What Type of Oil Is Used in Transformers Today

When considering what type of oil is in a transformer, the options include mineral oil, synthetic oil, natural esters, synthetic esters, and silicone oil. Understanding what type of oil is in a transformer is important because transformer oil is essential for both cooling and insulation, and the selection of oil affects safety and environmental impact. While mineral oil is still widely used, concerns about its toxicity and limited biodegradability have prompted a shift toward natural ester oils. These alternatives provide high temperature stability and environmental advantages, even though they have higher viscosity.

Key Takeaways

• Mineral oil is the most common transformer oil, known for its low cost and reliable insulation properties, but it poses environmental risks due to poor biodegradability.

• Natural ester oil is an eco-friendly alternative, offering high biodegradability and excellent fire safety, making it ideal for sensitive environments.

• Synthetic ester oil combines the benefits of natural esters with improved stability, suitable for high-voltage applications requiring long service intervals.

• Silicone oil provides exceptional fire resistance and thermal stability, making it a preferred choice in high-risk areas, despite its higher cost.

• Choosing the right transformer oil depends on factors like fire safety, environmental impact, and specific application needs, ensuring optimal performance and compliance.

Types of Transformer Oil

Transformer oil plays a vital role in the operation and safety of electrical transformers. The industry recognizes several transformer oil types, each with unique properties and applications. The following sections outline the main types of transformer oil used today.

Note: Mineral oil remains the most common transformer oil, but alternative transformer oil types are gaining popularity for specific applications.

Mineral Oil

Mineral oil dominates the global market for transformer oil types. Manufacturers produce mineral oil from petroleum, and it comes in two main forms: paraffinic and naphthenic. Paraffinic mineral oil offers better oxidation stability, while naphthenic mineral oil provides improved low-temperature performance. Mineral oil is classified as either Type 1 (uninhibited) or Type 2 (inhibited). Type 2 contains additives that enhance oxidation resistance. Over 65% of the global market relies on mineral oil due to its low cost and established supply chain, especially in Asia Pacific and North America.

Synthetic Oil

Synthetic oil represents a group of transformer oil types engineered for enhanced performance. These transformer oil types include synthetic hydrocarbons and other chemical blends. Synthetic oil provides superior oxidation stability and longer service life compared to mineral oil. Utilities select synthetic oil for demanding environments where transformer oil must withstand extreme temperatures or heavy loads.

Natural Ester Oil

Natural ester oil belongs to the environmentally friendly transformer oil types. Manufacturers derive natural esters from plant sources, such as soybean or rapeseed. This transformer oil type offers high biodegradability and excellent fire safety. Natural ester transformer oil types are often chosen for installations near water sources or sensitive ecosystems. The flashpoint and firepoint of natural ester oil are much higher than those of mineral oil, reducing fire risk.

Synthetic Ester Oil

Synthetic ester oil combines the benefits of natural esters with improved chemical stability. This transformer oil type resists oxidation and maintains performance over a wide temperature range. Synthetic ester transformer oil types suit high-voltage and high-temperature applications. Utilities use synthetic ester oil in transformers that require long service intervals and minimal maintenance.

Silicone Oil

Silicone oil stands out among transformer oil types for its exceptional fire resistance and thermal stability. This transformer oil type contains siloxane compounds, which provide a high flashpoint and firepoint. Silicone transformer oil types are non-biodegradable but have low toxicity. Utilities use silicone oil in transformers located in densely populated or hazardous areas.

• The types of transformer fluids listed above show distinct differences in fire safety, environmental impact, and cost.

Transformer Oil Types and Properties

Insulation and Cooling

Transformer oil provides insulation and cooling for electrical transformers. The ideal properties of transformer oil include high dielectric strength, low viscosity, and stable chemical composition. Mineral oil remains the most common choice due to its reliable insulation and moderate cooling performance. Utilities prefer mineral oil for small and medium transformers because it offers a breakdown voltage of 46 kV and a kinematic viscosity of 9.98 mm²/s at 40°C.

Natural ester transformer oil delivers superior insulation, with a breakdown voltage of 65 kV. Synthetic ester transformer oil exceeds 75 kV, making it suitable for high-voltage applications. Silicone oil achieves the highest breakdown voltage at 85 kV, supporting demanding environments. The ideal properties of transformer oil also include low moisture content and minimal acidity, which help maintain insulation quality.

