SOLID BREAKDOWN

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.

The breakdown of insulation can occur due to mechanical failure caused by the
mechanical stresses produced by the electrical fields. This is called "electromechanical" breakdown.
On the other hand, breakdown can also occur due to chemical degradation caused
by the heat generated due to dielectric losses in the insulating material. This process is cumulative and is more severe in the presence of air and moisture.
When breakdown occurs on the surface of an insulator, it can be a simple flashover
or formation of a conducting path on the surface. When the conducting path is formed,
it is called "tracking", and results in the degradation of the material. Surface flashover normally occurs when the solid insulator is immersed in a liquid dielectric. Surface flashover, as already mentioned, is the most frequent cause of trouble in practice.
Porcelain insulators for use on transmission lines must therefore be designed to have
a long path over the surface. Surface contamination of electrical insulation exists
almost everywhere to some degree. In porcelain high voltage insulators of the suspension type, the length of the path over the surface will be 20 to 30 times greater than that through the solid. Even there, surface breakdown is the commonest form of failure.
The failure of solid insulation by discharges which may occur in the internal voids
and cavities of the dielectric, called partial discharges, is receiving much attention today, mostly because it determines the life versus stress characteristics of the material. The energy dissipated in the partial discharges causes further eterioration of the cavity walls and gives rise to further evolution of gas. This is a cumulative process eventually leading to "breakdown. In practice, it is not possible to completely eliminate partial discharges, but a level of partial discharges is fixed depending on the expected operating life of the equipment Also, the insulation engineer should attempt to raise the discharge inception level, by carefully choosing electric field distributions and eliminating voids, particularly from high field systems. This requires a very high quality control during manufacture and assembly. In some applications, the effect of the partial discharges can be minimized by vacuum impregnation of the insulation.
For high voltage applications, cast epoxy resin is solving many problems, but great
care should be exercised during casting. High voltage switchgear, bushings, cables,
and transformers are typical devices for which partial discharge effects should be
considered in design.
So far, the various mechanisms that cause breakdown in dielectrics have been
discussed. 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.