Cooling method of transformers

Types of Transformer Cooling: 

Various losses taking place in a transformer are of the order of 1% of its full load kW rating. These losses get converted into heat. This heat increases the temperature of windings, core, oil as well as tank of the transformer. The heat is dissipated from the transformer tank and the radiator into the atmosphere. Various cooling techniques are used to maintain the temperature rise of various parts within the permissible limits. The transformer oil is used for cooling as well as insulation purpose. Cooling is provided by circulation of the coil. The smaller transformers do not use oil. So for them the cooling is provided by the natural thermal convection (this is called as air cooling). For larger transformers natural air cooling is not sufficient. Hence the forced air cooling is used where fans are used to take away the heat. For still larger transformers the cooling oil is circulated in the transformer parts by means of oil pumps. A number of different combinations of natural cooling, forced air or oil cooling are available and are used as per the size of the transformer and the demands of a particular application. 

Air Natural (A.N.) Type Transformer Cooling :
This method is used for small dry type transformers. The air | in the surrounding vicinity of the transformer is used for cooling. This method is suitable for transformers upto a rating of 25 kVA. 

Air Forced (A.F.) Type Transformer Cooling : 
This cooling method also is suitbale for the dry type transformers of slightly higher ratings. The air is forced upon the tank surface to increase the rate of heat dissipation. Fans are turned on automatically to facilitate forced air cooling whenever the winding temperature rises beyond the permissible limit.

Oil Natural Air Natural (O.N.A.N.) Type Cooling : This type of cooling is used for oil filled transformers upto a rating of about 30 MVA. As the oil in the tank gets heatec due to heated up core and windings it (oil) flows upwards ane the cold oil comes down as per the principle of convection. The heat is transferred from various parts of the transforme to the oil and the heated oil is cooled by the natural air. It is possible to increase the cooling area by providing th cooling tubes. The O.N.A.N. cooling is as shown i Fig.
Oil Natural Air Forced (O.N.A.F.) Cooling:
In this method, the transfer of heat from various parts of a transformer takes place naturally as explained for the O.N.A.N. type cooling. However the hot oil is now cooled by means of fans mounted near the transformer as shown in Fig. The forced air from the fans is directed to the cooling tubes to increase the rate of cooling. A thermostat is used to automatically sense the temperature of oil and windings and turns the fans on when it increases above the permissible limit.

Oil Forced Air Forced (O.F.A.F.) Cooling :
This type of cooling is preferred for the transforrers of ratings above 60 MVA. This type of cooling is illustrated in Fig. A separate cooler is mouted away from the transformer tank as shown. This cooler is connected to the transformer with pipes at the bottom and top. The oil is circulated from the 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 big transformer used at the substations and power stations.

Satish jagtap

Hello iam Satish jagtap Electrical engineer. From: Aurangabad, Maharashtra. Edu: MIT college of Engineering.

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