The functions of the lubricating oil in an electrical transformer are to insulate the windings and to carry away the heat generated when the transformer is in on-load. The oil must possess good dielectric properties and because efficient heat transfer depends on ready circulation it must be of low viscosity. Moreover, transformer oil to some extent is exposed to air. Consequently when the transformer is on-load such exposure coupled with elevated temperature, electrical stress and catalytic influence of copper tend to produced to attack the copper conductors, the transformer tank and the cotton insulation while sludge coats the windings and interferes with cooling thereby increasing the temperature. High temperature accelerates chemical change and also promotes the formation of more acids and sludge. The production of these injurious substances should be as small as possible, so highly refined mineral oils of high insulating quality optimum oxidation resistance and chemical stability are employed.
The insulating ability of oil may be measured by its “dielectric strength”, which represents the voltage which can be applied to electrodes of certain dimensions and distance apart immersed in the oil before a spark passes. According to British Standard Institute, a minimum of 30,000 volts with electrodes 4 mm apart is required. It may be pointed here that provided an oil of suitable quality is used the insulating ability of the oil depends almost entirely on the degree of absence of extraneous impurities such as dirt, fibers and moisture. Their presence, even in relatively small quantities may appreciable lower the dielectric strength of the oil. If this reduction proceeds too great, the oil may cease to be effective as an insulator, with consequent damage to the transformer in which it is being used for the reason, transformer oils should be properly filtered and dried before being put into use.