Natural Ester Transformer Fluid:
Benefits, Brands, and How to Order
Natural ester transformer fluid is a vegetable-based dielectric coolant that has become the dominant alternative to mineral oil for fire-safe, environmentally responsible transformer applications. Derived from seed oils — typically soybean, rapeseed (canola), or sunflower — natural ester fluids are manufactured by multiple suppliers worldwide and installed in millions of transformers across all voltage classes.
You may hear natural ester referred to by brand names like FR3 (Cargill), MIDEL eN (M&I Materials/Shell), BIOTEMP (Hitachi Energy), or BIOTRAN (DONGNAM). While these are different products from different manufacturers, all natural ester fluids conforming to ASTM D6871 and IEEE C57.147 share the same fundamental performance characteristics: dramatically higher fire safety than mineral oil (330°C flash point vs. 145°C), full biodegradability, and the ability to extend transformer insulation paper life by 5–8x.
TransformerParts.com supplies natural ester dielectric fluid in 55-gallon drums, 275-gallon totes, and full tanker loads with instant delivered pricing.
Get Natural Ester Pricing
What is Natural Ester Fluid?
Natural ester transformer fluid is manufactured from refined seed oils — typically soybean, rapeseed (canola), or sunflower — processed to achieve the electrical, chemical, and physical properties required for use as a dielectric coolant. The refining process removes impurities, reduces moisture content, and ensures consistent dielectric performance across production batches.
All natural ester fluids meeting industry specifications conform to:
-
ASTM D6871 — Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus
-
IEC 62770 — International standard for natural ester fluids
-
IEEE C57.147 — Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers
Because all compliant products meet the same rigorous specifications, the core performance characteristics — fire safety, biodegradability, moisture handling, paper life extension — are consistent across brands. The primary differences between products are the specific seed oil used (which affects pour point) and regional availability.
Natural Ester Brands and Manufacturers
Several manufacturers produce natural ester transformer fluids that meet ASTM D6871 / IEC 62770 specifications:
FR3 (Cargill)
FR3 — originally developed by Cooper Power Systems, now manufactured by Cargill — is the most widely installed natural ester fluid in North America. The name stands for "Fire Resistant, Food-grade, Fluid Replacement." FR3 is soybean-based, FM Approved, and UL Listed. Cargill reports over 3.5 million FR3-filled transformer installations worldwide.
MIDEL eN (M&I Materials / Shell)
MIDEL is the leading natural ester brand in Europe and increasingly available in North America. M&I Materials (now part of Shell) offers two natural ester products:
-
MIDEL eN 1204 — Rapeseed/canola-based with a pour point of –31°C, better suited for cooler climates
-
MIDEL eN 1215 — Soybean-based with a pour point of –18°C
MIDEL natural esters are FM Approved and UL Listed. The rapeseed-based MIDEL eN 1204 offers superior cold-weather performance compared to soybean-based alternatives.
BIOTEMP (Hitachi Energy)
BIOTEMP is a soybean-based natural ester originally developed by ABB, now part of Hitachi Energy. It received EPA Environmental Technology Verification confirming its performance and biodegradability. BIOTEMP is widely used in distribution and power transformers globally.
BIOTRAN-35 (DONGNAM Petroleum)
BIOTRAN-35 is manufactured by DONGNAM Petroleum in South Korea. It is UL Listed and FM Approved, and has been used in transformers up to 765kV. DONGNAM has been producing transformer oil since 1973 and developed BIOTRAN-35 as their natural ester offering.
bioTRANSOL-HF (Savita)
bioTRANSOL-HF is manufactured by Savita, one of India's largest transformer oil producers. It meets IEC 62770 specifications and holds FM Global approval. Savita has been producing transformer fluids for over 40 years.
Which Brand Should You Choose?
For most applications, the differences between compliant natural ester brands are minimal. All meet the same ASTM D6871 / IEC 62770 specifications and deliver equivalent fire safety, biodegradability, and paper life extension.
