How Does the Cooling Class Affect the Transformer Rating?
Quick answer — The cooling class (ONAN, ONAF, OFAF, OFWF/ODAF) directly sets the rated output. Natural cooling (ONAN) is the lowest rating, and adding fans (ONAF) or pumps (OFAF) raises the rating of the same unit for a given temperature rise. The nameplate gives the rating per cooling stage, e.g. 40 MVA ONAN and 60 MVA ONAF. A better cooling class gives a higher rating, but uses auxiliaries that need a supply.
How does the cooling class affect a transformer rating?
ONAN, ONAF, OFAF
ONAN is fully natural and lowest-rated; ONAF adds fans; OFAF/ODAF adds both fans and pumps, giving a higher rating for the same unit.
Multi-stage rating
A unit is often nameplated at different ratings per cooling stage (e.g. ONAN and ONAF), and the load is set by which stage is running.
The trade-off
A higher cooling class gives a smaller, lighter unit but requires auxiliary power and a loss of auxiliaries handling; the design and the loading guide set the safe use.
References & standards
- IEC 60076-2
- IEC 60076-7
Related
Related manufacturers
Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp
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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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