(1) The letters indicate the internal cooling medium in contact with the winding:

O -- mineral oil or synthetic insulating liquid with ignition point no more than 300 ℃;

K -- insulating liquid with ignition point greater than 300 ℃;

L -- insulating liquid with undetectable ignition point.

(2) The second letter indicates the internal cooling medium recycling mode:

N -- the oil flow through the cooling equipment and winding is a natural thermal convection cycle;

F -- the oil flow in the cooling equipment is forced circulation, and the oil flow through the inner winding is thermal convection circulation;

D -- the oil flow in the cooling equipment is forced circulation, and (at least) the oil flow in the main winding is forced guided circulation.

(3) The third letter indicates the external cooling medium:

A -- air;

W - water.

(4) The fourth letter indicates the circulation mode of external cooling medium:

N -- natural convection;

F -- forced circulation (fan, pump, etc.).

For example:

Oil immersed self cooling (onan)

Oil immersed air cooling (ONAF)

Forced oil circulation air cooling (ofaf)

Forced oil circulating water cooling (ofwf)