Power transformer cooling methods explained
Cooling keeps a power transformer inside its temperature limits, which is what determines how long the insulation lasts. The method is chosen from the size, rating and duty of the unit. Insulation ageing roughly doubles for every 6 to 8 K of extra hot-spot temperature, which is why a cooling decision is really a service-life decision.
ONAN cooling (oil natural air natural)
ONAN is the basic arrangement for oil immersed transformers. Heat leaves through natural convection of the oil inside the tank and natural air flow over the cooling surfaces.
- Simplest and most reliable cooling method
- No fans or pumps
- Low maintenance
- Suited to small to medium units, up to about 30 MVA
- Typical noise level: 55 to 65 dB
Because there are no moving parts, ONAN has the lowest failure rate and the lowest auxiliary power consumption. Its limit is physical: heat removal depends on the temperature difference between the tank surface and the air, so capacity cannot be increased without more surface area.
ONAF cooling (oil natural air forced)
ONAF adds fans mounted on the radiator, which raises cooling capacity by 30 to 50% over ONAN.
- Forced air fans on radiators
- Capacity 30 to 50% above ONAN
- Fan control driven by oil temperature, with automatic start and stop
- Suited to medium to large units, roughly 30 to 100 MVA
The practical attraction of ONAF is that it gives two ratings in one unit: the ONAN rating with fans off, and a higher ONAF rating with fans running. Utilities use this to cover seasonal peaks or a failed parallel unit without installing extra transformer capacity.
OFAF cooling (oil forced air forced)
OFAF uses oil pumps as well as fans, pumping oil through the coolers to remove heat faster.
- Oil pumps plus forced air fans
- Highest cooling capacity of the three
- Suited to large units of 100 MVA and above, including EHV and UHV transformers
- Compact for the rating
Beyond OFAF: OFWF and ODWF
OFWF (oil forced water forced)
Oil pumps circulate the oil, and water coolers remove the heat. Water cooling gives a higher heat transfer coefficient than air, so coolers are smaller, which helps where space is restricted or ambient temperatures are high. The trade-off is that a water supply and its treatment become part of the installation.
ODWF and ODAF
Directed oil flow (OD) routes oil specifically through the winding ducts rather than around them, which targets the hot spots directly. Directed flow is used on the largest transformers, where hot-spot temperature rather than average oil temperature is the limiting factor.
Cooling classes are multi-stage
A large unit is normally specified with several stages, for example ONAN/ONAF/OFAF. Each stage adds cooling capacity, and the nameplate carries a rating for each. This lets the transformer deliver more power at higher load while keeping losses and noise low at normal load.
Choosing a cooling method
Five factors usually decide it:
- Power rating. Small transformers run on ONAN, large ones on OFAF.
- Application. Power plants often need ONAF or OFAF for overload capability.
- Space constraints. OFAF allows a more compact design.
- Cost. ONAN is the cheapest to buy and to run.
- Maintenance. ONAN needs the least.
Additional factors worth checking
- Ambient temperature — a site hotter than the standard reference reduces effective capacity in every cooling class.
- Altitude — thinner air reduces air-side cooling performance, so derating applies.
- Noise limits — fans are the dominant noise source on an ONAF or OFAF unit, and low-noise fans cost more.
- Auxiliary power supply — pumps and fans need a reliable supply; a forced-cooled unit that loses its auxiliaries drops back to a much lower rating.
- Maintenance access — pumps, fans and coolers all need servicing, so clearances have to be designed in.
Cooling method comparison
| Cooling class | Oil movement | Air / water | Typical rating range | Maintenance |
|---|---|---|---|---|
| ONAN | Natural | Natural air | Up to about 30 MVA | Lowest |
| ONAF | Natural | Forced air (fans) | About 30 to 100 MVA | Fan servicing |
| OFAF | Forced (pumps) | Forced air (fans) | 100 MVA and above | Pumps and fans |
| OFWF | Forced (pumps) | Forced water | Large, space-restricted sites | Pumps, water circuit |
| ODAF / ODWF | Directed (pumps) | Air or water | Largest units | Highest |
How cooling interacts with specification
Cooling class cannot be chosen in isolation. It sets the temperature rise the unit will see, which sets insulation ageing, which sets expected life. If the load profile includes long periods near full load, a higher cooling class usually pays for itself in reduced ageing and lower losses; if the load is light and intermittent, the extra auxiliaries add running cost and failure points for little benefit. Our maintenance guide covers the checks that keep cooling equipment working, and our testing laboratory page lists the temperature rise tests that verify the design.
Key takeaways
- Cooling class follows rating and duty, not preference.
- ONAN is the most reliable and cheapest to run; OFAF and above are for large ratings and space-constrained sites.
- Multi-stage classes give more than one rating from the same unit.
- Ambient temperature, altitude and auxiliaries all reduce the cooling actually available.
- Temperature rise is verified by type test, so the test report is the evidence.
Explore our power transformers and oil-immersed transformers, or contact us with your load profile and site conditions.