Transformer Classification based on the Cooling Medium

Dry - Type:
These transformers use air as the cooling medium. These transformers are generally used for indoor applications. This kind of cooling can be applied for transformers up to 20 MVA.

These Transformers can be further classified into

a)Natural Cooled:
These transformers can be Natural Cooled with Air (Air - Natural). The natural convection of the air removes the heat generated by the transformers. The symbol for this type of cooling is AA.

b)Forced Air Cooling:
This involves cooling the windings of the transformers with forced air, usually through a fan. The symbol for Forced Air cooling in Transformer Nameplates is AF.

Gases such as Nitrogen or Sulphur HexaFluoride can also be used for cooling transformers. These transformers are also considered to be amongst the "dry" type. These transformers need to be placed in sealed containers

Oil Cooled Transformers:
These are transformers which are cooled by means of oil. These transformers are fitted with fins through which the oils pass through as they transfer the heat to the atmospheric air. The oil in the transformers needs to be periodically sampled and checked for integrity. For more on transformer oil analysis Click here.

Like their air cooled counterparts, these transformers too can be further classified on the basis of the flow of oil.

a)Oil Natural Cooling:
In this type of cooling, the oil circulates through the transformer by way of convection. The heat collected by the oil is transferred to the surrounding air by means of cooling fins. The cooling class symbol for this kind of cooling is OA

b)Forced Air Cooled:
In this kind of transformer, cooling is achieved by means of oil driven by convection. As the oil circulates through the fins, air is forced from the outside by means of fans on the fins. This enhances the process of heat exchange and increases cooling of the oil. The designation for this kind of cooling is OA/FA. The fans in this kind of cooling are controlled by a mechanism which switches them on when the transformer oil temperature increases beyond a specific limit.

c)Forced Oil, Forced Air Cooling:

This involves forcing the oil inside the transformer through a special heat exchanger by means of a pump. Air is forced through the other side of the heat exchanger by means of fans. The designation for this type of cooling is FOA (Forced Oil Air).

d)Water Cooled Transformers:
Water can also be used in the cooling of transformers. In this method, the oil which passes through the heat exchanger is cooled by water. This method of cooling is designated as FOA

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