Breakdown in Liquid Dielectrics:
Liquid are used in high voltage equipment to serve the dual purpose of insulation and heat condition. They have the advantage that a puncture path is self-healing. Temporary failures due to overvoltage are reinsulated quickly by liquid flow to the attacked area. However, the products of the discharges may deposit on solid insulation supports and may lead to surface breakdown over these solid supports and it is called as Breakdown in Liquid Dielectrics.
Highly purified liquids have dielectric strengths as high as 1 MV/cm. Under actual service conditions, the Breakdown in Liquid Dielectrics strength reduces considerably due to the presence of impurities. The breakdown mechanism in the case of very pure liquids is the same as the gas breakdown, but in commercial liquids, the Breakdown in Liquid Dielectrics mechanisms are significantly altered by the presence of the solid impurities and dissolved gases.
Petroleum oils are the commonest insulating liquids. However, fluorocarbons, silicones, and organic esters including castor oil are used in significant quantities. A number of considerations enter into the selection of any dielectric liquid.
Properties of Liquid Dielectrics:
The important electrical properties of the liquid include the dielectric strength, conductivity, flash point, gas content, viscosity, dielectric constant, dissipation factor, stability, etc. Because of their low dissipation factor and other excellent characteristics, polybutanes are being increasingly used in the electrical industry. However, in 1970s it was found that Askarels which more extensively used, exhibit health hazards and therefore most countries have legally banned their production and use.
Many new liquids have since been developed which have no adverse environmental hazards. These include silicone oils, synthetic and fluorinated hydrocarbons.
Applications of Liquid Dielectrics:
In practical applications liquids are normally used at voltage stresses of about 50-60 kV/cm when the equipment is continuously operated. On the other hand, in applications like high voltage bushings, where the liquid only fills up the voids in the solid dielectric, it can be used at stresses as high as 100-200 kV/cm.