In transformers, insulating oils (also known as transformer oils) are primarily used to insulate the windings and cool the system. Insulating oils also ensure efficient heat transfer and electrically insulate the transformer from its surroundings. Read more about the properties of insulating oils in this blog article.
What are insulating oils?
Insulating oils, also known as transformer oils, are highly refined mineral oils or low-viscosity silicone oils specifically developed for use in electrical high-voltage systems, transformers, and circuit breakers. They are thermally stable and provide both electrical insulation and thermal management. Typical areas of application include:
- Transformers
- Reactors
- Chokes
- Converters
- Switches
- High-voltage equipment subject to high thermal stress
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Insulating oils and transformer oils are true all-rounders
Insulating oils don’t just insulate – they also cool, lubricate, stabilise, offer dielectric strength, and possess medium to low viscosity. They fulfil a wide range of functions essential for the safe and efficient operation of your equipment:
1. Electrical insulation:
Insulating oils are widely used in transformers and switchgear to ensure efficient electrical insulation. Transformer oil has a high dielectric strength, including very high breakdown voltage. This prevents voltage arcs between live components inside the transformer. In other words, it electrically separates the components from each other.
2. Heat dissipation (cooling):
During operation, a transformer generates significant heat. Insulating oil has excellent thermal conductivity. It absorbs the heat and transports it to the outer surface of the transformer, where it is released into the environment – often via cooling fins or radiators. This helps maintain a safe operating temperature and extends the service life of the transformer.
3. Protection against oxidation and moisture:
Transformer oil surrounds the metallic parts and shields them from air and moisture, preventing corrosion.
4. Lubrication:
Insulating oils also have lubricating properties that help reduce wear on moving parts in transformers and switchgear. This contributes to extending the service life of the equipment and maintaining its performance.
5. Long-term stability:
Transformer oils must be chemically stable – even at high temperatures – to meet the operational demands of electrical equipment. They should not cause corrosion or break down under normal operating conditions.
6. Viscosity:
The viscosity of transformer oils is crucial to ensure even distribution over the electrical components. The right viscosity also ensures proper lubrication and efficient heat dissipation.
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Typical properties of insulating oils
- Good ageing stability
- High flash point (fire safety!)
- Low water content (even minimal moisture reduces breakdown voltage)
- Free from conductive or corrosive contaminants
It is essential that insulating oils are regularly and professionally tested and maintained to ensure the optimum performance of the transformer. This supports a long service life and consistent operational performance.
But: Insulating oils are not without risks – these disadvantages should be known
As powerful and versatile as insulating oils are, they also present certain challenges and potential risks that must be carefully considered during the maintenance and disposal of electrical equipment:
1. Ageing and Decomposition
Over time, insulating oils degrade due to heat, oxidation, and electrical stress. This leads to the formation of degradation products such as acids or sludge, which can impair the oil's insulating properties and potentially damage windings and paper insulation.
Learn more about this topic in the following article:
2. Moisture Absorption
Insulating oil can absorb moisture from the environment. Even small traces of water significantly reduce the breakdown voltage, which can lead to short circuits. Regular oil testing is particularly important in older systems with leaky seals.
Learn more about this topic in the following article:
3. Environmental and Health Hazards from PCBs
In older transformers—especially those from before the 1980s—insulating oil may contain polychlorinated biphenyls (PCBs). These substances are highly toxic, environmentally hazardous, and have been banned for years. Nevertheless, many old systems worldwide still contain PCB-contaminated oil. Disposal requires specialised procedures and the highest safety standards.
Learn more about this topic in the following article:
4. Risks of Improper Disposal
Transformer oils must not be disposed of carelessly—especially when contaminated with PCBs or other problematic substances. Proper analysis, storage, and disposal are essential.
Learn more about this topic in the following article:
Conclusion: High-quality insulating oils extend the service life of your transformers
Insulating oils are true all-rounders – they insulate, cool, lubricate, stabilise, and offer excellent dielectric properties. To maintain and regularly monitor these qualities, there are specialist transformer oil maintenance companies on the market. They support plant operators in extending the service life of their transformers by checking the quality and condition of the insulating oils through a simple oil sample.
You can find more tips in this blog article:
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This blog was first published on December 5, 2023 and updated on July 1, 2025.
