Every oil-immersed transformer converts a small fraction of its throughput into heat in the core and windings. The cooling class describes how that heat is carried away. IEC 60076-2 uses a four-letter code.
## Reading the code
- First letter — internal medium. O: mineral oil or synthetic liquid with a fire point up to 300 °C. K: insulating liquid with a fire point above 300 °C (for example many natural or synthetic esters).
- Second letter — internal circulation. N: natural thermosiphon flow. F: forced (pumped) circulation. D: forced and directed through the windings.
- Third letter — external medium. A: air. W: water.
- Fourth letter — external circulation. N: natural convection. F: forced (fans or pumps).
## ONAN
Oil circulates by natural convection through radiators or corrugated fins, and air flows over them naturally. There are no fans to maintain and no auxiliary supply needed, which is why ONAN dominates distribution and smaller power transformers.
## ONAF
Fans blow air across the radiators when temperature rises, raising the rating the same transformer can carry. Units are typically dual-rated, for example ONAN/ONAF, with the higher rating available once the fans are running. It is a cost-effective way to cover peak loads, at the cost of fan maintenance and an auxiliary supply.
## Choosing
Consider the load profile, site ambient, available auxiliary power and maintenance capability. For heavily cycled renewable plants, the thermal design should be checked against the real load profile rather than only the nameplate rating.
General engineering information.