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, slower biodegradation 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 (Shell MIDEL 7131), natural ester (Shell MIDEL eN 1215 and Shell MIDEL eN 1204) and mineral oil. It shows that Shell 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 their base oils are readily biodegradable according to OECD 301.

As a high performance fluid Shell 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).

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