9 Differences Between Dry Type and Oil Immersed Transformers

In power engineering construction, we often hear about dry type transformers and oil immersed transformers (also called oil filled transformers). But what are the main differences between dry type and oil immersed transformers in power engineering projects? And what are their respective advantages and disadvantages?

Shared Characteristics

Despite obvious differences, the two transformer types have similar properties in working principle and basic structure.

✅ All operate based on electromagnetic induction for voltage step-up or step-down;
✅ All include an iron core for the magnetic circuit and windings for the electrical circuit;
✅ All require heat dissipation;
✅ All are voltage transformation devices designed to transmit and distribute power at different voltage levels

Difference Between Dry and Oil Type Transformer

1. Is There Any Oil?

The biggest difference between oil immersed transformers and dry type transformers lies in the “oil type” and “dry type”. That is to say, their cooling media are different.

Oil immersed: Filled with transformer oil. Using transformer oil as the cooling and insulation medium. The core and windings are placed in an oil tank filled with transformer oil.

Dry type: Without liquid insulating oil, instead, it uses air or epoxy resin insulation. It uses air as the cooling medium. The core and windings are encapsulated with epoxy resin; there is also a more commonly used non-encapsulated type, where the windings are impregnated with special insulating paper and specific insulating paint to prevent the windings or core from getting damp.

2. Shape And Packaging Form

Oil immersed: Since oil is a liquid and has fluidity, an oil-immersed transformer must have an outer casing. Inside the casing is the transformer oil, through which the transformer’s coils are immersed. The coils of the transformer cannot be seen from the outside. Another characteristic is that an oil reservoir can be installed on the top of the oil-immersed transformer to respond to changes in oil temperature.

Dry type: Without oil, so there is no need for an outer casing. The iron core and coils of the transformer can be seen directly.

3. Heat Dissipation

Oil immersed: To facilitate heat dissipation, that is, to make it convenient for the insulation oil inside to dissipate heat, the oil-immersed transformer is externally equipped with a heat sink, which functions like a heat spreader.

Dry type: There is no such heat sink. Heat dissipation relies on the fan located beneath the transformer coil.

4. Wiring Form

Oil immersed transformer: Most use porcelain bushings

Dry type transformer: Most use silicone rubber bushings

5. Capacity and Voltage

Oil-immersed transformers: Can achieve the required full capacity, and the voltage levels can almost meet all voltage requirements;

Dry type transformers: Generally used for power distribution, with capacities mostly below 2500 kVA, and voltages are mainly medium voltage and high voltage.

6. Insulation and Heat Dissipation

Oil-immersed Transformer: It is insulated by insulating oil. Through the circulation of the insulating oil within the transformer, the heat generated by the coils is transported to the radiator (plates) of the transformer.

Dry type Transformer: Generally insulated with resin, it is cooled by natural air cooling. For large capacity, it is cooled by fans.

7. Application Areas

Oil-immersed transformers: Due to a higher fire risk compared to dry-type transformers, they are generally suitable for outdoor substations, open distribution points in industrial and mining areas, etc., in areas far from buildings.

Dry type transformers: They are mostly used in places that require fire prevention and explosion protection. They are generally suitable for large buildings and high-rise buildings.

8. The Cost

For transformers of the same capacity, the cost of purchasing a dry-type transformer is higher than that of an oil-immersed transformer. This is because the manufacturing process of dry-type transformers is more complex compared to oil-immersed transformers of the same voltage level and capacity.

9. Market

In terms of output and usage, the voltage levels of dry-type transformers are currently smaller compared to oil-immersed transformers. Most ultra-high voltage systems use oil-immersed transformers. Therefore, in terms of usage, oil-immersed transformers are still more common at present.

However, due to the environmental friendliness, flame retardancy, and impact resistance of the dry type transformers, they are often used in indoor high-demand power distribution areas such as hotels, office buildings, and high-rise buildings, etc.

Summary

Dry type transformers and oil-immersed transformers each have their own advantages and limitations.

Oil-immersed transformers are low in cost and easy to maintain, but they pose a risk of flammability and explosion.

Dry-type transformers have excellent fire resistance and can be deployed near the load center, effectively reducing voltage drop and power loss. However, they are relatively expensive and have a larger overall size.

The appropriate model can be selected based on the actual situation on site, budget and safety requirements.