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Transformer Impedance Reference

Transformer Impedance Reference

Transformer Impedance Reference

I have seen 3 different ways of displaying transformer impedance(Z%).

1. Z% based on reference temperature.
2. Z% based on temperature rise.
3. Z% based on capacity.

Are there additional ways to display Z%?
What determines which of these is displayed on the transformer nameplate?
Are these values interchangeable? If not, can you convert between the different values?


RE: Transformer Impedance Reference

Per IEEE C57.12.00-2015: The base kVA is the self-cooled rating and the temperature portion states the "impedance shall be referred to the reference temperature defined as 20 degrees C plus the rated average winding rise."

If you are in the IEC part of the world, I think they do things a bit differently.

RE: Transformer Impedance Reference

Z% (actual Ohms)
Z% based on a stated MVA base (which is not necessarily the Txf MVA rating)
IEC based country (Australia). The second is the more common

RE: Transformer Impedance Reference

% of actual ohms? 100%. Not very useful.

In my experience it is based on the lowest rating of a transformer. This is usually the self-cooled rating, but some transformers don't have such a rating. The mobiles I'm familiar with have a single FOA rating.

RE: Transformer Impedance Reference

Transformer impedance is generally expressed as % based on the rated KVA of the transformer and a reference temperature. The effect of reference temperature is negligible with Power Transformers as % R is extremely low compared to % X. % impedance= sqrt (square of %R + square of %X) Only R varies as per temperature.
IEEE - Rated KVA = output of TRF; % impedance based on lowest KVA rating ie ONAN rating; reference temp 20 C + average winding rise ( ie 85 or 95 C)
IEC - Rated KVA is the input of transformer; % impedance based on highest rating ie ONAF or OFAF rating; reference temp defacto 75 C; but option is there as per IEEE practice.
Impedance can also be expressed as per unit basis based on % impedance or in ohms referring to voltage side of trf.

RE: Transformer Impedance Reference

Comprehensive reply. Star for you, Sir!

RE: Transformer Impedance Reference

One more interesting point- %R is not ohmic resistance of winding alone; R from stray loss in metal parts inside tank are also added. % R = load loss / rating in KVA x 100. Ohmic resistance of winding will increase with increasing temperature. But stray losses ( may be 10- 40 % of load loss; hence R from it) will decrease with increasing temperatures. So net effect on R due to change in reference temperature is not appreciable.

RE: Transformer Impedance Reference

Thank you for the responses. This is making more sense now.

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