Since the early days of transformers the benefits of immersing coils in a dielectric fluid have been understood. The use of fluid allows designers to reduce electrical clearances and greatly improve cooling performance, thus making transformers more compact. For many years mineral oil has been seen as the fluid of choice for electrical transformers due to its favourable cooling and electrical performance. However, in more recent times the short comings of mineral oil are being realised in flammability, poor environmental performance, low moisture tolerance and corrosive sulphur.
The solution to all these problems is to use an ester based fluid, since they are fire safe, readily biodegradable, free from corrosive sulphur compounds and have excellent moisture tolerance. Ester fluids have also been shown to extend the life of cellulose insulation, bringing further benefits to operators.
By definition an ester is a reaction product from the combination of an acid and an alcohol. Esters come in many forms and are used for a vast array of applications. The ester based fluids used in transformers 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 has free access to oxygen from the air.
Natural esters, such as MIDEL eN, are derived directly from renewable natural sources, primarily seed oils such as soya bean, rapeseed oil or sunflower oil. 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 relatively poor 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.