ONAN, AN, AF: transformer cooling classes and how fans add 40 % capacity
The four-letter cooling code on the nameplate tells you how heat leaves the transformer. What ONAN and AN mean, why cast-resin units can be boosted with fans, and how temperature rise limits set the rating.
Every watt of loss becomes heat, and the rating of a transformer is nothing more than the load at which that heat can be removed without exceeding the temperature limits of the insulation. The cooling code on the nameplate – ONAN for oil units, AN or AN/AF for cast resin – describes how.
Reading the code
IEC 60076-2 uses four letters: the first pair for the internal cooling medium and its circulation, the second pair for the external medium.
| Code | Meaning | Used on |
|---|---|---|
| ONAN | Oil Natural, Air Natural – oil circulates by convection inside the tank, air by convection over the fins | PNACHO-O 25 – 2500 kVA |
| ONAF | Oil Natural, Air Forced – fans on the radiators | Larger oil units, typically > 5 MVA |
| KNAN | As ONAN, with a K-class (ester) liquid | PNACHO-O with MIDEL on request |
| AN | Air Natural – dry-type cooled by free convection | PNACHO-D 100 – 2500 kVA |
| AF | Air Forced – cross-flow fans under the windings | PNACHO-D with fan kit |
Temperature rise limits
- Oil-immersed: 60 K top-oil rise and 65 K average winding rise above a 40 °C ambient (IEC 60076-2). The hot-spot in the winding may reach about 98 °C at rated load – the design point for a "normal" insulation life of paper in oil.
- Cast resin, class F: 100 K average winding rise, i.e. a winding temperature around 140 °C at full load in a 40 °C room. The resin system is rated 155 °C (class F), with class H (180 °C) available for special designs.
The nameplate rating is therefore valid at 40 °C maximum ambient (30 °C daily average, 20 °C yearly average). Higher ambient temperatures mean a lower permissible load – see our article on derating.
AN/AF: the fan option
Cast-resin windings are cooled by air flowing through the ducts between the HV and LV coils. Forcing that air with fans mounted on the base frame raises the heat transfer enough for a continuous overload of about 40 %: a 1 000 kVA AN unit becomes a 1 400 kVA AN/AF unit. The fans are controlled by the temperature monitoring relay (PT100 sensors in the LV windings) and start automatically at a set winding temperature.
The AF rating is a genuine rating, not an emergency reserve – the transformer may run at 140 % continuously as long as the fans are running. Losses at 140 % load are, however, almost twice the rated load losses (1.4² = 1.96), so the AF mode is an economic choice only for peaks, not for permanent operation.
What this means when you specify
- For a load that will grow, a cast-resin unit with a fan kit is a cheap way to buy a 40 % reserve without buying iron you do not need today. The fan kit can be retrofitted.
- An oil-immersed distribution transformer has no fan option in the standard range – size it for the future load, or plan a parallel unit.
- Check the room: an indoor transformer needs ventilation openings large enough to remove the total losses. Rule of thumb for natural ventilation: free inlet area S ≈ 0.18 · P / √H (S in m², P = total losses in kW, H = height difference between inlet and outlet in m) – about 0.45 m² for a 630 kVA oil unit with a 5 m height difference; see our installation checklist.
- Any enclosure (IP21–IP44) around a dry-type transformer reduces its rating by 5–15 % unless fans are fitted.
Ratings, losses and the AF option for each size are listed in the PNACHO-D range; ONAN oil units are in the PNACHO-O range.
Further reading: How an oil-immersed transformer works? (energeks.com, EN) · Jak działa transformator olejowy? (energeks.pl, PL)
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