SELECTION OF COOLING SYSTEM OF POWER TRANSFORMER BASIC INFORMATION

The selection of a cooling system based on liquids permits a greater overload capability. Liquid-filled units are cooled in a variety of ways. Some of them protect the coolant from oxidation by sealing the transformer and inserting inert gas in the air space.

(1) Oil-Immersed Self-Cooled The insulating mineral oil circulates by natural convection within the tank, which has either smooth sides, corrugated sides, integral tubular sides, or detachable radiators.

(2) Oil-immersed self-cooled and forced-air cooled The same as type 1, but the addition of fans increases the rate of heat transfer from the cooling surfaces, thereby increasing the permissible transformer output.

(3) Oil-Immersed Self-Cooled and Forced-Oil–Forced-Air Cooled The rating of an oil-immersed transformer may be further increased by the addition of some combinations of fans and oil pumps.

(4) Oil-Immersed Forced-Oil-Cooled with Forced-Air Cooler Heat transfer from oil to air is accomplished in external oil-to-air heat exchangers with oil pumps and fans.

(5) Oil-Immersed Water-Cooled Cooling water runs through pipes that are in contact with the cooling oil of the transformer. The oil flows around the outside of these pipe coils by natural convection, thereby effecting the desired heat transfer to the cooling water.

(6) Oil-Immersed Forced-Oil-Cooled with Forced-Water Cooler External oil-to-water heat exchangers are used in this type of unit to transfer heat from oil to cooling water.

Depending upon the geometric duct dimensions and the pressure applied by the oil pumps, the oil velocities for laminar flow range from 0.005 m/s to 0.05 m/s. A great disadvantage of mineral oil is its flammability.

For this reason nonflammable synthetic oils were developed, such as those with the brand names Askarel, Inerteen, Pyranol (USA), Permitol (England), Aroclor (France), and Clophen (Germany). Unfortunately, most of these have proven to be undesirable from an environmental and health point of view, and are not used in new transformer designs.