Power Transformers with OFAF rating & % Impedance Volts 2

[RRaghunath]

Power Transformer impedance volts is specified at rated kVA rating as per IEC60076.
Rated kVA corresponds to OFAF rating when the transformer is equipped with multi-cooling.
Is this correct or should the impedance rating correspond to base rating of the transformer (ONAN).

Thanks.

 

[Marmite]

I've seen both, Raghun. It would be best to check the rating plate, and/or the spec if you have access to them.
Regards
Marmite

 

[jghrist]

I'm not familiar with IEC specs, but if it were ANSI, the impedance would be at the base ONAN rating.

 

[dpc]

As jghrist says, if the transformer is built to ANSI standards, the impedance is based on the self-cooled rating.

My understanding and experience is that IEC impedance is based on the highest forced-cooled rating of the transformer. But there may be exceptions to this, so you should verify.

This is generally spelled out on the nameplate.

 

[NCTHAI]

Assuming that OFAF rating will be higher than ONAN rating, It is desirable to use OFAF for all protection calculation and impedance should be measured as % of OFAF rating.

However, best way is to go through OEM factory test results / documentation and get absolute value and then use it.

NC

 

[ScottyUK]

NCTHAI,

Can you explain that a little more clearly? The ohmic impedance of the transformer doesn't change with either cooling method so, glancing at an SLD in front of me for some typical values, a 24/48MVA ONAN/ODAF transformer has an impedance of 9% on the 24MVA base and 18% on the 48MVA base. Either pair of values would yield the same results in a fault level study, so I can't see why it would be advantageous to use the OFAF value in calculations.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Agree with ScottyUK, dpc and jghrist.

 

[RRaghunath]

dpc,

"My understanding and experience is that IEC impedance is based on the highest forced-cooled rating of the transformer."

Thanks. I too think so. But my serach of IEC 60076 didn't yield any fruitful result. I am looking for help/confirmation.

 

[dpc]

I don't have any IEC specs. I have an old document from Waukesha that spells out the differences between ANSI and IEC standards. I'll see if I can dig that out on Monday. But the base kVA should be noted on the nameplate.

 

[prc]

Please refer to IEC 60076-1 General.( Ed2.1 2000-04)Under clause 3.7.1 it is said that rated percentage impedance refers to rated power. Under clause 4.1, "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."

 

[RRaghunath]

Thanks, prc. The references are of great value.
Thanks to you too, dpc. Your memory must be great.