Transformer Cooling Methods
Losses in the transformer are of the order of 1% of its full load kW rating. These losses get converted in the heat thereby the temperature of the windings, core, oil and the tank rises. The heat is dissipated from the transformer tank and the radiator in to the atmosphere. Transformer cooling helps in maintaining the temperature rise of various parts within permissible limits. In case of Transformer, Cooling is provided by the circulation of the oil. Transformer Oil acts as both insulating material and also cooling medium in the transformer. For small rating transformers heat is removed from the transformer by natural thermal convection. For large rating transformers this type of cooling is not sufficient, for such applications forced cooling is used.
As size and rating of the transformer increases, the losses increase at a faster rate. So oil is circulated in the transformer by means of oil pumps. Within the tank the oil is made to flow through the space between the coils of the windings.
Several different combination of natural, forced, air, oil transformer cooling methods are available. The choice of picking the right type of transformer cooling method for particular appilcatiion depends on the factors such as rating, size, and location.
Transformer Cooling Methods
Different Transformer Cooling methods are:
Air Cooling For Dry Type Transformers:
- Air natural Type (A.N.)
- Air Forced type (A.F.)
Cooling For Oil Immersed Transformers:
- Oil Natural Air Natural Type (O.N.A.N.)
- Oil Natural Air Forced Type (O.N.A.F.)
- Oil Forced Air Natural Type (O.F.A.N.)
- Oil Forced Air Forced Type (O.F.A.F.)
Oil immersed Water Cooling:
- Oil Natural Water Forced (O.N.W.F.)
- Oil Forced Water Forced (O.F.W.F.)
Air Natural Transformer Cooling:
This type of Transformer Cooling method applies to dry type transformer of small rating. The surrounding air in the vicinity of the transformer is used for cooling. Small transformers below 25kVA can be readily cooled by air natural cooling. However air natural cooling is also used for large dry type transformers. Cooling takes place by convection air currents.
Air Forced Transformer Cooling:
This type of cooling is provided for dry type transformers. The air is forced on to the tank surface to increase the rate of heat dissipation. The fans are switched on when the temperature of the winding increases above permissible level.
Oil Natural Air Natural Transformer Cooling:
This type of Transformer cooling is widely used for oil filled transformers up to about 30MVA. In Natural cooling, the oil in the tans gets heated due to the heat generated in the core and windings. The hot Oil flows upward and the cold Oil comes down according to the principle of convection. The oil flows in the transformer tank by the principle of natural convection hence this type of cooling is called Oil Natural Cooling. Heat is transferred from core and transformer windings to the oil and the heated oil is cooled by the natural air. Cooling area is increased by providing the cooling tubes.
Oil natural Air Forced Transformer Cooling:
In this method, air fans are mounted near the Transformer and the forced air is directed on to the cooling tubes to increase the rate of cooling. The fans are provided with automatic starting. When the temperature of the oil and windings increases above a permissible value the thermostats switch on cooling fans. This happens during heavy load condition and during higher ambient temperatures.
In higher rating transformers where the heat dissipation is difficult this type of cooling is used. Fans are used to forced and air blast on radiators. Forced air cooling increases the heat dissipation rate. In this type of cooling oil circulates by natural convection and the blast of air is directed towards the better heat dissipation rate.
Oil Forced Air Forced Transformer Cooling:
Transformers above 60 MVA employ a combination of Forced Oil and Forced Air Cooling. Oil Natural Air Forced type of cooling is not adequate to remove the heat caused by the losses which is approximately equal to 1% of the transformer rating (0.6MW). In case of Forced Oil and Forced air cooling system a separate cooler is mounted away from the transformer tank. This cooler is connected to the transformer with pipes at the bottom and the top. The oil is circulated from the transformer to the cooler through the pump. The cooler is provided with the fans which blast air on the cooling tubes. This type of cooling is provided for the higher rating transformers available at the Substations and Power Stations.
Oil Forced Water Forced Transformer Cooling:
This type of cooling system needs a heat exchanger in which the heat of the transformer oil is given to the cooling water. The cooling water is taken away and cooled in separate coolers. The oil is forced through the heat exchanger. The oil pump pumps the oil from transformer to the het exchanger though the top pipes. Oil from the heat exchanger is pumped back to the transformer through the bottom pipe.
This type of cooling Is provided for very large transformers which have ratings of some hundreds of MVA (Generating Transformer will have very high rating and rating equal to the rating of the generator). This type of transformers is used in large substations and power plants.
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