Transformer cooling classification — ONAN, ONAF, OFAF, ODAF per IEC 60076-2

The guide on the oil and winding temperature indicator (OTI/WTI) mentions in passing that a power transformer may have "ONAN or ONAF cooling", without explaining the four-letter code itself. This guide covers that code: the classification system standardised by IEC 60076-2 that unambiguously identifies the cooling system of an oil-filled transformer, and why a transformer often has multiple cooling stages, each permitting a different power rating.

The four-letter code

IEC 60076-2 assigns each cooling system a four-letter code, made up of two pairs:

A code such as ONAN therefore means: oil (O) circulates internally by natural thermosiphon action (N), cooled by air (A) that convects naturally (N) — the simplest and most common configuration, without fans or pumps, used for distribution transformers and smaller power transformers.

Multiple cooling stages: the same power rating at lower cost

A large proportion of power transformers at medium and large substations are fitted with more than one cooling stage, which come into operation in sequence as load and oil temperature rise. A common combination is ONAN/ONAF: at lower loads the transformer operates fully passively on natural convection (ONAN), and once the winding temperature crosses a set threshold, cooling fans switch on automatically and the transformer moves to forced-air cooling (ONAF) — permitting a higher rated power for the same temperature rise. Larger power transformers also use OFAF (forced oil circulation via pumps, forced air cooling via fans) and ODAF (directed oil circulation flowing directly along the windings, forced air cooling), each permitting a correspondingly higher power rating than the previous stage.

The nameplate of such a transformer therefore often lists several power ratings side by side, for example "40/53/67 MVA ONAN/ONAF1/ONAF2", where each rating corresponds to its associated cooling stage.

Why this matters for the OTI/WTI

The cooling fans and pumps of the higher cooling stages are in practice often automatically controlled by the same oil and winding temperature indicator (OTI/WTI) that also provides the alarm and trip contacts (see the guide on the OTI/WTI): a temperature threshold that switches on the next cooling stage is usually set lower than the alarm threshold, which in turn is set lower than the trip threshold. The failure of a cooling stage — for example a faulty fan motor on an ONAN/ONAF transformer — therefore limits the maximum permissible power to that of the lower, still functioning stage, even though the OTI/WTI itself may not yet report a temperature alarm.

Practical relevance

When assessing a power transformer, it is relevant to recognise the cooling code on the nameplate and to check whether all listed cooling stages actually function: a transformer nameplated as ONAN/ONAF/ONAF but with a fan group out of service can be structurally overloaded if operation still assumes the full, specified power rating.

Common mistakes

1. Confusing the cooling code with a performance or quality indicator — the code describes only the cooling principle, not the reliability or design quality of the transformer.
2. Assuming the highest power rating on the nameplate is always available — that rating only applies when the associated, highest cooling stage is actually operational.
3. Forgetting that a failed cooling stage reduces the permissible power without automatically triggering a temperature alarm on the OTI/WTI, as long as the load has been reduced to match the lower stage.
4. Mixing up the position of the letters — the first two letters always describe the internal (oil) system, the last two always the external (cooling) medium; swapping them leads to a wrong interpretation of the cooling configuration.

Related

• Oil and winding temperature indicator (OTI/WTI) of a power transformer
• Transformer differential protection (87T)
• Busbar differential protection (87B)