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Dielectric oil: what it is, types, and applications in transformers and industry
08 February 2024
Dielectric oil is a key fluid in the safe and stable operation of many kinds of electrical equipment. They are associated with the lubrication of transformers, but their functions also include insulating, cooling, and helping to preserve the reliability of critical installations in industry, electrical distribution, and high-voltage infrastructures.
When this fluid works well, it's barely noticeable. But its performance directly influences the life of the equipment, the safety of the installation, and the risk of breakdowns.
What is the purpose of dielectric oil?
Dielectric oil serves to prevent unwanted discharges between components subjected to different voltages and, at the same time, evacuate the heat generated during service. It is used in transformers, switches, capacitors, and other high-voltage equipment where electrical insulation and thermal control are inseparable.
Its main function is not only to "fill" the equipment but to create a stable working environment so that the electrical system operates safely. That is why, when choosing a dielectric oil, it it's not enough to look at a single property. The balance between dielectric strength, thermal behavior, chemical stability, material compatibility, and fire safety also matters.
Electrical insulation
The primary function of dielectric oil is electrical insulation. The fluid prevents arcing or discharge between active parts of the equipment, even when there are high voltages. To achieve this, you must maintain high resistance to electrical breakdown and a low content of moisture and contaminants.
Cooling and heat dissipation
The second function is refrigeration. The transformer's cooling system, whether by natural or forced convection, allows the heat generated to be dissipated, avoiding overheating and reducing the thermal aging of the solid insulation.
Technical properties of dielectric oil
A dielectric oil's performance depends on a set of physical, chemical, and electrical properties. Understanding them is important both in the initial selection of the fluid and in its subsequent maintenance.
Dielectric strength: how it is measured and what values are safe
Dielectric strength indicates the ability of the oil to withstand an electrical voltage without a breakdown. It is determined by standard laboratory tests, applying increasing voltage between separate electrodes at a defined distance.
There is no single universal "safe" value, because it depends on the type of equipment, the state of the oil, and the test method used. Despite this, high and stable values are expected in new oils, and any sustained decline during service is usually a warning sign. The presence of water, particles, oxidation products, or gases can significantly reduce this property.
Rather than looking at an isolated figure, it's better to compare results with the manufacturer-defined limits, the applicable standards, and the history of the equipment itself. In predictive maintenance, the trend is usually just as important as the absolute value.
Viscosity and an oil's flash point: highly relevant to safety
Viscosity directly influences an oil's ability to circulate within the equipment and transfer heat. If it is too high, the heat dissipation worsens. If it is too low, it can affect the overall behavior of the system according to its design.
The flash point, on the other hand, is a fundamental safety parameter. It tells us at what temperature the oil can generate flammable vapors in the presence of an ignition source. In critical installations, this property gains even more weight, since a fluid with a higher flash point helps reduce the risk of fire.
At this point, natural and synthetic esters often offer advantages over mineral oil, especially in environments where fire safety or environmental sensitivity condition the design of the installation.
Compatibility with materials: gaskets, varnishes, and insulation
A dielectric oil coexists with cellulosic paper, varnishes, gaskets, elastomers, metals, and other internal equipment materials. That's why chemical compatibility is essential.
A poorly selected fluid can accelerate the deterioration of joints, alter varnishes, favor swelling or bulging in the transformer tank walls, or affect solid insulation. In transformers, in addition, the interaction between oil and cellulose has a direct influence on the useful life of the system. As such, the substitution or change of oil type should be studied with caution, especially in retrofillings or conversions from mineral oil to alternative fluids.
Types of dielectric oil
There is no single dielectric oil valid for all applications. The choice depends on the design of the equipment, the working temperature, environmental requirements, and the applicable regulations or technical specifications. This is why the range of oils includes different formulations, each designed for a specific equipment profile and requirement.
Mineral oil: the most widespread - advantages, limits, and regulations (IEC 60296)
Mineral oil remains the most widely used oil in transformers and electrical equipment due to its wide implementation, its competitive cost, and its excellent balance between dielectric properties and thermal behavior. In addition, it has an extensive experience of use in the sector.
Its main international reference for transformers is the IEC 60296 standard, which establishes requirements for new insulating mineral oils. This standard defines quality criteria related to electrical, physical, and chemical properties before commissioning.
However, their biodegradability and flash point are lower than those of other types of fluids. For this reason, although it remains the most widespread solution, it is not always the most suitable in sensitive areas or installations with special security requirements.
