Although transformers can be classified by core construction (shell or core type), the more functional types of standardized classifications are based on how the transformer is designed for its specific application, and how the heat created by its losses is dissipated.
There are several types of insulating media available.
Two basic classifications for insulating media are:
- Dry-type and
- Liquid filled
We will talk here about heating of the dry-type transformers with occasionally comparison with oil type transformer.
What About Dry-type Transformer?
Dry-type transformers depend primarily on air circulation to draw away the heat generated by the transformer’s losses.
Air has a relatively low thermal capacity When a volume of air is passed over an object that has a higher temperature, only a small amount of that object’s heat can be transferred to the ah’ and drawn away.
Liquids, on the other hand, are capable of drawing away larger amounts of heat.
This is further complicated by the inherent inefficiency of the dry-type transformer. Transformer oils and other synthetic transformer fluids are capable of drawing away larger quantities of excess heat.
Dry-type transformers are especially suited for a number of applications. Because dry-type transformers have no oil, they can be used where fire hazards must be minimized. However, because dry-type transformers depend on air to provide cooling, and because their losses are usually higher, there is an upper limit to their size (usually around 10,000 kVA, although larger ones are constantly being designed).
Also, because oil is not available to increase the dielectric strength of the insulation, more insulation is required on the windings, and they must be wound with more clearance between the individual turns.
Dry-type transformers can be designed to operate at much higher temperatures than oil-tilled transformers (temperature rises as high s 150 °C).
Although oil is capable of drawing away larger amounts of heat, the actual oil temperature must be kept below approximately 100 “C to prevent accelerated breakdown of the oil.
Because of the insulating materials used (glass, paper, epoxy, etc.) and the use of air as the cooling medium, the operating temperatures of dry-type transformers are inherently higher. It is important that adequate ventilation be provided. A good rule of thumb is to provide at least 20 square feet of inlet and outlet ventilation in the room or vault for each 1,000 kVA of transformer capacity.
If the transformer’s losses are known, an air volume of 100 cfm (cubic feet per minute) for each kW of loss generated by the transformer should be provided. Dry-type transformers can be either self- cooled or forced-air cooled.
A self-cooled dry-type transformer is cooled by the natural circulation of air through the transformer case.
The cooling class designation for this transformer is AA. This type of transformer depends on the convection currents created by the heat of the transformer to create an air flow across the coils of the transformer.
Often, fans will be used to add to the circulation of air through the case. Louvers or screened openings are used to direct the flow of cool air across the transformer coils. The kVA rating of a fancooled dry-type transformer is increased by as much as 33 percent over that of a self-cooled dry-type of the same design.
Many other types of dry-type transformers are in use, and newer designs are constantly being developed. Filling the tank with various types of inert gas or casting the entire core assemblies in epoxy resins are just a few of the methods currently is use.
Two of the advantages of dry-type transformers are that they have no fluid to leak or degenerate over time, and that they present practically no fire hazard. It is important to remember that dry-type transformers depend primarily on their surface area to conduct the heat away from to core. Although they require less maintenance, the core and case materials must be kept clean.
A thin layer of dust or grease can act as an insulating blanket, and severely reduce the transformer’s ability to shed its heat.
Construction of Dry-Type Power Transformers (VIDEO)
Resource: Power transformer maintenence and acceptance testing