things for an 11kV distribution network. The first thing I have to do is

work out the zone sub transformer impedance. There are 2 x 25MVA subs for

the 11kV network. I have to choose an impedance so that the fault level does

not exceed 10.1kA and also so that the maximum voltage regulation of the

transformer is 10% (assume purely inductive on 0.9 lagging PF). I've done

thing so far but it's not making sense.

to satisfy fault level:

working on 100MVA base

fault level = 10.1kA = (root(3)

***11kV***10.1kA)MVA = 1.982pu

with 2 transformers, each will have fault level divide by 2 so 1.982/2 =

0.96pu

Z = 1/0.96 = 1.04pu

so for the fault level to not exceed 10.1kA, the transformer impedance

should be >= 1.04pu

to satisfy voltage regulation (10%):

P= 0.25pu,

with V = 1pu, I = 0.25pu

|V|=|I||Z|

0.1 = 0.25*Z

Z = 0.4pu

so for regulation to be