Three-Winding Transformers kVA calculation

[corvalan]

I have the following data for a three-winding, three-pahse transformer:

Primary: kVA: 1,100 Voltage: 12,470V
Secondary: Nominal Current: 250A, Voltage: 480V
Tertiary: Nominal Current 132A, Voltage: 4800V

Is the secondary kVA = 207.6 kVA?
Is the tertiary kVA = 1096 kVA?

If not, what formulas should I use to calculate the nominal kVA of the secondary and tertiary?

Thanks.

 

[davidbeach]

I'd say , first off, that you probably have your secondary and tertiary bass-ackwards; ;-) the secondary would be the 4800V winding and the 480V would be the tertiary. See below.

Second, I'd go with the nameplate ratings over any calculation, they should be there.

But, if no nameplate kVA ratings, I'd say that the secondary (4800V) is the full 1100kVA as that is 132.3A rather than the rounded 132A listed. The tertiary (480V) could be 208kVA, but that is a bit of an odd number; are you sure that the current rating of that winding isn't 240A instead of 250A? If so that would be a 200kVA rating and that would be closer to what one might expect, but ratings can be most anything.

In general, for three winding transformers with approximately equal ratings it is generally the middle voltage that is the secondary and the lower voltage that is the tertiary. Also, in general, for three winding transformers with two windings with the same kVA ratings and one winding with a much lower rating it will be the winding with the lower rating that is the tertiary. In this case both would point to the 480V being the tertiary, but like with the ratings many things are possible.