Transformer oil is a refined mineral oil obtained from the fractional distillation of crude petroleum. It is then treated to remove impurities and to obtain the most desirable properties to make it suitable as an insulating and cooling liquid.
The most important property for an insulating liquid is the high dielectric strength and for a cooling liquid the low viscosity. Other properties are important to show the absence of impurities and to show the degree of stability for long life. Properties indicating purity are also important in the consideration of the ability of the oil to be compatible with the other materials used in transformer construction.
The primary enemies of the transformer oil are oxidation, contamination, and excessive temperature. Processing and transformer construction are designed to minimize the effect of these enemies, and tests are conducted to evaluate these considerations.
ASTM Specification for Mineral Insulating Oil Used in Electrical Apparatus (D 3487) has been designed to cover the properties of transformer oil for use as an insulating and cooling medium. Quoting its scope: “This specification is intended to define a mineral insulating oil that is functionally interchangeable and miscible with existing oils, and is compatible with existing apparatus and with appropriate field maintenance, and will satisfactorily maintain its functional characteristics in its application in electrical equipment.”
Much effort is expended by manufacturers to obtain and maintain quality oil for use in their equipment.