Transformer 35kV: 480V and 240V

[living2learn]

I am trying to calcualte how much fault current this transformer can give. The image might be hard to read but it is 3-1phase 167kVA transformers. from the photo it looks like they are using one input winding and are tapping the windings of each center to get the 240V. never saw this before and suspect that it isn't actually wired up like it is shown in the photos due to the turns ratio.

it's easy to calcuate the fault current assuming infinite bus and etc. but never have seen a situation like this

Q1) what kVA shall i use for the 480V detla?
Q2) What kVA shall i use for the 240V delta?

thoughts?

 

[waross]

Three phase fault current, use 167 KVA x 3 = 500 KVA
Center tapped: The secondary winding from one end to the center tap will have half the voltage but half the impedance. Percent impedance voltage should be the same as for a full winding.
You have two windings in open delta. Due to the phase displacement between the two windings, the virtual transformer formed by the open delta should have the same percent impedance voltage as either of the real windings. But the A-B phase and the two half windings will also form a virtual transformer in parallel with the first virtual transformer.
Use 167 KVA x 2 = 334 KVA and 480 volts.

Bill
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