three single phase transformers impedance matching

[k2panman]

When installing a bank of utility transformers, say 100kVA 25kv:277/480V, using three single phase transformers, what effect does unmatched impedance have on the operation of the bank? If one transformer is 2.0% and the other two are 1.6% for example, what will this do?

K2ofKeyLargo

 

[davidbeach]

You will get unmatched fault currents and unmatched voltage regulation.

 

[cuky2000]

The transformer bank with unmachet impedance can not be loaded a full capacity.

 

[wfowfo]

Doesn't this depend on the type of hookup? I was under the impression from some old REA transformer guidelines that the only bank that really had to be watched on this was from a delta/delta connection. I understood that in wye/delta and wye/wye banks it wasn't that much of a concern.

 

[rbulsara]

I dont know the theoratical answer at this time, but you may be interested in the following link..

http://www.dstar.org/P_R_Software_GSVIC_1.htm

 

[waross]

Fault current; Divide the full load current by the %impedence to get available fault current. As long as it's less than you equipment rating no problem. If you can safely use three 1.6% transformers, you can use one 1.6% with two 2% transformers.

Voltage regulation.
277 x (1. - .02) = 271.46 volts
277 x (1 - 1.6) = 272.568 volts
difference, = 1.108 volts
One phase will run 1.108 volts higher than the others, the rare time that the incoming phases are exactly equal voltage.
I wouldn't worry.