Take base MVA as 100

System % Impedance = ( Base MVA x 100 )/ fault MVA

% source impedance = (100 x 100 ) /1517 = 6.59 %

% Impedance of T/F at Base MVA = (9.88 x 100)/12.5 = 79.04 %

Effective Impedance since T/F are in parallel = 79.02/2 = 39.52 %
Fault level at 11 KV Bus = ( base MVA x 100 )/% Z upto 11 KV bus
= (base MVAx100)/(6.59+39.52)
= 100x100/46.11

= 216.87 MVA

Fault current on 11 KV side = ( 216.87 x 10^6 )/ sq.root 3 x 11000 KA = 11.39 KA

Calculation for cross section of conductor

A = 11.1 x Is x sq.rt t

Is = SC Current , t= duration of fault in seconds.