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.

DISTRIBUTION TRANSFORMER COOLANTS BASIC AND TUTORIALS

DISTRIBUTION TRANSFORMER COOLANTS BASIC INFORMATION
What Are The Different Distribution Transformer Coolants?

Mineral Oil

Mineral oil surrounding a transformer core-coil assembly enhances the dielectric strength of the winding and prevents oxidation of the core. Dielectric improvement occurs because oil has a greater electrical withstand than air and because the dielectric constant of oil is closer to that of the insulation.

As a result, the stress on the insulation is lessened when oil replaces air in a dielectric system. Oil also picks up heat while it is in contact with the conductors and carries the heat out to the tank surface by self convection. Thus a transformer immersed in oil can have smaller electrical clearances and smaller conductors for the same voltage and kVA ratings.

Askarels

Beginning about 1932, a class of liquids called askarels or polychlorinated biphenyls (PCB) was used as a substitute for mineral oil where flammability was a major concern. Askarel-filled transformers could be placed inside or next to a building where only dry types were used previously.

Although these coolants were considered nonflammable, as used in electrical equipment they could decompose when exposed to electric arcs or fires to form hydrochloric acid and toxic furans and dioxins.

The compounds were further undesirable because of their persistence in the environment and their ability to accumulate in higher animals, including humans. Testing by the U.S. Environmental Protection

Agency has shown that PCBs can cause cancer in animals and cause other noncancer health effects. Studies in humans provide supportive evidence for potential carcinogenic and noncarcinogenic effects of PCBs (http://www.epa.gov). The use of askarels in new transformers was outlawed in 1977 (Claiborne, 1999).

Work still continues to retire and properly dispose of transformers containing askarels or askarel-contaminated mineral oil. Current ANSI/IEEE standards require transformer manufacturers to state on the nameplate that new equipment left the factory with less than 2 ppm PCBs in the oil (IEEE, 2000).

High-Temperature Hydrocarbons

Among the coolants used to take the place of askarels in distribution transformers are high-temperature hydrocarbons (HTHC), also called high-molecular-weight hydrocarbons.

These coolants are classified by the National Electric Code as “less flammable” if they have a fire point above 300˚C.

The disadvantages of HTHCs include increased cost and a diminished cooling capacity from the higher viscosity that accompanies the higher molecular weight.