Consider brand selection based on:
-
Regional availability and lead time — FR3 dominates North American distribution; MIDEL is stronger in Europe
-
Cold climate requirements — MIDEL eN 1204 (rapeseed) has a –31°C pour point vs. –18°C for soybean-based products
-
Transformer manufacturer preference — some OEMs have testing history with specific brands
-
Price and delivery — get quotes on multiple brands if availability allows
Brand | Manufacturer | Base Oil | Pour Point | Certifications | Notes |
|---|---|---|---|---|---|
FR3 | Cargill | Soybean | –18°C | FM Approved, UL Listed | Most widely installed in North America; 3.5M+ installations worldwide |
MIDEL eN 1204 | M&I Materials (Shell) | Rapeseed/Canola | –31°C | FM Approved, UL Listed | Better cold-weather performance; popular in Europe |
MIDEL eN 1215 | M&I Materials (Shell) | Soybean | –18°C | FM Approved, UL Listed | Standard natural ester option from MIDEL |
BIOTEMP | Hitachi Energy (formerly ABB) | Soybean | –18°C | EPA Environmental Technology Verified | Widely used in distribution and power transformers globally |
BIOTRAN-35 | DONGNAM Petroleum (South Korea) | Vegetable oil | –18°C | UL Listed, FM Approved | Used in transformers up to 765kV; manufacturer established 1973 |
bioTRANSOL-HF | Savita (India) | Vegetable oil | –18°C | FM Approved, IEC 62770 | One of India's largest transformer oil producers |
Natural Ester Brands and Manufacturers
All natural ester fluids meeting ASTM D6871 share these core characteristics:
Property | Natural Ester (typical) | Mineral Oil (for comparison) |
|---|---|---|
Base Chemistry | Vegetable oil (soybean, rapeseed, sunflower) | Petroleum hydrocarbon |
Flash Point | ≥330°C | ~145°C |
Fire Point | ≥360°C | ~160°C |
Pour Point | –18°C to –31°C (varies by seed oil) | –40°C to –60°C |
Dielectric Breakdown (ASTM D1816 2mm) | ≥75 kV | ≥56 kV |
Biodegradability (OECD 301B) | >97% (readily biodegradable) | <30% |
Moisture Saturation at 20°C | ~1100 ppm | ~55 ppm |
Paper Life Extension | 5–8x mineral oil baseline | Baseline (1x) |
Applicable Standards | ASTM D6871, IEC 62770, IEEE C57.147 | ASTM D3487 |
Fire Safety Advantages
The single most important difference between natural ester and mineral oil is fire resistance. Natural ester's flash point of 330°C is more than double that of mineral oil (145°C), which fundamentally changes the fire risk profile of transformer installations.
How Flash Point Affects Fire Risk
Flash point is the temperature at which oil vapor will ignite when exposed to an ignition source. In a transformer fault scenario — particularly an internal arc — temperatures at the fault location can exceed 2,000°C. If the tank is breached and hot oil is expelled, mineral oil vapor ignites almost immediately on contact with air.
Natural ester's high flash point means the expelled fluid is far less likely to ignite, and if it does ignite, the resulting fire is less intense and easier to suppress. In standardized pool fire tests conducted per IEEE C57.152, natural ester meets the criteria for "less-flammable" K-class fluid designation under IEC 61100.
Practical Implications
The fire safety difference translates to tangible project benefits:
Reduced fire suppression requirements. Indoor and vault-type installations using K-class fluids may eliminate or reduce requirements for deluge systems, foam suppression, CO2 systems, and fire-rated construction. These infrastructure savings often offset the fluid cost premium entirely.
Reduced clearance requirements. FM Global, IEEE, and local fire codes allow reduced clearances between K-class transformers and adjacent structures.
Lower insurance costs. Some insurers offer reduced premiums for facilities using less-flammable fluids.
Indoor installation feasibility. Many jurisdictions prohibit or restrict mineral oil transformers inside buildings. Natural ester makes indoor transformer installations practical where they would otherwise require expensive dry-type alternatives.
Insulation Paper Life Extension
Beyond fire safety, natural ester's ability to extend transformer insulation paper life is a significant economic advantage.
The Science
Transformer insulation paper degrades through three mechanisms: thermal aging, oxidation, and hydrolysis — chemical decomposition caused by water. Of these, hydrolysis is the dominant degradation mechanism.
The key difference between natural ester and mineral oil is moisture saturation capacity:
-
Mineral oil saturates at approximately 55 ppm at 20°C
-
Natural ester saturates at approximately 1,100 ppm at 20°C
Because natural ester can hold 20x more dissolved water than mineral oil, it actively pulls moisture out of the cellulose paper insulation and into the fluid. This reduces the paper's moisture content, which directly slows the hydrolysis reaction.
The Result
Research published in IEEE and CIGRE technical brochures demonstrates that natural ester extends cellulose insulation life by a factor of 5–8x compared to mineral oil under equivalent thermal conditions.
