Hi i am Naqibullah student in lovely professional university i would like to get information about cooling methods of transformer which i got this topic in my term paper .

As the size and capacity of the transformer increased, the associated cooling arrangement become more powerful and sophisticated. So, by definition, the transformer cooling system is such arrangement for power transformers, which limits the generated heat into a safe value by means of proper dissipation of generated heat. Different cooling system is used for different types of transformers, and they are discussed as follows.

Generally, two types of transformers are there according to the use of insulating oil, namely Dry Type Transformers and Oil Immersed Type Transformers. In oil immersed type, the transformer core is immersed into the transformer oil. Different types of cooling are needed for these two categories. In dry type transformers, air is used as the coolant medium but in oil immersed transformer (as the size and ratings both are high), both air and transformer oil are used as the coolant medium.

Air Natural Type:

This cooling method is used in dry type transformer with smaller ratings. As the name implies, the natural circulation of atmospheric air is used in this technique. This type of transformers is also referred as self-cooled transformer. When the transformer is operated in full load, then the temperature of the transformer becomes greater than the ambient air temperature. So, by convection process, the light and heated air gets replaced by the heavy and comparatively cool surrounding air. In this way, the generated heat is dissipated via the natural air circulation process. But this type of cooling arrangement provides satisfactory operation for low voltage transformers only.
Air forced type:

This cooling method is also used in dry type transformer but the application is also implemented in oil immersed transformers. As the name implies, in addition with natural air circulation, cool air with high velocity is provided to the core. High speed fans are provided with the transformer, and by the rotation of this fan high velocity of air is subjected to the transformer. This additional air force ensures quicker heat dissipation of the transformers. The fans are automatically controlled, that is when the temperature of the transformer core goes beyond the safe limit than all the fans are switched ON. Air forced cooling method provides better performance than natural air cooling, but additional cost is associated for the fans.
Oil Natural Air Natural (ONAN) Cooling Of Transformer:

This cooling system is used in oil immersed type transformers. This is the simplest way of cooling of oil immersed transformers. We know that, the transformer core is immersed in transformer oil. When the transformer is heated up, then temperature of oil near to the transformer core is also raised. So, the light and heated oil flows in upward direction and comparatively cool and heavy oil takes the vacuum places by natural convection process. And the heated oil releases its temperature into the atmosphere. In this way, a natural oil circulation cycle is generated and this cycle continues until the transformer temperature is tapped into a safe limit. In this method, the surface area of the oil tank is usually larger, as more surface area provides more quick heat dissipation process. But to provide more surface areas, several hollows tubular plates are attached with the transformer and they are termed as radiator. When the hot oils are circulating through the radiators, they get more surface area so the cooling rate is much higher. Also the light and heated air gets replaced by the heavy and comparatively cool surrounding air by natural process. In this way, the generated heat is dissipated via the natural air circulation process as well as natural oil circulation.