Cooling efficiency depends on the transformer oil and the cooling method. ONAN systems use transformer oil for moderate cooling in small units. ONAF systems increase efficiency for larger transformers. OFAF and OFWF methods deliver very high and extremely high cooling, using transformer oil in extra-large installations.

• ONAN: Moderate cooling, low maintenance.

• ONAF: High cooling, medium maintenance.

• OFAF: Very high cooling, high maintenance.

• OFWF: Extremely high cooling, high maintenance.

Fire Safety

Fire safety is a critical factor in transformer oil selection. Mineral oil has a flash point between 140°C and 160°C, which limits its fire resistance. Silicone oil offers a flash point above 300°C and a fire point near 370°C, making it suitable for hazardous locations. Synthetic ester transformer oil provides outstanding fire resistance and high dielectric strength. Natural ester transformer oil also features a high flash point of 330°C and a fire point of 362°C, reducing fire risk in sensitive environments.

• Mineral oil: Low fire resistance, less oxidation-resistant.

• Silicone oil: High flash and fire points, tolerates high temperatures.

• Synthetic ester oil: Excellent fire resistance and stability.

• Natural ester oil: High flash and fire points, preferred for fire-sensitive areas.

Environmental Impact

Transformer oil impacts the environment through biodegradability and toxicity. Natural ester transformer oil is plant-based and biodegradable, making it environmentally friendly. In case of leaks, natural ester transformer oil causes less harm to soil and water. Synthetic ester transformer oil also offers high biodegradability and low toxicity, reducing contamination risks. Silicone oil is non-biodegradable but has low toxicity. Mineral oil has poor biodegradability and poses greater risks during spills.

• Natural esters: Biodegradable, non-toxic, safe for sensitive ecosystems.

• Synthetic esters: High biodegradability, minimal greenhouse gas emissions.

• Silicone oil: Non-biodegradable, low toxicity.

• Mineral oil: Poor biodegradability, higher environmental risk.

Pros and Cons of Types of Transformer Oils

Mineral Oil Pros and Cons

Mineral oil remains the most widely used transformer oil. Utilities choose mineral oil for its low cost and reliable insulating properties.

• Pros:

• Affordable and easy to source

• Good dielectric strength

• Well-understood performance in transformer oil testing

• Cons:

• Low fire resistance

• Moderate toxicity risk, including skin and respiratory irritation

• Poor biodegradability, which can lead to soil and water contamination

• Less oxidation resistance, requiring frequent transformer oil testing

Mineral oil costs much less than silicone oil, making it attractive for large-scale installations. However, its environmental impact and safety concerns drive the search for alternatives.

Synthetic Oil Pros and Cons

Synthetic oil offers enhanced performance for transformer oil applications.

• Pros:

• Superior oxidation stability

• Longer service life

• Performs well in extreme temperatures

• Reliable results in transformer oil testing

• Cons:

• Higher cost than mineral oil

• May present mild irritation risks

• Not always biodegradable

• Less common in standard transformer oil testing procedures

Ester Oil Pros and Cons

Ester oils include both natural and synthetic types. They have become popular for eco-friendly transformer oil solutions.

• Pros:

• Outstanding fire resistance

• High biodegradability

• Minimal toxicity, often food-grade origins

• Positive impact on solid insulation aging

• Reliable performance in transformer oil testing

• Cons:

• Higher cost than mineral and silicone oils

• Natural esters lose effectiveness at higher winding temperatures

• Synthetic esters may still cause mild irritation

• Require specialized transformer oil testing methods

Ester oils provide a cost-effective alternative in the long run due to their environmental benefits.

Silicone Oil Pros and Cons

Silicone oil stands out for its fire-resistant properties in transformer oil applications.

• Pros:

• High ignition point, excellent flame resistance

• Exceptional thermal stability

• Low toxicity risk

• Performs well in transformer oil testing for hazardous locations

• Cons:

• Very high cost, about six times more expensive than mineral oil

• Non-biodegradable

• Lower dielectric strength compared to other transformer oil types

• Less common in routine transformer oil testing

Silicone oil prices reach $7 per quart, making it suitable only for specialized transformer oil applications.

