ONAN vs ONAF vs OFAF: What Is the Difference?

What do the letters mean?

IEC 60076 classifies cooling by two letters: the first describes the cooling medium (O = oil, F = water), the middle letter the medium in contact with the windings (N = natural, F = forced), and the last letter the secondary / external medium (N = natural air, F = forced air). So ONAN reads "oil-natural / air-natural", ONAF "oil-natural / air-forced", and OFAF "oil-forced / air-forced".

How each cooling regime works

In ONAN the oil circulates by natural convection and heat leaves through the tank and radiator by natural convection and radiation. ONAF adds banks of cooling fans that blow air across the radiators, roughly increasing airside heat transfer by a factor of 1.5 to 2. OFAF adds an oil pump (and often directed oil flow through the windings) so that both the oil circulation and the airside are forced.

Effect on rating

The same physical unit is often nameplated at different ratings per cooling stage, e.g. 40 MVA ONAN and 60 MVA ONAF. The cooling stage is chosen by how the transformer is expected to be loaded and whether the auxiliaries (fans/pumps) can lose supply.

Considerations and trade-offs

ONAN is simple, quiet and fails passive (still works without auxiliaries) but is the biggest, heaviest option. ONAF/OFAF are smaller and lighter for a given rating but consume auxiliary power and reduce some of the overload margin if the fans/pump are lost. OFAF also lets you support higher and more uniform winding temperature for very large transformers that cannot be cooled practically by natural oil flow.

References & standards

• IEC 60076-2: Power transformers — Temperature rise
• IEC 60076-1: General requirements and cooling-code definitions

Related

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

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