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established. As an example, for a reference insulation class rated at 15 kV, the BIL is 110 kV.
3.2.4 Cooling. Removal of heat caused by losses is necessary to prevent excessive internal
temperature which would shorten the life of the insulation. The transformer life is cut in half by
a 10C rise in temperature. The general classes of cooling are: self-cooled, forced-air or
forced-oil-cooled, and water-cooled. The basic types of cooling are referred to by standard
designations.
3.2.4.1 Oil Immersed Self-Cooled (OA). In this type of transformer the insulating liquid
circulates by natural convection and heat is dissipated over the radiating surface. The OA
transformer is a basic type and serves as a standard for capacity rating.
3.2.4.2 Oil Immersed Self-Cooled/Forced-Air-Cooled (OA/FA). This type of transformer
is basically an OA unit with the addition of external fans to increase heat dissipation from the
radiators. It is usually possible to obtain between 15 percent - 33 1/3 percent more capacity
relative to the OA rating with the fans running. The OA/FA transformer is applicable in
situations that require short-time peak loads to be carried recurrently, without affecting normal
expected transformer life.
3.2.4.3 Oil Immersed Self-Cooled/Forced-Air Cooled/Forced-Oil (OA/FA/FOA). The
rating of an OA transformer can be increased by the addition of fans and oil pumps. Automatic
controls responsive to liquid temperature are normally used to start the fans and pumps in a
selected sequence as transformer loading increases. The increase in capacity over the OA rating
is usually obtained in two stages: first stage - 33 1/3 percent and second stage - 66 2/3 percent. A
variation of these triple-rated transformers is the OA/FOA/FOA type.
3.2.4.4 Oil Immersed Forced-Oil Cooled Forced-Air Cooled (FOA). This type of
transformer uses external oil-to-air heat exchangers requiring both air fans and fluid pump for all
operating conditions. FOA transformers cannot, without pumps and fans, carry rated loads.
3.2.4.5 Oil Immersed Water Cooled (OW). The cooling water runs through coils of pipe
which are in contact with the insulating oil of the transformer. The oil flows around the coils of
pipe by natural convection, thereby effecting the heat transfer to the cooling water. This type of
transformer has no self-cooled rating. The OW/A type is water cooled and self cooled.
3.2.4.6 Oil Immersed Forced-Oil Cooled with Forced-Water Cooled (FOW). This type of
transformer is similar to the FOA unit, except that external oil-to-water heat exchangers are used.
3.2.4.7 Dry-Type Transformers. In dry-type transformers, available at voltage ratings of
15 kV and below, cooling is accomplished primarily by internal air flow. Three classes of
dry-type transformers are: self-cooled (AA), forced-air cooled (AFA) and self-cooled/forced-air
cooled (AA/FA).
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