What is the difference between ONAN and ONAF?

Both use natural internal oil circulation. ONAN uses natural external air flow, while ONAF uses fans to force air across the radiators.

Can fans be added to increase any transformer rating?

No. A higher ONAF rating is valid only when the complete transformer was designed, guaranteed and tested for that rating, including windings, leads, bushings and other current-carrying parts.

What happens if ONAF fans fail?

The response depends on actual load, temperature and the nameplate cooling ratings. Operation may need to be reduced to the available ONAN rating in accordance with the protection and operating procedure.

ONAN vs ONAF: Transformer Cooling Methods Explained

ONAN and ONAF describe how heat moves through a liquid-immersed transformer. ONAN uses natural oil circulation and natural external air flow. ONAF retains natural oil circulation but uses fans to force air across the radiators. The cooling designation must be read together with the guaranteed MVA rating and temperature-rise limits.

Code meaning: O identifies the transformer insulating liquid in this cooling designation, N means natural circulation, A means air, and F means forced circulation. The first two letters describe the internal coolant circuit; the last two describe the external cooling medium and its circulation.

How ONAN removes heat

Losses in the core and windings become heat. Warmer oil rises from the active part toward the upper tank and radiator inlet. Heat transfers through radiator surfaces to ambient air, while cooler, denser oil returns toward the bottom of the tank. This thermosiphon process needs no oil pump or radiator fan.

ONAN transformer natural oil and natural air cooling path — ONAN heat path: natural oil circulation inside the transformer and natural air movement around the radiators.

What changes in ONAF operation

In ONAF mode, radiator fans increase air velocity and heat rejection. A transformer may therefore have staged ratings such as an ONAN base rating followed by one or more ONAF ratings. Those ratings are design-specific: they must be stated on the nameplate, guaranteed by the manufacturer and demonstrated under the applicable temperature-rise requirements. Adding fans to an ONAN transformer does not by itself create an approved higher rating.

Illustration of ONAN and staged ONAF transformer cooling ratings — Illustrative cooling stages only. Actual MVA values, fan groups and changeover temperatures are project-specific.

Operating issue	ONAN	ONAF
External air movement	Natural convection	Radiator fans provide forced airflow
Auxiliary supply	No fan supply required	Control and fan power required
Noise	No fan contribution	Fan noise must be included in the site assessment
Failure response	Thermal alarms still apply	Loss of a fan group may require load reduction to the stated available rating
Maintenance focus	Radiator condition, oil path and cleanliness	Also fan motors, guards, contactors, controls and stage logic

Temperature limits are not one number

Transformer loading is governed by the thermal design, ambient conditions and ageing risk. Top-oil temperature, average winding rise and winding hot-spot temperature describe different parts of the thermal condition. A normal top-oil indication does not prove that every local winding hot spot is within limit. Load cycle, previous loading, ambient temperature and cooling availability all influence the result.

Cooling data to include in an enquiry

Required ONAN and ONAF ratings, including each cooling stage
Maximum ambient temperature, daily average and site altitude
Expected load cycle, emergency loading policy and harmonics
Fan supply voltage, redundant groups and preferred control arrangement
Automatic start/stop setpoints, manual control and remote indications
Noise limit, radiator orientation and space available for airflow
Alarm and trip contacts for temperature and cooling-system failure

Common cooling mistakes

Do not block radiator airflow with walls, acoustic panels or storage. Do not assume every fan failure requires an immediate trip; the operating response should follow the transformer’s stated ONAN capability, actual load and temperature protection scheme. Likewise, do not specify a higher ONAF rating without confirming winding, lead, bushing, tap-changer and other current-carrying limits.

Practical conclusion: Specify cooling as a complete operating system—rating stages, ambient conditions, auxiliaries, controls, noise and failure response—not as a four-letter code alone.