Transformers:Transformer Categories and Type


This chapter covers information on the maintenance and testing of power transformers. To ensure trouble-free service over the life of the transformer, it has to be maintained regularly, but equally important it must be operated properly. Therefore this chapter provides information on the basic design, construction, application, and operation of power distribution transformers with the expectation that this information will help toward better care and maintenance of transformers.

A transformer is an energy transformation device that transforms alternating current (AC) or voltage at one level to AC and voltage at another level. A trans- former can economically convert voltage or current from low to high levels, or from high to low levels. The transformer usually consists of two or more insulated windings on a common iron core. In industrial and commercial applications, transformers are used to step down voltages from utility service voltage to lower distribution voltage levels or lower utilization voltages that may be required for a facility or a plant. Transformers are very reliable devices and can provide service for a long time if maintained and serviced regularly. Transformer failures, when they occur, are usually of a very serious nature, which may require costly repairs and long downtime. The best insurance against transformer failure is to ensure that they are properly installed and maintained.

Transformer Categories and Type

For consideration of maintenance requirements, transformers can be divided into the following categories:

Insulating medium Construction Application and use

Insulating Medium

The transformer’s insulating medium can be subdivided into two types: dry and liquid filled.

Dry-type transformers are usually air cooled with winding insulation of class A, B, C, or H. The dry-type transformer can be either self-cooled or forced air cooled.

Selfcooled: A self-cooled transformer of the dry type is cooled by natural circulation of air through the transformer case. The cooling class designation for this transformer is AA.

Forced air cooled: A forced air-cooled transformer of dry type is cooled by means of forced circulation of air through the case. Transformers of this type have air-blast equipment such as fans with louvered or screened openings. These transformers are rated at 133% of the rating of the self-cooled dry-type transformers. The cooling class designation for this transformer is FA. Dry- type transformers can be obtained with both self-cooled and forced air-cooled rating. The designation for such a transformer is AA/FA. Dry-type trans- formers can also be cooled by gas instead of air. For such transformers, a sealed tank is required.

Liquid-Filled Transformer

In this type of transformer, the windings and core are totally immersed in a liquid contained in the transformer tank. The tank is equipped with cooling fins for circulation of the transformer liquid. The transformer liquid provides an insulating medium for the coils as well as for dissipation of heat. Two liquids have been used extensively in the past for transformers: mineral oils and polychlorinated biphenyls (PCB), commonly known as askarel. Askarel was extensively used in transformers for indoor applications because it is a nonflammable synthetic insulating fluid. Askarel is a nonbiodegradable and toxic. Environmental Protection Agency (EPA) banned the use of askeral in transformers and other electrical equipment, and its availability for reuse or for use in new applications is almost nonexistent. Newer fluids have been introduced, such as silicone, RTemp, Wecosal, and Alpha 1 for replacement of askarel. Others are still in developmental stages. Regardless of what new fluids come on the market for transformer applications, they would still have to be maintained and tested to assure transformer integrity.

Several cooling methods are used for liquid-filled transformers.

Selfcooled: A self-cooled transformer uses the natural circulation of the insulating liquid. Heat in the transformer tank is dissipated by convection currents set up in the liquid, which circulates through the tank and cooling fins. The cooling class designation for this transformer is oil natural, air natural OA.

Forced air cooled: In this type of transformer, air is forced over the cooling surface of the tank to supplement the self-cooled rating. The supplemental air is provided by fans that are mounted on the transformer tank and which can be manually or automatically controlled. The cooling class designation for this type of transformer is OA/FA.

Forced air cooled and forced oil cooled: This transformer uses a pump to circulate oil through a heat exchanger to increase heat dissipation, which supplements the self-cooling and forced air cooling. The cooling class designation for this transformer is OA/FA/FOA.

Water cooled: This transformer uses water instead of air to provide the cooling. The cooling system consists of a heat exchange by means of water pumped through a pipe coil installed inside or outside the transformer tank. The cooling class designation for this transformer is FOW.


Transformers can be classified by tank construction and core construction.

Tank Construction

Several types of transformer tank construction are used to prevent exposing liquid to the atmosphere. These types are as follows:

Free breathing: This type is open to the atmosphere (i.e., the airspace above the liquid is at atmospheric pressure). The transformer breathes as the air pres- sure and temperature change outside the tank. Some of these transformers can be equipped with dehydrating compounds in the breather.

Conservator or expansion-tank: These transformers are equipped with small expansion tanks above the transformer tank. The transformer tank is com- pletely filled with oil, and the transformer breathes by means of this small tank, usually through a dehydrating compound. The purpose of the small tank is to seal the transformer fluid from the atmosphere and to reduce oxidization and formation of sludge.

Sealed tank: These transformers are equipped with an inert gas, such as nitrogen that is under pressure above the liquid in the transformer tank. Generally, the pressure range for this type of transformer is −8 to +8 lb/in.2

Gas-oil sealed: These transformers have an auxiliary tank to completely seal the interior tank, containing transformer liquid, from the atmosphere.

Vaporization: This type of transformer uses a special nonflammable insu- lating fluid, such as florocarbon (General Electric R-113), which is nonflam- mable, and a special condenser assembly welded on top of the transformer tank. The cooling tube ends are swaged and welded to tube headers. This transformer uses the technique of sprayed liquid on core and coil assembly (i.e., vaporization cooling known as pool boiling). The purpose of the con- denser is to cool the boiling vapor into liquid for continued circulation of the fluid.


Core Construction

Transformers employ basically two types of core construction techniques.

Core type: In core-type construction, the transformer winding surrounds the laminated core. The coils can be cylindrical, flat, or disk shaped. They can be arranged to fit around the rectangle or square cross section of the core, as shown in Figure 5.1. Core-type construction provides a single-path magnetic circuit through the magnetic core. Most small distribution transformers are of this construction.

Shell type: In shell-type construction, the magnetic core surrounds the wind- ings, as shown in Figure 5.2. The primary and secondary windings may be interspaced side by side or circularly stacked one above the other. Some large power transformers have this form of construction. One advantage of the shell type is that it offers a separate path for the zero-sequence currents through the core, as compared to the core type in which the zero-sequence path exists only through the transformer tank and end connections.

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