What Type of Oil Is in a Transformer Today

Industry Trends

The transformer oil market is experiencing rapid change. Utilities and manufacturers now face stricter environmental regulations in major markets such as the United States, Europe, and Asia. These rules encourage the use of eco-friendly transformer oil, especially in new installations. The demand for mineral oil remains strong, accounting for over three-quarters of global transformer oil consumption. However, the industry is shifting toward synthetic and bio-based oils due to their environmental benefits and improved performance.

Several factors drive this transition:

• Consumer demand for sustainable solutions pushes companies to adopt greener transformer oil.

• Bio-based transformer oil offers better fire resistance and cooling efficiency, making it suitable for high-performance transformers.

• Governments provide tax breaks, subsidies, and research grants to promote renewable technologies, motivating investment in bio-based transformer oil.

• Companies focus on corporate social responsibility, using bio-based transformer oil to reduce carbon footprints and meet regulatory requirements.

Environmental regulations also play a major role. In the U.S., strict standards promote the use of biodegradable transformer oil. Europe follows IEC standards, while North America relies on ANSI/IEEE guidelines. Neglecting these standards can result in equipment failing local acceptance tests. The modernization of power grids and the integration of renewable energy sources further increase the need for transformer oil with enhanced oxidation stability.

The market for transformer oil is projected to grow from $4.7 billion in 2025 to over $11 billion by 2035, with a notable shift toward synthetic and bio-based oils in sensitive environments.

Application Choices

Selecting what type of oil is in a transformer depends on the application and regulatory requirements. Utilities choose transformer oil based on fire safety, environmental impact, and performance needs. In urban areas, regulations often favor dry-type transformers due to fire safety concerns. Industrial settings prefer oil-immersed transformers for their high power capacity.

The most common transformer oil types used today include:

• Mineral oil: Cost-effective and reliable, preferred for power distribution and standard industrial applications.

• Synthetic oil: Offers superior thermal stability and fire resistance, chosen for high-voltage and high-temperature transformers.

• Ester oils (natural and synthetic): Biodegradable and fire-resistant, ideal for environmentally sensitive or fire-critical installations.

• Silicone oil: Used in locations with high fire risk, valued for its exceptional fire resistance and long service life.

Transformer manufacturers recommend selecting transformer oil based on transformer design and application context. The table below summarizes these choices:

Maintenance requirements also influence what type of oil is in a transformer. Mineral oil requires regular testing for dielectric strength, moisture, and acidity. Silicone oil needs less frequent testing due to its high oxidation stability. Natural ester oil demands monitoring for moisture content, while synthetic ester oil allows extended intervals between maintenance.

Utilities and manufacturers consider several factors when choosing transformer oil for new installations or retrofitting existing transformers:

Modern transformers in high-voltage, environmentally sensitive, or fire-critical applications often use bio-based transformer oil, high fire point transformer oil, or natural esters. These oils provide high fire points, biodegradability, and good dielectric strength. Utilities select these transformer oil types to reduce environmental impact and improve safety.

The choice of what type of oil is in a transformer directly affects reliability and failure rates. High-quality transformer oil with antioxidants improves oxidative stability and extends transformer lifespan. Mineral oil remains popular for its electrical and thermal properties, but environmental concerns have led to increased use of biodegradable alternatives.

One of the biggest drivers behind the popularity of natural and synthetic esters is their high flash point and fire point. These properties make them safer for both indoor and outdoor transformer installations.

Mineral oil remains the most widely used transformer oil, but silicone-based and bio-based options are gaining ground due to stricter regulations and environmental priorities. Performance, safety, environmental impact, and compliance drive oil selection. Recent case studies highlight that silicone oil excels in high-temperature settings, while natural esters offer eco-friendly benefits. When choosing transformer oil, users should review key specifications:

Regular monitoring and timely replacement of transformer oil help maintain reliability and extend equipment life.

FAQ

What is the main function of transformer oil?

Transformer oil insulates electrical components and removes heat from the transformer core and windings. It prevents electrical breakdown and supports safe, efficient operation.

How often should transformer oil be tested?

Experts recommend testing transformer oil every 12 to 24 months. Regular testing helps detect moisture, acidity, and other contaminants that can reduce performance or cause failures.

Can transformer oil be recycled or reused?

Many facilities recycle transformer oil using filtration and chemical treatment. Recycled oil must meet strict standards before reuse in electrical equipment.

Which transformer oil is safest for the environment?

Natural ester oil offers the highest biodegradability and lowest toxicity, making it the safest choice for the environment.