Due to the heat that a transformer generates during operation, it must always be subject to some type of cooling. The types of cooling in transformers are always distinguished by the type of internal cooling system and the type of external cooling. The internal cooling system comes into contact with the windings and cools them. With external cooling, the heat is removed from the transformer. This means that the cooling system in this case does not have to be designed with regard to the electrical insulation properties. In the case of dry-type transformers operated by convection of ambient air, there is only one cooling system. The types of cooling are distinguished according to the cooling media and the type of coolant, and are subdivided as follows.
A – Air
G – Gas (SF6)
O – OIL
K – liquid with fire point above 300°C
A – Air
W – water
Differentiation type of conduction and transport of the cooling medium
N – Natural convection by heating.
F – Forced flow. In case of forced flow in an oil circuit, the pump is placed in such a way that all the flow also passes through the cooler, since the back pressure is a critical point in the circuit.
D – Forced flow directed specifically at the windings. E.g. high current transformer.