An insulating fluid in the transformer tank, transformer oil surrounds the core and windings. Two major roles for this oil: working as insulation between electrical elements, and cooling. Without this service, transformers would both overheat and suffer electrical breakdowns due to poor insulation.

Given the very high voltages at which transformers operate and the amount of heat they generate, it is essential that the oil exhibit very good chemical and physical properties. Among the requirements: the oil must resist oxidation, remain stable under varying temperatures, and prevent the occurrence of electrical discharges between the internal parts. It must flow easily to allow the heat generated in the internal parts to escape through the transformer tank, which is often aided by radiators or cooling fans.

 

Mineral Oil

Mineral oil is obtained from the refining process of crude oil and is therefore easily available all over the world. Despite new competitors in the market, mineral oil remains popular because it’s inexpensive and offers a good balance between performance and cost.

It doesn’t easily conduct electricity, even under high voltages. Doing this reduces the chances of having short circuits and internal flashovers inside the transformer, which can lead to damage. It also has decent cooling properties that allow the efficient transfer of heat from the transformer.

It is flammable. Hence, special safety measures must be taken to prevent fire hazards. Furthermore, mineral oil can degrade over time due to oxidation, which formation of acids and sludge impairs insulation quality. Transformers filled with mineral oil should therefore undergo regular maintenance, including oil sampling and testing.

Many utilities and industries favor mineral oil transformers because of their proven track record and relatively simple maintenance. Modern technologies have also improved their oxidation stability and extended oil life through purification techniques.

 

Synthetic Oils

Synthetic transformer oils are man-made fluids designed to overcome some limitations of mineral oil. Common types include silicone-based oils and polyalphaolefin (PAO) oils. These oils offer better thermal and electrical properties, making them suitable for transformers operating in harsh environments or with special requirements.

Silicone oils have exceptional thermal stability, meaning they can handle higher temperatures without breaking down. This quality allows transformers to operate at greater loads without overheating, boosting efficiency and lifespan. They have low volatility, reducing evaporation losses and minimizing the need for top-ups.

Unlike mineral oil, these fluids do not degrade rapidly when exposed to oxygen at high temperatures. As a result, transformers filled with synthetic oils require less maintenance and experience fewer insulation problems over time.

Synthetic oils are more expensive and less widely available than mineral oils. Their environmental impact is also a consideration, as some synthetic fluids can be less biodegradable. Their use is often reserved for specialized transformers or applications where specific performance criteria demand it.

 

Natural Ester Oils

Power Transformer OilsIn recent years, natural ester oils derived from vegetable oils (such as soybean or sunflower oil) have gained traction as environmentally friendly alternatives to traditional transformer oils. These oils offer a unique combination of electrical insulating properties and biodegradability, along with enhanced safety features.

One of the key advantages of natural ester oils is their high flash point and fire point, which make transformers much less likely to catch fire under fault conditions. This reduces safety risks in populated or sensitive areas and aligns with more stringent fire safety regulations.

Natural esters provide excellent moisture tolerance. They can absorb water without a significant drop in dielectric strength, unlike mineral oil, which is more sensitive to moisture contamination. This means transformers using natural ester oils can perform better in damp or humid environments, extending asset life and reliability.

From a sustainability standpoint, natural esters are biodegradable and derived from renewable sources, making them attractive to energy companies aiming to lower their environmental footprint. They may be slightly more viscous, which can affect flow and cooling, and their oxidation stability, although good, may require careful monitoring.

Natural ester oils strike a solid balance between performance, safety, and eco-friendliness, leading to their growing use in modern transformers.

 

Functions of Transformer Oils Beyond Insulation and Cooling

Transformer coils and cores carry high voltages and currents, often within a few millimeters of one another. Transformer oil fills this space to prevent electrical arcs or shorts. If the oil’s dielectric strength weakens, the likelihood of electrical faults goes up dramatically. The oil’s purity and condition directly affect transformer insulation integrity.

Inside transformers, tiny electrical discharges called corona or partial discharges can occur when voltages jump across gaps or imperfections. These discharges generate heat and degrade materials over time. Transformer oil acts as a barrier that helps suppress and minimize these small electrical events, prolonging equipment longevity.

Transformer oils can slow down aging in insulation paper wrapped around windings by absorbing impurities and moisture that accelerate breakdown. This protection extends the life of costly winding materials.

Transformers generate heat during operation, and failure to remove it quickly causes damage to windings and insulation. Transformer oil circulates through cooling systems, transferring internal heat to radiators or external cooling units. This prevents hot spots and thermal stresses.

 

Proper Handling and Maintenance of Transformer Oils

Keeping transformer oil in good condition is critical for the overall reliability of power transformers. Regular testing and maintenance schedules help detect problems early.

Oil samples are taken periodically to check for signs of contamination, moisture content, acidity, and dielectric strength. Measurements such as Dissolved Gas Analysis (DGA) can identify internal faults by detecting gases generated from electrical or thermal stress in the oil.

Purification methods like oil filtering, dehydration, and regeneration help remove impurities and restore oil properties without complete replacement. Proper storage and handling reduce the risk of contamination by dust, water, or air.

When replacing or topping up oils, the compatibility of the new fluid with existing oil and transformer materials must be checked to avoid chemical reactions or insulation damage.

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