All about ester fluids

The benefits of immersing transformer coils in a dielectric fluid have long been understood. The use of such a fluid allows designers to reduce electrical clearances and greatly improve cooling performance, thus making transformers more compact. For many years, mineral oil has been used in electrical transformers due to its cooling and electrical performance. However,the shortcomings of mineral oil are now acknowledged in terms of its flammability, poor environmental performance, low moisture tolerance and corrosive sulphur.

The solution is to use ester based fluids, since they are fire safe, readily biodegradable, free from corrosive sulphur compounds and have excellent dielectric characteristics. Ester fluids have also been shown to extend the life of cellulose insulation, bringing further benefits to operators.

Transformer ester fluids can be split into two groups – synthetic and natural.

Synthetic esters are manufactured from carefully selected raw materials to give a finished product that is tailored to the specific application. In the case of MIDEL 7131 it has been developed specifically to be used as a transformer dielectric; hence it is a high performance fluid which is suitable for use in all types of application. Crucially, MIDEL 7131 has a very low pour point and excellent oxidation stability, making it suitable for cold climates and breathing systems where the fluid is exposed to oxygen from the air.

Natural esters are made from renewable natural sources, for example MIDEL eN 1204 (rapeseed/canola) and MIDEL eN 1215 (soybean). The base oil is chosen to give the best possible fit to the application; however unlike synthetic esters the properties of these base oils cannot be significantly altered. Therefore in order to get a natural ester dielectric fluid that remains liquid at low temperatures a compromise has to be made, and a base oil with reduced oxidation stability is usually chosen. This means that natural esters are only suitable for sealed equipment. Natural esters also have high pour points when compared to synthetic esters, and so are best suited to temperate locations or indoor applications.