Characterisation of insulating oils

Power transformers contain a substantial amount of insulating fluid. A 400 KV 1000 MVA unit may contain 150000 Litres of mineral oil. The fluid has both insulating and cooling function. For the former the fluid must be free from water and suspended matter. For the latter it must have a low viscosity and volatility plus good heat transfer capabilities. A number of liquids may be used. Some synthetics have value where fire risk is important. However, the most common fluild is mineral oil. It satisfies the basic requirement and is relatively cheap. This paper discusses the use of mineral oils and diagnostic testing relating to its use.