Difference between Dry-Type and Oil-Filled Transformers

The most fundamental difference between dry-type and oil-filled transformer is that a dry-type transformer has a solid insulating material to provide insulation between conducting parts of the transformer, whereas an oil-filled transformer is one in which dielectric oil is used as the insulating medium.

Both transformers have their own advantages, disadvantages, and applications. In this article, we will learn about the major differences between dry-type transformer and oil-filled transformer. But before that let us get an overview of an electrical transformer, dry-type transformer, and the oil-filled transformer.

What is an Electrical Transformer?

An electrical transformer is a static electromagnetic equipment used in electrical power systems to change the value of voltage and current.

A typical electrical transformer consists of three major parts namely, a magnetic core, a primary winding, and a secondary winding.

The magnetic core is a piece of laminated silicon steel that provides a low reluctance path for the flow of magnetic field between primary and secondary windings.

The primary winding is made up of copper wires and takes input supply from the source. While, the secondary winding is also made up of copper wires and supplies the transformed electricity to the electrical load.

The primary function of the electrical transformer is to increase or decrease the supply voltage and current in an electrical circuit.

Types of Transformers

Depending on the insulating material used, the electrical transformers can be classified into the following two types −

• Dry-Type Transformer

• Oil-Filled Transformer

Let us discuss about each type of electrical transformer in detail.

What is a Dry-Type Transformer?

A type of electrical transformer in which a solid insulating material is used for providing insulation between conducting components like windings, core, etc. is called a dry-type transformer.

In most practical dry-type transformers, epoxy resin or polyester resin is used as the insulating material. Thus, these transformers are also known as cast resin transformer or epoxy resin transformer.

These solid insulating materials have good thermal stability and dielectric strength. Also, these materials have non-flammable and self-extinguishing capabilities. Hence, these transformers are relatively safer from the fire safety point of view.

The dry-type transformers are mainly used in medium voltage applications where oil-filled transformers cannot be used. Some common examples of applications of dry-type transformers include indoor substations, commercial buildings, underground substations, etc.

What is an Oil-Filled Transformer?

A type of electrical transformer in which the core and the winding assembly is immersed in a dielectric oil to provide insulation and cooling capabilities is called an oil-filled transformer. It is also known as an oil-immersed transformer.

The oil-filled transformer consists of a steel tank which is filled with transformer oil and encloses the internal parts of the transformer.

When an oil-filled transformer is in operation, a significant amount of heat is generated due to losses in the transformer windings and the core. Since, the transformer is immersed in the oil, the heat is transferred to the oil and hence to the external environment.

The oil-filled transformers are commonly used in generating stations, distribution systems, transmission systems, outdoor substations, etc.

This is all about electrical transformer, dry-type transformer, oil-filled transformer. Let us now understand the differences between dry-type and oil-filled transformers using a comparison table.

Difference between Dry-Types and Oil-Filled Transformer

The following table highlights all the significant differences between dry-type and oil-filled transformers −

Parameter	Dry-Type Transformer	Oil-Filled Transformer
Insulating Material	In dry-type transformers, solid insulating materials like epoxy resin or polyester resin are used.	In oil-filled transformers, dielectric oil is used as the insulating material.
Alternate name	Dry-type transformer is also called cast resin transformer or epoxy resin transformer.	Oil-filled transformer is also called oil-immersed transformer.
Cooling medium	In dry-type transformers, air is used as the cooling medium.	In oil-filled transformers, oil and air both are used as the cooling medium.
Maintenance	Dry-type transformers require less maintenance.	Oil-filled transformers require regular maintenance.
Capital cost	The capital cost of dry-type transformer is higher.	The capital cost of oil-filled transformer is relatively lower than that of the dry-type transformer.
Operational cost	The operational cost of dry-type transformer is lower due to need of less maintenance.	The operational cost of oil-filled transformer is higher, as it requires regular maintenance and monitor.
Risk of fire and explosion	In dry-type transformer, non-flammable and self-extinguishing insulating materials are used. Hence, these transformers have lower risk of fire and explosion.	In oil-filled transformer, the oil used is an inflammable material. Thus, they have risk of fire that needs extra care.
Suitability	Dry-type transformers are suitable for low and medium voltage and indoor applications.	Oil-filled transformers are suitable for medium and high voltage and outdoor applications.
Efficiency	Dry-type transformers have lower efficiency.	Oil-filled transformers have higher efficiency.
Weight	Dry-type transformers are lighter.	Oil-filled transformers are heavier.
Size	Dry-type transformers have compact size and hence require less space.	Oil-filled transformers are larger in size and consume more space.
Installation cost	The installation cost dry-type transformers is lower.	Oil-filled transformers involve high installation cost.
Impact on environment	Dry-type transformers produce less waste. Thus, they are environment friendly.	Oil transformers have risk of leakage or spill of oil that can harm environment.
Dielectric strength	Dry-type transformers have solid insulation material that provides high dielectric strength.	Oil-filled transformers have oil as the insulation material that provides excellent dielectric strength.
Lifespan	Dry-type transformers have longer lifespan.	Oil-filled transformers have relatively shorter lifespan.
Transport	The transportation of dry-type transformers is easier due to absence of liquid.	The oil-filled transformers are difficult to transport.

Conclusion

In this article, we explained the major differences between these two types of transformers in the above comparison table. From the above discussion, we can state that the dry-type transformers are suitable for small to medium voltage applications and require less space for installation. On the other hand, oil-filled transformers are suitable for medium to high voltage applications and need a larger space for installation.

FAQs Related to Dry-Type Vs Oil-Filled Transformers

Here is a set of some of the most commonly asked questions related to the difference between dry-type transformer and oil-filled transformer.

1. What is the difference between liquid and dry type transformers?

In dry-transformers, a solid insulating material is used, whereas, in a liquid transformer, dielectric oil is used as the insulating material.

2. What is the difference between air and oil cooled transformers?

In an air-cooled transformer, natural air is used for keeping the transformer cool, while in an oil cooled transformer, the insulating oil provides the cooling mechanism.

3. What is the difference between wet type and dry type transformers?

A wet transformer is one in which the insulating and cooling medium is used in the liquid form, whereas a dry transformer uses a solid material as an insulating medium.

4. What are the advantages of dry type transformer over oil type?

Dry-type transformers are easy to handle and transport. They are compact and lightweight. They have lower operational cost and less maintenance requirement. Dry-type transformers have longer lifespan.

5. What are the disadvantages of dry type transformer?

Dry-type transformers are not suitable for high-voltage applications. They are highly expensive and difficult to repair and they have high operational cost.

6. Why is it called a dry type transformer?

A transformer is said to be a dry type transformer if it is cooled by normal air ventilation and does not require any oil or liquid for cooling its windings and core.

7. Where is dry type transformer used?

Dry type transformers are used in indoor substations, inside buildings and tunnels, in mines, on ships and offshore platforms, food processing industries, nuclear power plants, etc.