What Are Dry Type Transformers?

A dry-type transformer is one in which the insulating medium surrounding the winding assembly is a gas or dry compound. Basically, any transformer can be constructed as “dry” as long as the ratings, most especially the voltage and kVA, can be economically accommodated without the use of insulating oil or other liquid media.

Many perceptions of dry-type transformers are associated with the class of design by virtue of the range of ratings or end-use applications commonly associated with that form of construction Of course, the fundamental principles are no different from those encountered in liquid-immersed designs.

Dry-type transformers compared with oil-immersed are lighter and nonflammable. Increased experience with thermal behavior of materials, continued development of materials and transformer design have improved transformer thermal capability.

Upper limits of voltage and kVA have increased. Winding insulation materials have advanced from protection against moisture to protection under more adverse conditions (e.g., abrasive dust and corrosive environments).

Dry Type Transformer Taps
Transformers may be furnished with voltage taps in the high-voltage winding. Typically two taps above and two taps below rated voltage are provided, yielding a 10% total tap voltage range (ANSI/IEEE, 1981 [R1989]; ANSI/IEEE C57.12.52-1981 [R1998]).

Cooling Classes for Dry-Type Transformers
American and European cooling-class designations are indicated in Table 2.5.1. Cooling classes for drytype transformers are as follows (IEEE, 100, 1996; ANSI/IEEE, C57.94-1982 (R-1987)):

Ventilated — Ambient air may circulate, cooling the transformer core and windings
Nonventilated — No intentional circulation of external air through the transformer
Sealed — Self-cooled transformer with hermetically sealed tank
Self-cooled — Cooled by natural circulation of air
Force-air cooled — Cooled by forced circulation of air
Self-cooled/forced-air cooled — A rating with cooling by natural circulation of air and a rating with cooling by forced circulation of air.

Winding Insulation System
General practice is to seal or coat dry-type transformer windings with resin or varnish to provide protection against adverse environmental conditions that can cause degradation of transformer windings. Insulating media for primary and secondary windings are categorized as follows:

Cast coil — The winding is reinforced or placed in a mold and cast in a resin under vacuum pressure. Lower sound levels are realized as the winding is encased in solid insulation. Filling the winding with resin under vacuum pressure eliminates voids that can cause corona. With a solid insulation system, the winding has superior mechanical and short-circuit strength and is impervious to moisture and contaminants.

Vacuum-pressure encapsulated — The winding is embedded in a resin under vacuum pressure. Encapsulating the winding with resin under vacuum pressure eliminates voids that can cause corona. The winding has excellent mechanical and short-circuit strength and provides protection against moisture and contaminants.

Vacuum-pressure impregnated — The winding is permeated in a varnish under vacuum pressure. An impregnated winding provides protection against moisture and contaminants.

Coated — The winding is dipped in a varnish or resin. A coated winding provides some protection against moisture and contaminants for application in moderate environments.

Below are two photographs of dry-type transformer assemblies.

Nonventilated and sealed dry-type transformers are suitable for indoor and outdoor applications (ANSI/IEEE, 57.94-1982 [R-1987]).

As the winding is not in contact with the external air, it is suitable for applications, e.g., exposure to fumes, vapors, dust, steam, salt spray, moisture, dripping water, rain, and snow.

Ventilated dry-type transformers are recommended only for dry environments unless designed with additional environmental protection. External air carrying contaminants or excessive moisture could degrade winding insulation.

Dust and dirt accumulation can reduce air circulation through the windings (ANSI/IEEE, 57.94-1982 [R 1987]). Table 2.5.2 indicates transformer applications based upon the process employed to protect the winding insulation system from environmental conditions.

All energized parts should be enclosed to prevent contact. Ventilated openings should be covered with baffles, grills, or barriers to prevent entry of water, rain, snow, etc. The enclosure should be tamper resistant.

A means for effective grounding should be provided (ANSI/IEEE, C2-2002). The enclosure should provide protection suitable for the application, e.g., a weather- and corrosion-resistant enclosure for outdoor installations.

If not designed to be moisture resistant, ventilated and nonventilated dry-type transformers operating in a high-moisture or high-humidity environments when deenergized should be kept dry to prevent moisture ingress.

Strip heaters can be installed to switch on manually or automatically when the transformer is deenergized for maintaining temperature after shutdown to a few degrees above ambient temperature.
Operating Conditions
The specifier should inform the manufacturer of any unusual conditions to which the transformer will
be subjected. Dry-type transformers are designed for application under the usual operating conditions
indicated in Table 2.5.3.
Gas may condense in a gas-sealed transformer left deenergized for a significant period of time at low
ambient temperature. Supplemental heating may be required to vaporize the gas before energizing the
transformer (ANSI/IEEE, C57.94-1982 [R1987]).

Limits of Temperature Rise
Winding temperature-rise limits are chosen so that the transformer will experience normal life expectancy for the given winding insulation system under usual operating conditions. Operation at rated load and loading above nameplate will result in normal life expectancy.

A lower average winding temperature rise, 80°C rise for 180°C temperature class and 80°C or 115°C rise for 220°C temperature class, may be designed providing increased life expectancy and additional capacity for loading above nameplate rating.

The winding-temperature indicator can be furnished with contacts to provide indication and/or alarm of winding temperature approaching or in excess of maximum operating limits. For sealed dry-type transformers, a gas-pressure switch can be furnished with contacts to provide indication and/or alarm of gas-pressure deviation from recommended range of operating pressure.

Surge Protection
For transformers with exposure to lightning or other voltage surges, protective surge arresters should be coordinated with transformer basic lightning impulse insulation level, BIL.

The lead length connecting from transformer bushing to arrester—and from arrester ground to neutral—should be minimum length to eliminate inductive voltage drop in the ground lead and ground current (ANSI-IEEE, C62.2-1987 [R1994]).

Lower BIL levels can be applied where surge arresters provide appropriate protection. At 25 kV and above, higher BIL levels may be required due to exposure to overvoltage or for a higher protective margin (ANSI/IEEE, C57.12.01-1989 [R1998]).

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