Fluids Comparison

When choosing a transformer fluid it is important to evaluate all the options and so mitigate the risk for your application. In the past mineral oil has been the main fluid specified for transformers, but since the 1970s two further options have been introduced in the shape of synthetic and natural esters which bring many benefits to the users and specifiers.

The main risks associated with using mineral oil are flammability, high environmental impact and poor moisture tolerance. Ester based fluids provide the solution to these risks, while still offering excellent dielectric strength and cooling properties.

The table below shows a comparison of the properties of three insulating oils, synthetic ester (MIDEL 7131), natural ester (MIDEL eN 1215 and MIDEL eN 1204) and mineral oil. It shows that MIDEL esters are K-class, meaning they have much higher fire points than mineral oil. In addition, both esters have high moisture saturation points and are readily biodegradable.

As a high performance fluid MIDEL 7131 is recommended for the full range of transformer applications. It is perfectly suited to innovative compact designs of the type used on rolling stock and in wind turbines, which require higher temperature stability from the dielectric fluid. It also benefits from having the highest possible fire safety classification (K3).

MIDEL 7131 is also suitable for both breathing and sealed applications due to excellent oxidation stability, whereas MIDEL eN 1204 and MIDEL eN 1215, as with all natural esters, are only recommended for use in sealed distribution and power transformers.

eN 1215
eN 1204
Fire Safety Class K2 K2 K3 O1
Readily / Fully Biodegradable
Breakdown Voltage kV >75 >75 >75 >70
Moisture Saturation (ppm) at 20 °C 1100 1100 2700 55
Kinematic Viscosity at 40°C 32 37 29 8.7
Density (g/cm³) at 20 °C 0.92 0.92 0.97 0.88
Expansion Coefficient 0.00074 0.00074 0.00075 0.00075
Relative Permittivity 3.1 3.1 3.2 2.2
Pour Point °C -18 -31 -56 <-50
High Temperature Good Good Excellent Poor


All properties quoted in this table are typical values and do not constitute a specification.