impedance of traformer

20MVA which is the 100% rating of the tx....The ONAF rating is 125% of the normal tx rating.

But when you specify for ONAN/ ONAF transformer in your specification, Manufacturer will design transformer ( ie conductor size ,core size etc and hence resistance and reactance ) based on ONAF rating ,hence i think impedance is based on ONAF rating?

What do you say?

Per IEEE red book and ANSI C57 you always use the self cooled rating for all calculations. 20 MVA

But Transformer design (that is determination of resistance and reactance and hence impedance ) will be on which rating ONAN or ONAF?. As higher MVA rating (ONAF) will cause more current. As current will decide decide conductor size hence resistance , and Higher MMF (Ampere turns ) will also affect the reactance (ie design of core, and clearances etc).

You use the self cooled rating 20 MVA. Do you know how the %Z is determined?

It is always the base rating ie 20MVA

The % impedence whether on 20 MVA or 25 MVA depends upon which standard (IEC or ANSI) is used by transformer manufacturer. % impedence is on the base rating (self cooled rating ) as per IEEE red book or C57.12.01. Even the rated KVA of transformer is output power at rated secondary voltage for transformer manufactured as per ANSI. For transformers manufactured as per IEC (almost entire world except USA) the rated KVA is the highest KVA rating (In our case 25 MVA) and it is the input KVA of the transformer. As per IEC manufacturer has to specify the impedence alongwith KVA (on which it is based). Normally it is based on rated KVA which is highest KVA rating of the transformer for IEC based transformers.