Mineral oil or ester fluid? Fire safety, environment and cost in oil-immersed transformers

The liquid in an oil-immersed transformer does two jobs: it insulates and it cools. For more than a century that liquid has been mineral oil refined from crude – cheap, well understood and standardised in IEC 60296. In the last twenty years ester fluids have taken a growing share of installations where fire safety or water protection is critical. Here is how to decide.

The properties that matter

Property	Mineral oil (IEC 60296)	Synthetic ester (e.g. MIDEL 7131)	Natural ester
Fire point	≈ 170 °C	> 300 °C	> 350 °C
Fluid class (IEC 61039)	O	K	K
Biodegradability	Low	Readily biodegradable	Readily biodegradable
Water tolerance	Low (≈ 50 ppm saturates)	High (> 2 000 ppm)	High
Viscosity at 40 °C	≈ 9 mm²/s	≈ 29 mm²/s	≈ 33 mm²/s
Cost per litre	1×	≈ 4–6×	≈ 3–4×

What changes in the transformer

The higher viscosity of esters slows natural convection, so a transformer designed for mineral oil runs a few kelvin hotter when filled with ester. Manufacturers compensate with slightly more cooling surface – a somewhat larger tank – or accept a small reduction in rating. The paper insulation, on the other hand, ages slower in ester because the fluid absorbs moisture that would otherwise stay in the cellulose. In practice: same ratings, 5–10 % more mass, and a longer insulation life.

When ester is worth it

• Transformers inside or close to buildings where a fire-protection concept would otherwise require a separate fire-rated room, sprinklers or a cast-resin unit. A K-class liquid often satisfies the requirement at lower cost than switching to dry-type.
• Water protection zones, near rivers, wells and in agricultural areas: a leak of biodegradable ester is an incident, a leak of mineral oil is an environmental case.
• Offshore, ships, wind turbines – space is tight and fire load is a design criterion.
• Overloaded units: the higher thermal class of ester-paper insulation allows a higher hot-spot temperature for the same life.

Specifying

PNACHO-O transformers are filled with mineral oil to IEC 60296 as standard and can be supplied with synthetic ester (MIDEL 7131) on request; the option is listed on every product page in the oil-immersed range. State the fluid in your enquiry together with the installation site – our engineers will tell you whether the fire-protection benefit is needed or whether a standard unit meets the local regulations.