Distribtuion network design assignment

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)*11kV*10.1kA)MVA = 1.982pu
with 2 transformers, each will have fault level divide by 2 so 1.982/2 =
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
0.1 = 0.25*Z
Z = 0.4pu
so for regulation to be
Reply to
Warren Thai
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Hello Warren
I make that 1.92 pu
OK but they are 25 MVA transformers, so you have to correct back to a 25 MVA base I make it 26% or 0.26 pu on the 25 MVA rating.
Reply to
John Rye

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