When modeling multi-rating transformers using SKM, do you guys stick with the base values or the highest values? For instance, if you have a transformer that has a 30/40/50MVA rating, which would you use in your model?

I've been running them with both sets of data, but I am curious how others see this.

Use the rating for which you have the impedance. The impedance should change at each rating (different ratings have different secondary FLA, so need different levels of primary voltage in the impedance test).

Not sure why you'd have different values unless you are reading the values with/without fans. I always put them in as the highest condition that they will operate at. So if you have fans but never or "hardly ever" use them then you might have to have a separate scenario for overload conditions or simply have a policy/procedure to deal with this case and not use it as part of the model.

I was indeed refering to the ratings with the open air and forced air ratings. If I understand transformer testing at all, it seems that the values given for impedance are at the base values and not the forced air values.

I was really curious to how you guys approached it.

The maximum current the secondary can provide is not a function of fans (which is really what the impedance gives you, the "strength" of the transformer as a source).

Let's me explain with an example.
Transformer: 315 kV to 34.5 kV
%Z: 9.2%
At 30°C ambient:
ONAN: 110 MVA
ONAF: 183 MVA
At 0°C ambient:
ONAN: 137.5 MVA
ONAF: 217.31 MVA

If you model it as something else than 110 MVA (correct for 30°C ambient, ONAN), then you need to adjust the %Z. Else, you change the strength of the transformer.

I fairly certain that the impedance is for the base rating, at the rated taps (if the transformer has primary taps). As VincentB. mentioned, you should use the base rating for SC calculations.

The nameplate %Z is based on the nominal (base) rating (i.e. no fans and no temperature rise) of the transformer. In your case, the 30 MVA rating is what should be used with the nameplate %Z in order to calculate the impedance of the transformer. The higher ratings relate to transformer heating, not its impedance.

Hi, I have a system in which i need to model 1000kv automatic voltage regulator using PTW 6.5v. But i dont see any provision for that. How can i do it and if not possible can i ignore the device or what are the implications and how to solve it. Thanks in advance please help.

Gautham - your question is not germane to the OP's original question and subsequent replies. This should be a new thread in this forum section and I have sent a request to the moderator to re-direct it to a new thread.