Natural and synthetic ester oils: biodegradable and high flash point
Natural and synthetic esters have gained prominence for two very clear reasons: their superior environmental profile and their high fire safety. Many of these fluids are biodegradable and have significantly higher flash or burn points than mineral oils.
Natural esters are generally known for their renewable origin and rapid biodegradability. Synthetic esters, on the other hand, provide great stability and very solid performance in demanding applications. Both can be an especially interesting solution in transformers located indoors, urban areas, critical infrastructures, or areas with high environmental sensitivity.
Applications: transformers, switches, capacitors and high voltage equipment
The best known application of dielectric oil is the electrical transformer, where it acts as an insulating and cooling medium. In these kinds of equipment, the fluid is decisive in containing the thermal and electrical stress to which the coils and core are subjected.
It is also used in high-voltage switches, where it assists in insulation and, depending on the design, switching off the electric arc. In certain capacitors, dielectric oil contributes to improving internal insulation and stabilizing equipment performance. In addition, it is present in other maneuvering, measuring, or electrical distribution equipment where continuity of service and safety are a priority.
Whatever the application, the quality of the fluid and its compliance with the corresponding standard are critical. In this regard, it is always advisable to review the technical specifications and the regulatory context. If you want to go deeper into this approach, here is a guide to international lubricant regulations that will help you understand why approvals and standards are so important.
Maintenance of dielectric oil: predictive analysis and when to replace it
Modern maintenance in transformers and electrical installations is increasingly supported by predictive analysis because it allows the type of fault to be detected before the fault appears.
The objective is not only to check if the oil "works" but to understand what is happening inside the equipment. A change in results may indicate oxidation, moisture ingress, localized overheating, cellulose aging, or even incipient electrical failures.
Parameters for analysis: acidity, humidity, dielectric strength, and dissolved gases
Some of the most common parameters are:
- Acidity, which reflects the degree of oxidation of the fluid.
- Moisture, critical because it reduces dielectric strength and accelerates insulation deterioration.
- The dielectric strength itself, which allows the insulating capacity of the oil to be evaluated.
- Dissolved gases, whose analysis identifies internal faults such as overheating, partial discharges, or arcs, depending on the type and concentration of gases present.
Additionally, the dielectric loss factor, interfacial tension, furan content, or particulate pollution status are also usually monitored, depending on the criticality of the equipment.
When to replace dielectric oil
There is no single answer. Sometimes it is enough to filter, degas, or dehydrate the oil. Other times, if chemical degradation is advanced or if the fluid can no longer be safely recovered, substitution is the most reasonable option. The important thing is to decide based on data and according to routine.
What are PCBs (polychlorinated biphenyls)?
PCBs are a family of synthetic organic compounds derived from biphenyl to which chlorine atoms have been incorporated. They were used for years as dielectric fluids due to their chemical stability and superb electrical behavior. The problem is that they are also persistent, bioaccumulative compounds that are hazardous to your health and to the environment.
Today, any old facility must treat this matter with the utmost caution. If there is suspicion of the presence of PCBs in our oil (and, therefore, in our transformer), an oil change is not enough: analytical characterization, management as hazardous waste, and strict compliance with applicable regulations are necessary.
FAQs about dielectric oil
What is dielectric oil used for in electrical transformers?
It is used for two essential functions: electrically insulating the stressed components and evacuating the heat generated during operation. In other words, it protects against shocks and helps keep the temperature under control.
What is the difference between mineral and synthetic dielectric oil?
Mineral oil is the most common and strikes a good middle ground between cost and performance. Synthetic fluids, such as esters, usually provide greater fire safety, better performance in specific environments, and in some cases a better environmental profile. The choice depends on the equipment and the environment of use.
How often should the oil in a transformer be replaced?
It is not usually replaced but treated. The decision must be based on an analysis of its condition: humidity, acidity, dielectric strength, dissolved gases, and treatment and use history. In many cases, good maintenance makes it possible to extend the life of the oil without replacing it prematurely. The guide for mineral oils IEC 60422 details the treatment to be carried out.
Can dielectric oil burn? What is its flash point?
Yes, dielectric oil can burn, although not all fluids behave the same. Mineral oil has a lower flash point than esters. That is why, in installations with high safety requirements, solutions with higher flash points or combustion points are used.
What is biodegradable dielectric oil?
It is an insulating fluid formulated to offer adequate electrical and thermal performance with a lower environmental impact in the event of a spill.