Whenever the transformer is loaded, the current in the windings inherently produces the heat. This heat, if not dissipated, will cause damage to the windings cores and other accessories in the tank. Sometimes nearly the tank surface is enough to dissipate the heat without extra provisions but it is only for small transformers. In large-capacity transformers, extra means are also employed. These are the following methods of cooling:
(i) Natural air cooling. It is also known as dry type cooling. In this method the outer container dissipates the heat. This type of cooling is only suitable for the transformers up to 5-10 k VA, extra means of cooling are not provided, only the natural air is sufficient.
Fig. 18.6. Natural air cooling.
(ii) Oil immersed natural aircooled. Transformers up to 500 kVA ratings are designed for Oil immersed natural air cooled. In this case the cotes and winding are immersed in the tank of oil. The heat developed in the windings and cores, is conducted by the circulation of oil to the surface of the tank. This heat is then dissipated to the atmosphere. The area of heat dissipation is also increased by providing the corrugated sheets and other tubular constructions. The oil after getting hot, circulates and comes up, naturally the cold oil from the walls and tubes of the tank will occupy the empty space. Thus the circulation of oil will help in cooling the transformer. The oil also provides the additional insulation for the windings. For small transformer the smooth surface is sufficient, for medium transformer the ribs are provided to increase the dissipating area and for large transformers the dissipating area is increased by providing the tubes and radiators.
Fig. 18.7. Oil immersed forced air cooling.
Oil immersed forced air cooling. In this system the windings and cores are immersed in Oil and cool air is forced on the tank surface to increase the rate of heat dissipation It is used with the transformers of big capacities.
Fig. 18.8. Oil immersed forced water cooling,
(iv) Oil immersed forced warer cooling. In this case also the cores, windings are immersed in oil. The cooled water is then circulated in the tank through the circular tubes provided. Thus the cold water brings out the heat from the oil and oil is cooled which in other words will control the temperature of oil, core and windings. In this case the main drawback is that the leakage may cause the serious effect over the dielectric strength of the transformer oil. Obviously the dielectric strength is inversely proportional to the moisture.
Oil immersed forced oil cooled. In this case the cores and windings are immersed in the oil. In this type instead of the cold water the cooled oil is forced through the circular tubes. Thus this oil brings out the heat and cooling is obtained. In this case the danger of water leakage is also eliminated because the oil is also having the same dielectric strength, which will not effect the transformer oil in case of any leakage if it is caused.
Air blast cooling. In this case air is passed through the cores and windings of the transformer. The air brings out the heat because the air velocity is high enough. Thus heat is dissipated. It is a dry transformer. In this case air should be dry and free from moisture and dust particles.
The transformers up to 10 MVA are oil immersed radiator type natural air cooled, and above 10 MVA generally employed with air blast cooling radiator type.