Oil-Type Transformers

General Overview

Power transformers are generally produced in two main types: oil-type transformers and dry-type power transformers. Oil-type transformers themselves are also produced in two different categories.

These are:

Hermetically sealed oil-type transformers (fully closed type)
Expansion tank oil-type transformers (open to atmosphere type)

Both types of oil transformers have their windings completely immersed in oil. Hermetically sealed types are airtight and therefore do not require regular maintenance due to the absence of air contact.

Oil-filled transformers form the basis of electrical systems.

Hermetically Sealed Oil-Type Transformers

Hermetically sealed oil transformers do not include an expansion tank due to their compact size. The tank is fully filled with oil.

These transformers are sealed from the atmosphere by applying hermetic pressure adjustment at the factory. As a result, they have no air contact.

Oil expansion is controlled through flexible and corrugated tank structures. When the oil heats up, heat is dissipated through the corrugated walls, and the expansion is absorbed by the tank structure.

Thanks to the absence of air contact, oxidation and moisture problems do not occur in the oil, which increases service life compared to expansion tank types.

Expansion Tank Oil-Type Transformers

In expansion tank transformers, the oil is in direct contact with air. Therefore, over time, oxidation may occur, and insulation performance may decrease.

For this reason, periodic maintenance is required, including:

Oil replacement
Moisture removal from the tank
Silica gel replacement (for air drying systems)

These transformers generally have a shorter service life compared to hermetically sealed types.

Regular maintenance of transformers is crucial for a sustainable electricity infrastructure.

Structure of Oil-Type Transformers

Oil-type transformers generally consist of the following components:

Core (magnetic core)
Tank and cover
Windings
Terminals
Bushings
Tap changer (voltage regulator)
Cable boxes
Expansion tank (not present in hermetic types)
Other accessories

Technical Characteristics

Oil-type transformers are generally produced in power ranges between 50 – 20,000 kVA and system voltages up to 36 kV.

Three-phase oil-immersed and naturally cooled (ONAN) transformers can be used both indoors and outdoors.

The main function of oil is to dissipate heat generated inside the transformer, ensuring safe operating temperature levels.

Oil-Type vs Dry-Type Transformers

Dry-type transformers operate in air or gas and are not immersed in oil. They are insulated with epoxy resin.

Oil-type transformers are widely used in:

Urban distribution centers
Construction sites
Factories
Irrigation systems
Power plants
Industrial zones

What Is an Oil-Type Transformer?

An oil-type transformer is a transformer that removes heat using insulating oil, making it suitable for high-power applications where thermal management is critical.