When transformer is connected to the supply, some heat is always produced due copper losses (or I2R losses) and core losses (iron losses). The output of the transformer limited by the rise in temperature. So, there becomes a necessity to cool down the transformer for their normal operation.
In rotating electrical machines, cooling fan provided on their shaft. When the machine rotates and thus causes the circulation of our and hence provides cooling. But Transformer is a static device. Therefore, it is difficult to cool down the inner parts of a transformer. The removal of heat from the transformer is called cooling.
The different methods of cooling the transformer are given below

Methods of Cooling of Transformers

Air Natural Cooling

Small transformer of a rating 10 to 15 KVA is cooled by the circulation of natural air through the core and transformer winding. The heat generated in the transformer dissipated by conduction, convection and radiation.

Oil Immersed Natural Cooling (ON Type)

In this case, the assemble of core and winding of a transformer is placed in a tank filled with oil known as transformer oil or insulating oil. The insulating oil or transformer oil performs two function.

  • It provides the necessary insulation to the winding.
  • It helps to cool the transformer. It takes the heat from the core and winding and by natural circulation of the oil it cool down the transformer.
Cooling of Transformers
Cooling of Transformers

The hot oil become lighter in weight and goes up from where comes down through the pipes to the bottom of the tank after cooling.

Oil Immersed Forced Oil-Circulation Natural Cooling (OFN)

In this type, transformer core and windings are immersed in transformer oil and oil is circulated through the transformer with the help of pump and heat extracted by the oil from the core and winding and gives it to the tank surface from where the air takes away the heat.

Air Blast Cooled Transformer (AB TYPE)

In this method of cooling, forced air flow through the core and winding. The transformer is placed over a chamber and air is maintained under pressure with the help of compressor.

Oil Immersed Water-Cooled Transformer (OW TYPE)

The rating above 500KVA forced water cooling is employed. In this method of cooling, the core and transformer winding is placed in transformer oil and cold water passed through the pipe kept in the transformer oil. The cold water absorbs the heat from the oil and carries away the heat of the oil.