Natural Ester Transformer Fluid
How natural ester dielectric fluids based on vegetable oil work, which standards apply and where they are used.
What is a natural ester transformer fluid?
Natural esters are dielectric liquids based on vegetable oils, chemically triglycerides. Refined and additised for electrical use, they insulate and cool transformers as an alternative to mineral oil. Their high fire point and high moisture tolerance are the main reasons for choosing them.
Unused natural esters are specified in IEC 62770 and ASTM D6871. Guidance on maintenance and use is given in IEC 62975 and IEEE C57.147.
Key properties
| Property | Natural ester (typical) | What it means in practice |
|---|---|---|
| Fire point | Above 300 °C | K-class fluid according to IEC 61039 |
| Moisture tolerance | Very high water saturation | Absorbs water from the cellulose insulation while dielectric strength stays high |
| Oxidation stability | Lower than synthetic ester or mineral oil | Mainly used in sealed transformers or with suitable air protection |
| Pour point | Moderate | Cold-start conditions have to be considered in cold climates |
| Viscosity | Higher than mineral oil | Considered in the cooling design |
Values for IsoTec NE 350 are on the product page. A comparison with synthetic ester and mineral oil is in our synthetic ester guide.
Applications
- Distribution transformers – ground-mounted, pole-mounted and in compact substations
- Power transformers in new installations
- Urban substations and indoor installations with fire safety requirements
- Retrofilling of mineral-oil transformers
Sealed or free-breathing?
Because natural esters oxidise more readily than synthetic esters, contact with air should be limited. In practice they are mostly used in hermetically sealed transformers or in designs with a membrane or other air protection.
Biodegradability
IsoTec NE 350 is readily biodegradable according to the OECD 301 test criteria (≥ 60 % biodegradation within 28 days).
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 natural ester fluid
- IsoTec NE 350 – natural ester transformer fluid based on vegetable oil, K-class
Frequently asked questions
What is the difference between natural and synthetic ester?
Natural esters are based on vegetable oil; synthetic esters are produced from a polyol and selected fatty acids. Synthetic esters are more oxidation-stable and have a lower pour point; both are K-class.
Why are natural esters mostly used in sealed transformers?
They oxidise more readily in contact with air. Sealed designs or air protection keep the fluid in good condition over its service life.
Which standards apply?
IEC 62770 and ASTM D6871 for unused fluid; IEC 62975 and IEEE C57.147 for maintenance and use.
Considering natural ester for your transformers?
We support fluid selection, laboratory testing and retrofilling. IsoTec products are supplied to customers outside Germany.