# Homework Help: Max Fault Current XFMR-need help quickly if possible

1. Apr 11, 2013

### Fisher92

Hi, I have a power systems protection assignment and I wanted to get some simple calcs verified.

XFMR - 30MVA, 132kV:22kV; Star/Delta respectively. star neutral is floating, not that it matters for this part.

reactance is 8% and resistance is 0.5% on the 30MVA base.

Basically I am trying to choose CT's and I don't want them saturated so I need the max line current (obviously the XFMR will trip on OC so I need max current up to 140%30MVA = 42MVA)

I have: $$I= \frac{42MVA}{\sqrt{3}*22kv} = 1.102kA$$ on the LV delta side. I'm not sure if this is correct though???? why have the specified the transformer impedance???
I also need the max line current on the star side for diff protection but I think that that should just be reduced by the turns ration,,,, may have to account for the star delta configuration????

Thanks for any help.