In practical terms, a transformer designed for a 30-year life with mineral oil could potentially achieve 40–50+ years with natural ester. For asset managers, this life extension represents substantial capital deferral value.
Environmental Benefits
Natural ester is classified as "readily biodegradable" under OECD 301B testing, meaning it degrades >97% within 28 days under aerobic conditions.
Spill Impact
Mineral oil spills require active remediation: excavation, soil treatment, groundwater monitoring. Cleanup costs for significant spills can reach six figures.
Natural ester spills still require response and notification, but the fluid breaks down naturally in soil and water. Remediation scope is dramatically reduced.
Regulatory Drivers
Environmental regulations increasingly favor biodegradable transformer fluids:
-
Installations near waterways, wetlands, or groundwater sources
-
Sites with SPCC plan requirements
-
LEED and sustainable building certifications
-
Jurisdictions with local ordinances restricting petroleum-based fluids
Applications
Where Natural Ester is Commonly Specified
Indoor substations and vault transformers — Fire safety is the primary driver.
Urban substations — Reduced fire risk and environmental impact in densely populated areas.
Data centers — Indoor installations, 24/7 critical operation, and corporate sustainability commitments.
Renewable energy installations — Wind farms, solar facilities, and battery storage projects with environmental commitments.
Environmentally sensitive locations — Near water, wetlands, or protected habitats.
Where Mineral Oil May Still Be Preferred
Extreme cold climates — Natural ester's pour point (–18°C to –31°C) limits use in unheated outdoor installations in extreme northern climates. Synthetic ester or mineral oil are better choices.
Cost-constrained projects — Natural ester costs 2–3x more per gallon than mineral oil.
Existing mineral oil fleets — Some utilities prefer to maintain a single fluid type for inventory simplicity.
Retrofilling Existing Transformers
Natural ester can replace mineral oil in existing transformers through a retrofill process.
The Retrofill Process
-
De-energize and isolate the transformer
-
Drain the existing mineral oil
-
Flush the tank with natural ester to remove residual mineral oil
-
Refill with natural ester
-
Process the fluid (vacuum dehydration, degassing) if required
-
Test dielectric strength and moisture content before re-energizing
IEEE C57.147 provides guidelines for retrofill procedures. Residual mineral oil content below 7% is generally acceptable.
Retrofill Considerations
-
Gasket compatibility — Most modern gasket materials (nitrile, Viton) are compatible. Older cork or neoprene gaskets may require replacement.
-
Manufacturer approval — Major transformer manufacturers (ABB, Siemens, GE, Hitachi Energy, Prolec, Delta Star) approve natural ester in their equipment.
Ordering Natural Ester Fluid
TransformerParts.com supplies natural ester dielectric fluid in:
-
55-gallon drums — for small retrofills, top-offs, and maintenance
-
330-gallon totes — for mid-size projects and inventory stocking
-
Tanker loads (6,000+ gallons) — for large retrofills and new transformer fills
All natural ester fluid ships with certificates of analysis confirming compliance with ASTM D6871 specifications.
Get Natural Ester Pricing
Frequently Asked Questions
Is FR3 the same as natural ester?
FR3 is a brand name for a specific natural ester product manufactured by Cargill. "Natural ester" is the generic category that includes FR3, MIDEL eN, BIOTEMP, BIOTRAN, and other products. All compliant natural ester fluids meet the same ASTM D6871 / IEC 62770 specifications and deliver equivalent core performance.
Are all natural ester brands interchangeable?
For most purposes, yes. All ASTM D6871-compliant natural ester fluids share the same fundamental chemistry and performance characteristics. However, you should not mix different brands in the same transformer without verifying compatibility, and you should follow your transformer manufacturer's recommendations.
Can I mix natural ester with mineral oil?
Not intentionally. Retrofills require draining and flushing to below 7% mineral oil contamination per IEEE C57.147.
Which natural ester brand is best?
There is no universally "best" brand — all compliant products deliver equivalent fire safety, biodegradability, and paper life extension. Choose based on regional availability, cold climate requirements (rapeseed-based products have better pour points), transformer manufacturer preference, and price.
How does natural ester perform in cold weather?
Natural ester's pour point varies by seed oil source. Soybean-based products (FR3, MIDEL eN 1215, BIOTEMP) typically have pour points around –18°C. Rapeseed-based products (MIDEL eN 1204) offer –31°C pour points. For extreme cold (below –25°C), synthetic ester or mineral oil may be better choices.
An engineer is online — click to chat