SOLUTION

The transformer oil should have low Volatility and low Viscosity.

Viscosity: The ease with which this convection flow can be induced clearly is very dependent on the viscosity of the fluid and it is therefore important for a transformer oil to have a low viscosity.

Low viscosity will assist in the penetration of oil into narrow ducts and assist in the circulation through windings to prevent local overheating which would result from poorer flow rates in the less accessible areas. Therefore transformer Oil having low viscosity i.e greater fluidity will cool transformers at a much better rate.

Mineral oils, like most other fluids, increase in viscosity as their temperature is reduced until they become semi-solid, at which stage their cooling efficiency is virtually nil. The pour point of a fluid is the lowest temperature at which the fluid is capable of any observable flow. For many transformers used in cold climates, the oil must not approach this semi-solid condition at the lowest temperatures likely to be experienced and so the oil must have a low pour point.

Volatility:- Normally transformers are expected to have a life of at least 30 years. It is desirable not to have to constantly think of making good evaporation losses during this lifetime, nor is it acceptable that the composition of the oil should change due to the loss of its more volatile elements. Low volatility is therefore a desirable feature. Volatility is quantified by the tendency of a substance to vaporize. Volatility is directly related to a substance’s vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with lower vapor pressure

It will be recognized that fire and explosion are to some extent potential risks whenever petroleum oils are used in electrical equipment. It is, therefore, necessary that the temperature of the oil in service should be very much lower than the flashpoint.