Overview of Transformer Coolants

Oil-filled transformers require coolants that meet specific performance criteria, including thermal conductivity, dielectric strength, and fire safety. Each coolant offers unique advantages suited for different transformer designs and operating environments.

Mineral Oil

Mineral oil is the most commonly used coolant due to its cost-effectiveness and strong dielectric properties. It efficiently dissipates heat, but its flammability and environmental impact have led to the exploration of alternative fluids.

FR3 Ester

FR3 ester-based fluids are biodegradable and provide enhanced fire resistance. Their environmentally friendly properties and lower toxicity make them a safer choice. However, they are more expensive than mineral oil.

Synthetic Liquids

Synthetic coolants, such as polyalphaolefins, offer excellent thermal stability and function well in extreme temperatures. They are used in high-performance applications requiring precise temperature control and extended equipment life.

Silicone Liquids

Silicone-based transformer coolants have a wide operating temperature range and strong insulating properties. However, their higher cost and potential material compatibility issues can limit their use.

Technical Dielectric Fluid Comparison

Below is a summary of key characteristics to consider when comparing transformer coolants:

• Dielectric Strength: Determines the coolant’s ability to resist electrical breakdown.
• Thermal Conductivity: Affects heat dissipation efficiency, with synthetic and silicone liquids performing well under high loads.
• Fire Resistance: FR3 ester fluids have a higher fire point than mineral oil, improving safety.
• Environmental Impact: Biodegradability and toxicity are critical for sustainable transformer operations.
• Cost and Availability: Mineral oil is generally the most cost-effective, while specialty fluids are more expensive.

The choice of transformer coolant depends on balancing performance, safety, environmental impact, and cost.

Whether your system requires robust thermal management or prioritizes fire safety and sustainability, selecting the right dielectric fluid is essential. Review the detailed technical comparison table below to match the best coolant to your transformer’s needs.

	Mineral Oil	FR3® Dielectric Fluid Natural Ester	Envirotemp™ 360 Fluid Synthetic Ester	Silicone Transformer Oil
Base Fluid	Petroleum oil	Vegetable seed oil	Hydrocarbons	Polydimethylsiloxanes
Voltage Class Applicable	Any, incl. AC and HVDC	Any, incl. AC and HVDC	Any, incl. voltage AC	limited to 66 kV
Fire Point	160°C	360°C	>315°C	343°C
Flash Point	140°C	330°C	>260°C	324°C
Density at 20°C	880 kg/cm3	920 kg/m3	960 kg/m3	960 kg/cm3
Pour Point	-35°C	-21°C	-48°C	-55°C
Kinematic Viscosity 100°C 40°C -20°C	2 – 3 mm2/sec 8 – 9 mm2/sec 230 mm2/sec	7.7 – 8.3 mm2/sec 32 – 34 mm2/sec 500 mm2/sec	6 mm2/sec 34 mm2/sec 1700 mm2/sec	20 mm2/sec 45 mm2/sec 200 mm2/sec
Toxicity	Potentially toxic	Non-toxic, non-hazardous in soil & water	Not classified as hazardous according to GHS	Potentially toxic
Biodegradability	Non-biodegradable	Ultimately biodegradable	Readily biodegradable	Not biodegradable (only oxidegradable)
Biobased	No	Yes	No	No
Oxidation Stability Non-free-breathing Free-breathing	Very good Good	Very good Not recommended	Very good Good (moisture control recommended)	Very good Not recommended
Temp. Limits for Normal Lifespan (TUK Paper)	65°C avg. winding rise 110°C hottest spot	85°C avg. winding rise 130°C hottest spot	65°C avg. winding rise 110°C hottest spot	65°C avg. winding rise 110°C hottest spot
Degradation Effects/Consequences	Average; reduces dielectric capacity and produces sludge	Best; long-term impact in viscosity with no formation of sludge	Better; may produce minimal sludge	Average; may produce sludge
Transformer reliability	Declines with aging as moisture content in paper and oil increases	Preserves dielectric capacity due to continuous drying of insulation system	May decline as moisture content in paper increases	Typically only used up to 66 kV; no available information
Application Location	Indoor & outdoor; requires complex containment & fire mitigation systems	Indoor & outdoor; highly simplified containment & fire mitigation systems	Indoor & outdoor; highly simplified containment & fire mitigation systems	Indoor & outdoor; requires containment system