Power Transformers with OFAF rating & % Impedance Volts
Power Transformers with OFAF rating & % Impedance Volts
(OP)
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.
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.






RE: Power Transformers with OFAF rating & % Impedance Volts
Regards
Marmite
RE: Power Transformers with OFAF rating & % Impedance Volts
RE: Power Transformers with OFAF rating & % Impedance Volts
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.
RE: Power Transformers with OFAF rating & % Impedance Volts
However, best way is to go through OEM factory test results / documentation and get absolute value and then use it.
NC
RE: Power Transformers with OFAF rating & % Impedance Volts
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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RE: Power Transformers with OFAF rating & % Impedance Volts
RE: Power Transformers with OFAF rating & % Impedance Volts
"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.
RE: Power Transformers with OFAF rating & % Impedance Volts
RE: Power Transformers with OFAF rating & % Impedance Volts
RE: Power Transformers with OFAF rating & % Impedance Volts
Thanks to you too, dpc. Your memory must be great.