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Describe any two methods for transformer cooling.

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Methods of Cooling of Transformer: - 

i) Oil Natural Air natural (O.N.A.N.):- This type of cooling is used for transformer upto rating of 30 MVA. The basic structure of transformer is immersed completely in the oil kept in transformer tank. When transformer gets loaded, the windings and core gets heated, the generated heat is absorbed by the oil as per the principle of convection. The heated oil is being cooled by the natural air. For effective cooling of oils is made by providing cooling tubes to the transformer tank 

ii) Oil Natural Air forced cooling (O.N.A.F):- In this method the transfer of heat from the various parts of transformer takes place naturally like O.N.A.N. type cooling. However the cooling fans are used which are mounted below or near the transformer, the forced airs from these fans are directed to the cooling tubes of transformer tanks. These improve the rate of cooling.

iii) Air Natural cooling (A.N):- This type of cooling is used for small dry type transformers. The air in the surrounding vicinity of the transformer is used for cooling. This type is suitable for transformers upto a rating of 25 KVA. 

iv) Air forced cooling (A.F):- This type of cooling is suitable for dry type transformers of slightly higher ratings. The air is forced upon the bank surface to increase the rate of heat dissipation. 

v) Oil forced Air forced cooling (O.F.A.F):- This type of cooling is used for the transformers of ratings above 60MVA. A separate cooler is mounted away from the transformer tank and this cooler is connected to the transformer with pipes at the bottom and top. The oil is circulated from transformer to the cooler with the help of an oil pump. This oil is then subjected to forced air cooling with the help of fans installed inside the cooler. The O.F.A.F type of cooling is used for the high rating transformer at the substation and power station.

vi) Oil forced Water forced cooling (O.F.W.F):- This type of cooling is used for the large transformers, and cooling needs a heat exchanger. Inside heat exchanger the heat from the oil is transferred to the cooling water. The cooling water is taken away and cooled in separate coolers. The oil is forced to circulate through the heat exchanger by using a pump. This type of cooling is used for transformers having rating of few MVA (generating transformer)

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