Dielectric 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.
Highly purified liquids have dielectric strengths as high as 1 MV/cm. Under actual service conditions, the breakdown 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 mechanisms are significantly altered by the presence of the solid impurities and dissolved gases.
Petroleum oils are the commonest insulating Dielectric Liquid. 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. 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 Dielectric Liquid have since been developed which have no adverse environmental hazards. These include silicone oils, synthetic and fluorinated hydrocarbons.
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 Dielectric Liquid only fills up the voids in the solid dielectric, it can be used at stresses as high as 100-200 kV/cm.
Gas/Vacuum as Insulator
Air at atmospheric pressure is the most common gaseous insulation. The breakdown of air is of considerable practical importance to the design engineers of power transmission lines and power apparatus. Breakdown occurs in gases due to the process of coliisional ionization. Electrons get multiplied in an exponential manner, and if the applied voltage is sufficiently large, breakdown occurs. In some gases, free electrons are removed by attachment to neutral gas molecules; the breakdown strength of such gases is substantially large. An example of such a gas, with larger dielectric strength, is sulphur hexafluoride (SF6).
High pressure gas provides a flexible and reliable medium for high voltage insulation. Using gases at high pressures, field gradients up to 25 MV/m have been realized. Nitrogen (N2) was the gas first used at high pressures because of its inertness and chemical stability, but its dielectric strength is the same as that of air. Other important practical insulating gases are carbon dioxide (CO2), dichlorodifluoromethane (CC12F2) (popularly known as freon), and sulphur hexafluoride (SF6). The breakdown voltage at higher pressures in gases shows an increasing dependence on the nature and smoothness of the electrode material.
However, in recent years pure SF6 gas has been found to be a green house gas causing environmental hazards and therefore research efforts are presently focussed on finding a replacement gas or gas mixture which is environmentally friendly. Pure nitrogen, air and SF6/N2 mixtures show good potential to replace SF6 gas in high voltage apparatus. In the next few years, SF6/N2, SF6 gas has to be replaced by a new gas and lot of research is being done to find such a gas.
Ideally, vacuum is the best insulator with field strengths up to 107 V/cm, limited only by emissions from the electrode surfaces. This decreases to less than 105 V/cm for gaps of several centimetres. Under high vacuum conditions, where the pressures are below 10-4 torr, the breakdown cannot occur due to collisional processes like in gases, and hence the breakdown strength is quite high. Vacuum insulation is used in particle accelerators, x-ray and field emission tubes, electron microscopes, capacitors, and circuit breakers.