Ionization of occluded gases in high-tension insulation

Abstract

This paper deals with the ionization of occluded gases in solid insulation from the standpoint that these gas spaces are the weakest part of an insulation design and should receive first consideration. The stress at which ionization starts in different types of built-up insulation, such as used in cables and coils, is measured and from these measurements a safe working stress determined. The paper brings out, more than anything else, the importance of reducing the gas spaces to a minimum size, and using materials of the lowest possible permittivity, since the higher the permittivity the greater the stress on the gas spaces. If an insulation design is worked at a stress so high that internal ionization occurs it should be considered as “over-stressed” and its life will certainly be shortened. Just how much it will be shortened depends upon such factors as; type of insulation used, degree of over-stressing, looseness and porosity, etc. Roughly, the life of an over-stressed design is any where from six months to six years, judging from numerous tests made. This shows that the damage is very gradual.