The following methods are generally used to cool transformers.
1. Air Natural (AN) Cooling
In a dry-type self-cooled transformer, the natural circulation of the surrounding air is used for its cooling. The windings are protected from mechanical injury by a sheet metal enclosure. This type of cooling is satisfied tory for low-voltage small transformers to a few kVA.
2. Air Blast (AB) Cooling
The dry-type forced air-cooled transformer is similar to that of the dry-type self-cooled transformer with the addition that continuous blast of filtered cool air is forced through the core and windings for better cooling. The blast is produced by a fan.
3. Oil Natural (ON) Cooling
The majority of trans large rating have their windings and core immersed in oil which acts both as a cooling medium and an insulating medium. Oil-immersed transformers are enclosed in a sheet-steel tank. The heat produced in the cores and windings is passed to the oil. Heated oil becomes lighter and rises to the top and its place is taken by cool oil from the bottom of the tank. The heat of the oil is transferred to the walls of the tank by the natural circulation of the oil. The heat is then transferred to the surrounding atmosphere through natural radiation and convection. The oil gets cooler and falls to the bottom. Thus, a continuous natural circulation of oils takes place.
Plain tanks are economical to a rating of 25 kVA. Above this rating, the large cooling surface is generally provided by using corrugations, fins, tubes (circular or elliptical) and radiator tanks.
4. Oil Blast (OB) Cooling
In this type of cooling, forced air is directed over cooling elements of the transformer immersed in oil.
5. Forced Oil and Forced Air Flow (OFB) Cooling
Oil is circulated from top of the tank of the transformer tank to a cooling plant. Cool oil is then returned to the bottom of the tank.
6. Forced Oil and Water (OFW) Cooling