The transformers get heated due to iron and copper losses occurring in them. It is necessary to dissipate this heat so that the temperature of the windings is kept below the value at which the insulation begins to deteriorate. The cooling of transformers is more difficult than that of rotating machines because the rotating machines create a turbulent air flow which assists in removing the heat generated due to losses. Luckily the losses in transformers are comparatively small. Nevertheless the elaborate cooling arrangements have been devised to deal with the whole range of sizes.
As far as cooling methods are concerned, the transformers are of following two types:
- Dry type.
- Oil immersed type.
Dry Type Transformers. Small transformers upto 25 kVA size are of the dry type and have the following cooling arrangements:
(i) Air natural. In this method the natural circulation of surrounding air is utilized to carry away the heat generated by losses. A sheet metal enclosure protects the winding from mechanical injury,
(ii) Air blast. Here the transformer is cooled by a continuous blast of cool air forced through the core and windings (Fig. 12). The blast is produced by a fan. The air supply must be filtered to prevent accumulation of dust in ventilating ducts.
Oil Immersed Transformers. In general most transformers of oil immersed types. The oil provides better insulation than air and it is a better conductor of heat than air. Mineral oil is used for this purpose.
Oil immersed transformers are classified as follows:
(i) Oil immersed self-cooled transformers. The transformer is immersed in oil and heat generated in cores and windings is passed to oil by conduction. Oil in contact with heated parts rises and its place is taken by cool oil. From the bottom. The natural oil transfers its heat to the tank walls from where heat is taken away by the ambient air. The oil gets cooler and falls to the bottom from where it is dissiptated into the surroundings. The tank surface is the best dissipator of heat but a plain tank will have to be excessively large, if used without any auxiliary means for high rating transformers. As both space and oil are costly, these auxiliary means should not increase the cubic capacity of the tank. The heat dissipating capacity can be increased by providing (i) corrugations, (ii) fins (iii) tubes (Fig. 13) and (iv) radiator tanks.
The advantages of ‘oil natural’ cooling is that it does not clog the ducts and the windings are free from effects of moisture.
(i) Oil Immersed forced air-cooled transformers. In this type of cooling, air is directed over the outer surfaces of the tank of the transformer immersed in oil.
(ii) Oil immersed water-cooled transformers. Heat is extracted from the oil by means of a stream of water pumped through n metallic coil immersed in the oil just below the top of the tank. The heated water is in turn cooled in a spray pond or n cooling- tower.
(iii) Oil immersed forced oil cooled transformers. In such transformers heat is extracted from the oil by pumping the oil itself upward through the winding and then back by way of external radiators which may themselves be cooled by fans. The extra cost of oil pumping equipment must of course be economically justified but it has incidentally the advantages of reducing the temperatures different between the top and bottom of enclosing tank.
Fig. 14 shows the cooling of transformers having capacities from 10000 kVA and higher. In such cases air blast cooling of radiator is used.