Cooling method for transformer

Why cooling system for transformer very important? The amount of heat from a transformer typically dissipated  is about 12.5 watts per square meter of surface area per degree C.It also can prolong the transformer life span if it running in normal temperature without exceed the maximum limit.

Normally transformer installed in special area or room for power distribution such as sub station or power room.The heat generated from transformer must be removed by circulate the air by ventilation or by the cooling system.

Commonly manufacture use a method of lettering is used to denote the form of cooling for a particular transformer.Four upper case letters are used.The first pair of letters are for the heat removal from the windings and core.

Transformer cooling code

For more detail,please refer the code of cooling method below :-



  • AIR-NATURAL :- Natural cooling by the internal air circulating amongst the winding and core by natural convexion.


  • OIL-NATURAL :- Natural cooling by oil that circulates amongst the windings and core by natural convexion.


  • LIQUID-NATURAL :- As for ON but a synthetic liquid is used.


  • OIL-FORCED :- The oil is circulated by the use of an oil pump, which is usually mounted externally in the lower interconnecting pipework between the external radiator bank and the side of the tank. This method is seldom used in the oil industry because it pertains to very large ratings of transformers.


  • LIQUID-FORCED :- As for OF but a synthetic liquid is used.The second pair of letters are for the external surface heat removal,


  • AIR-NATURAL:- Natural cooling by atmospheric air circulation. The windings and core are directly exposed to the air, as in the case of a dry-type or resin insulated transformer.



  • AIR-FORCED:- Air forced cooling is arranged by using fans and trunking on the outside of the transformer. These can be applied to dry-type or liquid insulated transformers


For safety purpose,normally transformers are usually fitted with devices to indicate the temperature of liquid, windings and the core.Usually it used a thermometers,temperature detectors or thermocouples.

Signals from these devices are used to trip the feeder switchgear in the event of excessive temperature.It can avoid from the serious damage for the transformer.


  1. haitham hashim says

    I like your style in summarizing the topic

  2. Air Natural cooling (A.N. Cooling)
    Air Blast Cooling (A.B. Cooling)
    oil immersed Natural cooling ( ON cooling)
    Oil Immersed forced air cooling (OB type)
    Oil immersed water cooling ( OW type)
    oil forced natural cooling (OFN type)

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