I'm working on a university assignment where I have to calculate a few
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)

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 <= 10%, Z should be <=0.4pu

so with the 2 results for Z, there aren't any values for Z that satisfy both conditions. What am I doing wrong? (please help, I'm so confused) -Warren

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 >= 1.04puto 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 <= 10%, Z should be <=0.4pu

so with the 2 results for Z, there aren't any values for Z that satisfy both conditions. What am I doing wrong? (please help, I'm so confused) -Warren