Synthetic Ester Transformer Fluid
Properties, standards and applications of synthetic ester dielectric fluids – and how they compare with natural ester and mineral oil.
What is a synthetic ester transformer fluid?
A synthetic ester is a dielectric liquid produced by reacting a polyol (typically pentaerythritol) with selected fatty acids. The result is a tailored molecule that insulates electrically and carries heat away from the core and windings of a transformer. Synthetic esters are used instead of mineral oil where fire safety, moisture tolerance and long-term oxidation stability matter.
Unused synthetic esters for electrical purposes are specified in IEC 61099. Guidance for their maintenance and use in equipment is given in IEC 61203.
Key properties
| Property | Synthetic ester (typical) | Why it matters |
|---|---|---|
| Fire point | Above 300 °C | Classifies the fluid as K-class (IEC 61039) – less flammable, which can reduce fire protection requirements |
| Moisture tolerance | Water saturation many times higher than mineral oil | Dielectric strength stays high even when the fluid absorbs water |
| Oxidation stability | Very high | Suitable for free-breathing transformers with a conservator |
| Pour point | Very low | Usable in cold climates and outdoor installations |
| Viscosity | Higher than mineral oil | Has to be considered in the thermal design of the transformer |
Exact values differ by product. For IsoTec SE 300, see the product page and data sheet.
Synthetic ester, natural ester or mineral oil?
| Synthetic ester | Natural ester | Mineral oil | |
|---|---|---|---|
| Base | Synthesised polyol ester | Vegetable oil (triglycerides) | Refined petroleum |
| Specification | IEC 61099 | IEC 62770, ASTM D6871 | IEC 60296 |
| Fire class (IEC 61039) | K | K | O |
| Oxidation stability | Very high – free-breathing possible | Lower – mainly sealed designs | High |
| Pour point | Very low | Moderate | Low |
| Moisture tolerance | Very high | Very high | Low |
Natural esters are covered in detail in our natural ester guide.
Applications
- Distribution transformers in buildings, tunnels and other locations where fire safety is critical
- Transformers in wind turbines, onshore and offshore
- Traction transformers in rail vehicles
- Power transformers and new designs that use the higher fire point
- Retrofilling of existing mineral-oil transformers
Retrofilling
When a mineral-oil transformer is retrofilled with synthetic ester, residual mineral oil remains in the insulation. A higher residual content lowers the fire point of the mixture, so the procedure, drainage time and target values need to be planned. Our laboratory checks material compatibility and the fluid before and after the retrofill.
Biodegradability
IsoTec SE 300 is readily biodegradable according to OECD 301 F: the finished product reached 89 % biodegradation after 28 days (pass level 60 %).
Biodegradability does not mean that a fluid is harmless. Requirements for containment depend on the site and are determined case by case by the competent authority.
IsoTec synthetic ester fluids
- IsoTec SE 300 – synthetic ester transformer fluid, K-class
- IsoTec RSE 300 – synthetic ester regenerated from used fluid to IEC 61099 specification
Frequently asked questions
What does K-class mean?
IEC 61039 classifies insulating liquids by fire point. K-class liquids have a fire point above 300 °C; mineral oil is O-class.
Is synthetic ester miscible with mineral oil?
Yes. Residual mineral oil after a retrofill mixes with the ester, but it lowers the fire point of the mixture. This is why the residual content is planned and measured.
Can synthetic ester be used in free-breathing transformers?
Synthetic esters have very high oxidation stability and are commonly used in free-breathing designs with a conservator. The final decision depends on the transformer manufacturer.
Which standard applies to unused synthetic ester?
IEC 61099 specifies unused synthetic organic esters for electrical purposes; IEC 61203 gives guidance on maintenance and use.
Planning a project with synthetic ester?
Our laboratory supports fluid selection, material compatibility tests and retrofilling. IsoTec products are supplied to customers outside Germany.