A good solid dielectric should have some of the properties mentioned earlier for gases and liquids and it should also possess good mechanical and bonding strengths. Many organic and inorganic materials are used for high voltage insulation purposes. Widely used inorganic materials are ceramics and glass. The most widely used organic materials are thermosetting epoxy resins such as polyvinyl chloride (PVC), polyethylene (PE) or cross linked polyethylene (XLPE). Kraft paper, natural rubber, silicon rubber and polypropylene rubber are some of the other materials widely used as insulants in electrical equipment.
If the solid insulating material is truly homogeneous and is free from imperfections, its breakdown stress will be as high as 10 MV/cm. This is the `intrinsic breakdown strength’, and can be obtained only under carefully controlled laboratory conditions. However, in practice, the breakdown fields obtained are very much lower than this value. The breakdown occurs due to many mechanisms. In general, the breakdown occurs over the surface than in the solid itself, and the surface insulation failure is the most frequent cause of trouble in practice.
In many engineering applications, more than one types of insulation are used together, mainly in parallel, giving rise to composite insulation systems. Examples of such systems are solid/gas insulation (transmission line insulators), solid/vacuum insulation and solid/liquid composite insulation systems (transformer winding insulation, oil impregnated paper and oil impregnated metallised plastic film etc).
In the application of composites, it is important to make sure that both the components of the composite should be chemically stable and will not react with each other under the application of combined thermal, mechanical and over the expected life of the equipment. They should also have nearly equal dielectric constants. Further, the liquid insulant should not absorb any impurities from the solid, which may adversely affect its resistivity, dielectric strength, loss factor and other properties of the liquid dielectric.
It is the intensity of the electric field that determines the onset of breakdown and the rate ‘of increase of current before breakdown. Therefore, it is very essential that the electric stress should be properly estimated and its distribution known in a high voltage apparatus. Special care should be exercised in eliminating the stress in the regions where it is expected to be maximum, such as in the presence of sharp points.