IEC 60076-2 describes liquid-immersed cooling with a four-letter code: the internal cooling medium, how it circulates, the external medium, and how that circulates. ONAN — oil natural, air natural — covers most distribution transformers, ONAF adds fans, and dry-type units use the two-letter codes AN and AF.
Decoding the four letters
The first letter is the internal liquid: O for mineral oil or a synthetic liquid with a fire point at or below 300 °C, K for a liquid with a fire point above 300 °C — typically a synthetic or natural ester — and L for a liquid with no measurable fire point. The second letter is its circulation: N for natural thermosiphon flow, F for forced by pump, D for forced and directed through the windings.
The third and fourth letters repeat the exercise for the external medium: A for air or W for water, then N, F or D. KNAN is therefore an ester-filled transformer with natural circulation on both sides, while ODAF describes a large power transformer with pumped, directed oil flow and fan-cooled radiators.
What each class is good for
- ONAN — silent, no auxiliaries, nothing to fail; the default for distribution units up to several MVA.
- ONAF — the same active part with fans, typically 25 – 33 % more rating when they run, at the cost of noise and an auxiliary supply.
- OFAF and ODAF — pumped oil for large power transformers, where natural flow can no longer remove the heat.
- KNAN and KNAF — ester fluid, chosen for fire safety and biodegradability, indoors or near water.
- AN and AF — cast-resin dry-type with natural or fan-assisted air; AF typically adds around 40 % rating.
- OFWF — water-cooled, found on ships and in plants that already have a cooling circuit.
Dual ratings and what they cost
A dual-rated transformer is written as ONAN/ONAF with two power figures, for example 1 600/2 000 kVA. The first is what it delivers with no auxiliaries running, the second with the fans energised. This is an economical way to buy future headroom: the core and windings are already there, and fans plus control can often be retrofitted if the tank was prepared for them.
The catch is that the second rating depends on equipment that can fail. Fans need an auxiliary supply, a winding-temperature controller and periodic maintenance. Set the protection so that losing the fans reduces the permitted load rather than causing an unnoticed overheat, and remember that ONAF operation adds noise — a real constraint in urban substations.
The surroundings decide whether the class works
A cooling class is a promise about heat transfer, and it only holds if the room cooperates. A substation needs enough free ventilation area to carry away the total losses, clearance around the radiators, and no recirculation of hot exhaust air back to the inlet. The IEC reference conditions are a 40 °C maximum ambient, a 30 °C daily average and a 20 °C yearly average; above roughly 1 000 m altitude the rating must be corrected downwards.
Temperature-rise limits go with the class. For mineral-oil units they are typically 60 K top-oil rise and 65 K average winding rise. Exceeding them trips nothing immediately — it simply ages the paper insulation faster, which is why continuous temperature monitoring pays for itself on any unit that matters.
Specify ONAN wherever the load allows it: nothing to power, nothing to service, nothing to hear. Move to ONAF when the peak is short and occasional, or when you want a documented path to more capacity in the same footprint, and consider KNAN where fire load or spill risk drives the decision. Protrafo Insights measures top-oil and winding temperature continuously, so you can see whether the cooling class you specified is delivering what the nameplate promises.
