Why is oil added to a transformer?

Adding oil to a transformer (usually transformer oil, such as mineral oil or synthetic oil) is mainly to achieve four key functions: 

**cooling and heat dissipation**, **insulation**, **arc quenching**, and **antioxidation**. The following is a detailed introduction for you:

 1. Cooling and Heat Dissipation: The "Cooling System" to Ensure the Stable Operation of the Transformer

When the transformer is in operation, the resistance losses of the windings and the eddy current losses of the iron core will 

generate a large amount of heat. If the heat accumulates continuously, it may cause the insulation materials to age and even burn 

out the equipment.

- **Working Principle**: Transformer oil has good thermal conductivity. It can absorb the heat generated by the windings and the

 iron core, and then, through **natural convection** or the circulation of the oil pump, carry the heat to the radiator outside the 

transformer, and finally dissipate the heat into the air, thus achieving the effect of cooling.

- **Actual Situation**: Large transformers often come with **radiators**, **fans**, or **oil circulation cooling devices** to enhance 

the heat dissipation efficiency.

 2. Insulation Function: Building an Internal Safe "Insulation Barrier"

Reliable insulation is required between the windings inside the transformer, between the windings and the iron core, and between

 the windings and the casing. The insulation performance of transformer oil is much better than that of air.

- **Specific Performance**:

  - The **breakdown voltage** of transformer oil is very high (usually can reach above 40kV), which can effectively prevent short

 circuits between components with different potentials.

  - It can fill the gaps between the windings and eliminate air bubbles (because the insulation performance of air is poor), thereby improving

 the overall insulation strength.

- **Related Comparison**: Dry-type transformers are insulated by air or epoxy resin. However, their heat dissipation capacity is

 relatively weak, so they are generally only suitable for small-capacity scenarios.

 3. Arc Quenching Function: The "Safety Guard" to Deal with Failures

In parts such as the **on-load tap changer** of an oil-immersed transformer, an arc will be generated when the contacts open

 and close.

- **Arc Quenching Mechanism**: The high temperature of the arc will decompose the transformer oil, generating a large amount

of gases such as **hydrogen**. These gases will form a high pressure locally, thus quenching the arc and preventing the accident

 from further expanding.

- **Significance**: This function is particularly important for switchgear that is frequently operated, as it can extend the service 

life of the equipment.

# 4. Antioxidation and Sealing: The "Protective Clothing" to Delay Equipment Aging

- **Isolating Oxygen**: Transformer oil can cover the iron core and windings, preventing them from direct contact with air,

 thereby slowing down the rate of oxidation and moisture absorption (moisture will reduce the insulation performance).

- **Function of the Breather**: A **silica gel breather** is usually installed on the top of the transformer, which can absorb the

 moisture in the air and ensure the dryness of the oil.

 Supplementary: Requirements for the Characteristics of Transformer Oil

- **High Insulation Performance**: High breakdown voltage and low dielectric loss.

- **Good Thermal Conductivity**: High specific heat capacity and low viscosity, which is conducive to the transfer of heat.

- **Stable Chemical Properties**: High flash point (not easy to burn), low freezing point (can remain liquid in a low-temperature 

environment), and strong antioxidant capacity.

 Conclusion

Transformer oil is like the "blood" of a transformer. It integrates multiple functions such as cooling, insulation, arc quenching, and 

protection, and is the key to the safe and efficient operation of oil-immersed transformers. Since dry-type transformers do not

 use oil, they usually rely on air cooling or epoxy resin insulation and are mostly applied in places with high fire protection

 requirements, such as office buildings, subways, etc.