When the transformer is in operation, heat is generated due to copper losses in the windings and iron losses in the core. The removal of heat from the transformer is known as cooling of the transformer.
For the dry type transformers
For oil immersed type transformers
The air natural (AN) cooling is used in dry type, self-cooled transformers. In this method, the natural circulation of surrounding air is used for cooling of the transformer. The windings of the transformer are protected against the mechanical damages by a sheet metal enclosure. The air natural cooling is used for low voltage small transformers upto a few kVA.
The air forced (AF) cooling is used in the dry type, forced-air cooled transformers. In this method, a continuous blast of filtered cool air is forced with the help of a fan through the core and windings of the transformer for cooling. This method is used for cooling of the transformers upto 15 MVA
The oil natural air natural cooling is used in oil immerged type transformers. The most of the transformers of the medium and large ratings have their core and windings immerged in dielectric oil, which acts both as cooling medium and insulating medium.
The oil immerged transformers are enclosed in a sheet steel tank. The heat generated in the core and windings being transferred to the oil. The heated oil becomes lighter and rises to the top and the cool oil takes its place from the bottom of the transformer tank.
The heat of the oil is transferred to the walls of the tank by natural circulation of the oil and the heat is then transferred to the surrounding air through the natural radiation and convection. Hence, the oil gets cooler and falls to the bottom. Therefore, a natural circulation of oil takes place for the cooling of the transformer.
In oil natural air forced cooling of the transformer, the heat generated by the core and windings of the transformer is transferred to the walls of the tank and to the radiator through the natural circulation of the oil. Now, the forced air is directed over cooling elements (tank, radiator, tubes, fins etc.) of the transformer. Hence, the transformer is cooled by the natural circulation of oil and blast of air.
This method of cooling is suitable for the large transformers upto 60 MVA.
In this method of cooling, the heated oil is circulated from the top of the transformer tank to a heat exchanger and the blast of air is forced through the heat exchanger by turning on a fan. Then, the cool oil is returned to the bottom of the transformer tank.
This method of cooling is used for higher rating transformers like transformers which are used in substations and generating stations.
In the oil forced water forced cooling method, the heated oil is circulated from the top of the transformer tank to a heat exchanger where the pressurised water is used to separate the heat from the oil. The cool oil is then returned to the bottom of the transformer tank.
This type of cooling is mainly used for very large transformers with a 100 MVA ratings like at generating stations.