Transformer MVA rating
Transformer MVA rating
(OP)
I'm not an electrical engineer, but looking into a transformer issue. Does MVA rating refer to primary or secondary side of transformer? I know there should be some losses through windings. Thanks!






RE: Transformer MVA rating
RE: Transformer MVA rating
RE: Transformer MVA rating
RE: Transformer MVA rating
RE: Transformer MVA rating
Primary side:
72KV
162.2A
Seconday side:
4.092Kv
2693.4A
I looked some formulas online (for 3 phase systems), and calculated Primary and Secondary MVA as I mentioned above. Did I make a mistake in calculation? If not, is that much loss unrealistic for 28MVA size transformers?
RE: Transformer MVA rating
The transformer you are presenting is a 20MVA transformer. Whatever the losses, depends on how much your load is as well as the no-load (core loss) of the transformer.
RE: Transformer MVA rating
RE: Transformer MVA rating
RE: Transformer MVA rating
I don't believe anything was stated that he is dealing with a two winding. But for more than two winding transformers, each winding will have it's own rating. Most three winding transformers have a third winding for stablizing, which will have a rating, but is normally not used. Which would explain both high and low winding with the same rating.
Just beware there are transformers with different ratings for each winding.
RE: Transformer MVA rating
RE: Transformer MVA rating
RE: Transformer MVA rating
Another consideration though, is the action of the tapchanger. By way of example, consider a 33/11kV, 28MVA transformer (sorry I live in the IEC world so ANSI voltage levels do not readily come to mind). Nominal tap = tap7 = 33000 to 11000. Turns ratio is 3.
Maximum tap = tap 17 and corresponding nameplate voltage is 28875kV, i.e. turns ratio is now 2.625.
Now suppose the LV is loaded to rated capacity, i.e 28MVA. ILV = 1470A (assuming the LV voltage is about nominal voltage). IHV = 560A. HV voltage is at system voltage which is 33kV. Thus MVA in HV winding is 1.732 * 560 * 33kV = 32MVA. 4MVA is thus lost in the transfer of power from HV to LV! Admittedly this is a hypothetical question and in reality the actual discrepancy will be determined by %Z, power factor of %Z, tap position, power factor of load, etc. It may not be as much as 4MVA but 1MVA would be quite possible.
Regards.
RE: Transformer MVA rating
RE: Transformer MVA rating
I've tested this with two engineering software packages (CAPE and Powertools) a while ago and it confirms what I explained.
The HV (source)voltage is not a function of the tap position but the source. LV voltage (load end) is regulated to within a certain deadband by the AVR. current and subsequently the MVA is a product of the volts and amps on each side, where the amps must differ by the turns ratio.
RE: Transformer MVA rating
Yes it is possible to exceed the ratings of the LTC before you exceed the ratings of the transformer. Or if the NLTC is not on the center tap. This is a minor point as most transformer peak loading is less than 4 hours.
Even on generator step-up transformers can operate over ratings for some time before there is a problem, or significent loss of life (Only happens if the generator and turban are also overloaded).
A good maintenance program can extend the life of a transformer by years, which is what should happen if the transformer is at an attended facility. And if not, it is likely a smaller transformer where the cost of a program can't be justified.
RE: Transformer MVA rating