Impedance on IEC transformer nameplate 2

[guiyermo]

I have a question about the impedance value in percent that is found on transformers under IEC standards.

As I understand a 32/40 MVA ONAN/ONAF, 20/6.9 kV with and Z=12%, had its impedance refered to lowest power rating, 32 MVA in this case.

For ANSI transformers the previous is true, but I have found reviwing a short circuit study that the impedance value was referred to the highest power rating (40 MVA), the reason: it was built under IEC standards.

I am not sure about this statement, I am currently reviewing the IEC 60076 standards and I could not found anything related.

I would appreciate your support or a reference that could help me with this issue.

Thank you very much

 

[dpc]

I have run into this in the past and my understanding is that the impedance given for an IEC transformer is generally based on the highest forced cooled rating on the nameplate.

The nameplate often gives the base kVA for the impedance in any event.

I have an old ANSI/IEC comparison chart from Waukesha Transformer that confirms this.

I'm sure someone will have a ready reference.

 

[lz5pl]

According to me in your case transformer impedance should be referred to 40 MVA value. 32 MVA is just the maximum power allowed in case of operation without forced cooling. Using transformer in ONAN mode doesn't change physical parameters of the machine.
I cannot confirm this from 60909 standard anyway, it is just my understanding of the problem.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Marmite]

Text from IEC60076:-
4.1 Rated Power
If different values of apparent power are assigned under different circumstances, for example, with
different methods of cooling, the highest of these values is the rated power if not otherwise agreed between
manufacturer and purchaser before an order is placed.

5.5 Specification of short circuit impedance.
Percentage values may lead to
confusion because of varying practices concerning reference values. Whenever percentage values are given
it is advisable that the corresponding reference power and reference voltage values be explicitly indicated.

The rating plate is meant to indicate the reference voltage and power at which the percent impedance is calculated so as to avoid confusion.

Regards
Marmite

 

[slavag]

At Siemens power transformer cataloge show impedance for ONAN and for ONAF separetly.
Regards.
Slava