2. Mineral oils

The good oxidation stability and low viscosity combined with the immediate supply and cost-effectiveness of mineral oils have made them the insulating liquid of choice for over a century.

Mineral insulating oils are obtained by refining a fraction of hydrocarbons collected during the distillation of crude oil, freeing them of various materials (light gases, hydrogen sulfide, water, etc.). Their properties can vary considerably from one batch to another, even within the same brand. The degree of distillation and refining of the crude and the type of processes involved can considerably modify the constituents. Crudes may contain varying amounts of compounds containing sulfur (S), nitrogen (N) and oxygen (O), or various metals such as nickel (Ni), vanadium (V) and iron (Fe). The quantities of these impurities determine the refining techniques best suited to these crudes,...