Chapter 4 Physical and Chemical Properties of Mineral Insulating Oils

Abstract

Dielectric liquids are a major part of the electrical insulation system in many types of electrical equipment-transformers, circuit breakers, bushings, cables, and capacitors. Often the insulation liquid provides additional functions. It is also the cooling medium in transformers and cables, the arc extinguishing phase in switchgear, or even the lubricant in tap changers and circulating pumps. Each application then demands an insulating liquid with somewhat different electrical, chemical, and physical characteristics. In transformers, for example, the dielectric liquid must possess high specific heat and thermal conductivity along with low viscosity and pour point in order to provide effective heat transfer. Electrical stresses are not extremely high. However, these stresses are high in capacitors, and the dielectric liquid must provide exceptional resistance to partial electrical discharges (high discharge inception voltage, high gas adsorption). Less heat is generated in normal operation of a capacitor, and the physical characteristics of the liquid related to heat transfer are of lesser concern. Still another set of characteristics is required in circuit breakers. The electrical arc associated with operation must die abruptly as the breaker is opened or closed. The initial degradation in an arc in the liquid must not produce a situation in which the arc is sustained as the voltage across the liquid falls.