Max Fault Current XFMR- quickly if possible

[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: [tex] I= \frac{42MVA}{\sqrt{3}*22kv} = 1.102kA [/tex] 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.

 

[jbsteele]

Yes, your calculation for the maximum line current on the LV delta side is correct. For the maximum line current on the star side, you can use the turns ratio and the delta side line current to calculate it. The turns ratio is 132kV/22kV = 6, so the maximum line current on the star side is 6 times the delta side line current, which is 6.612 kA. However, the star side voltage is higher than the delta side voltage, so you will need to account for the change in voltage and power factor when calculating the actual current draw of the transformer.