Dry-Type or Liquid-cooled Transformer

Oil Cooled & Dry Type Transformer:

In oil cooled transformer the cooling medium is transformer oil whereas the dry type transformer is air cooled.

Dry type transformer never uses any insulating liquid where its winding with core be immerged. Rather windings with core are kept within a sealed tank that is pressurized with air.

The dry type transformer is of two types. They are

A-   Cast Resin Dry Type Transformer ( CRT)

B-   Vacuum pressure Impregnated Transformer ( VPI)

Transformers change voltages from one level to another. Most commonly, that change involves very high power line transmission voltages (500 kV) being reduced to the much lower levels used in heavy industry (as much as 30 kV) and households (120-240 V). Dry type, or air-cooled, transformers accomplish this function so safely and efficiently that they are commonly used for indoor applications where other transformer types are considered too risky.

Basically, there are two distinct types of transformers: Liquid insulated and cooled (liquid-filled type) and non liquid insulated, air or air/gas cooled (dry type). Also, there are sub categories of each main type.

For liquid-filled transformers, the cooling medium can be conventional mineral oil. There are also wet type transformers using less flammable liquids, such as high fire point hydrocarbons and silicones.

Comparision:

Dry-type transformers with ratings exceeding 5MVA are available, but the vast majority of the higher-capacity transformers are liquid-filled. For outdoor applications, wet-type transformers are the predominate choice.

Liquid-filled transformers are normally more efficient than dry-types, and they usually have a longer life expectancy. Also, liquid is a more efficient cooling medium in reducing hot spot temperatures in the coils. In addition, liquid-filled units have a better overload capability.

Dry Type transformers use air as the cooling medium. Oil Type transformers are considered a potential fire and safety hazard. Oil Type transformers require the development and maintenance of reliable fire safety and extinction procedures.

Oil Type transformers may require periodic sampling of the oil and more exhaustive maintenance procedures.

Dry Type Transformers can be located closer to the load unlike oil transformers which require special location and civil construction for safety reasons. Locating the transformers near the loads may lead to savings in cable costs and reduced electrical losses.