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What is the purpose of transformer oil?

The oil utilised within a transformer is an important element that enables equipment users to check a unit’s physical condition. Today, mineral-based oils like transformer oil are frequently employed in a wide variety of different transformer types for their dielectric strength and impressive electrical properties. Oil used within a transformer works both as an insulator and as a dedicated cooling agent. In the following sections, we’ll take an in-depth look at this specialised lubricant and explore how it functions.

Defining transformer oil

A transformer oil can be defined as a special type of lubricant that has exceptional electrical insulating capabilities, which is why it is also often referred to by many as insulating oil. Operating at high temperatures, transformer oil is able to maintain stability, and for this reason, it is used within electrical power transformers, where it works to dissolve heat and prevent arcing. Both the transformer’s core and windings are submerged within the lubricant, allowing the transformer oil to act as a coolant and protect these critical components.

The function of transformer oil

As mentioned previously, the main roles of transformer oils are to act as a coolant and to insulate. It is understood that different types of materials will have dissimilar levels of dielectric strength. For this reason, materials must be able to maintain a voltage that is equal to their dielectric strength. When the voltage of a material is enhanced beyond its particular dielectric strength, it can handle the electrical current flow present.

It’s worth noting that transformer oil absorbs moisture exceptionally quickly. When the oil absorbs dampness, the oil’s dielectric strength deteriorates. For this reason, a solution such as silica gel may be used inside a transformer to ensure any external moisture present is captured, so it can’t harm functionality.


The main function of transformer oil is to work as a coolant. The design of transformer coils can involve copper that is capable of carrying a high current, which, in turn, makes these components extremely hot. Transformer oil is an exceptional heat conductor and can effectively reduce the copper coils’ temperature, stopping them from burning.


Transformer oil also acts as a powerful insulator. Due to its high dielectric strength, which is able to resist high voltages, it provides ideal insulation within a transformer.

What are the different types of transformer oil?

Finally, it’s important to remember there are two different types of transformer oil available that are designed for different applications – naphthenic oil and paraffinic oil.

Naphthenic oil is a mineral-insulating oil that is derived from specific crudes that are exceptionally low in n-paraffin. The oil’s pour point is considerably lower than its paraffinic counterpart due to its lower wax content, and it has a boiling point of around 425°C.

Paraffinic oil is also a mineral-insulating oil, but it comes from particular crudes that contain a substantial amount of n-paraffin. As they have high-levels of n-paraffin present, these oils have a higher pourpoint than naphthenic oil, and their boiling point is 530°C.

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