Even though the basic functions of the oil used in transformers are (a) heat conduction and (b) electrical insulation, there are many other properties which make a particular oil eminently suitable. Organic oils of vegetative or animal origin are good insulators but tend to decompose giving rise to acidic by-products which attack the paper or cloth insulation around the conductors.
Mineral oils are suitable from the point of electrical properties but tend to form sludge. The properties that are required to be looked into before selecting an oil for transformer application are as follows:
INSULTING PROPERTY: This is a very important property. However most of the oils naturally fulfill this. Therefore deterioration in insulating property due to moisture or contamination may be more relevant.
VISCOSITY: It is important as it determines the rate of flow of the fluid. Highly viscous fluids need much bigger clearances for adequate heat removal.
PURITY: The oil must not contain impurities which are corrosive. Sulphur or its compounds as impurities cause formation of sludge and also attack metal parts.
SLUDGE FORMATION: Thickening of oil into a semisolid form is called a sludge. Sludge formation properties have to be considered while choosing the oil as the oil slowly forms semi-solid hydrocarbons. These impede flows and due to the acidic nature, corrode metal parts. Heat in the presence of oxygen is seen to accelerate sludge formation. If the hot oil is prevented from coming into contact with atmospheric air sludge formation can be greatly reduced.
ACIDITY: Oxidized oil normally produces CO2 and acids. The cellulose which is in the paper insulation contains good amount of moisture. These form corrosive vapors. A good breather can reduce the problems due to the formation of acids.
FLASH POINT AND FIRE POINT: Flash point of an oil is the temperature at which the oil ignites spontaneously. This must be as high as possible (not less than 160◦C from the point of safety). Fire point is the temperature at which the oil flashes and continuously burns. This must be very high for the chosen oil (not less than 200◦C).
Inhibited oils and synthetic oils are therefore used in the transformers. Inhibited oils contain additives which slow down the deterioration of properties under heat and moisture and hence the degradation of oil. Synthetic transformer oil like chlorinated di-phenyl has excellent properties like chemical stability, non-oxidizing, good dielectric strength, moisture repellent, reduced risk due fire and explosion.
It is therefore necessary to check the quality of the oil periodically and take corrective steps to avoid major break downs in the transformer.
There are several other structural and insulating parts in a large transformer. These are considered to be outside the scope here.