Estimation of partial discharge inception voltages due to voids in solid sheet insulation

Defects like formation of voids, cavities, and cracks or inclusion of dust particles/small metallic turnings in the solid insulation of modern power cables may not be completely avoided even under the most stringent manufacturing conditions. These defects or weak points are the vulnerable sites of occurrence of Partial Discharges (PD). A continuous dissipation of energy due to discharges at afore-mentioned sites causes a gradual but definite damage to insulation and brings about its premature failure. Since PD adversely affects the insulation life, knowledge of the voltage at which PD start, Partial Discharge Inception Voltage (PDIV) is very important for proper and safe stress for design as well as reliable operation of the equipment. In this paper we report a technique for estimating the PDIV in case of discharges occurring in voids in solid sheet insulation in the ambient medium of air. Since occurrence of PD is a phenomenon dependent on upon many factors such as the type of gas in the void, the gas pressure, surface properties of the void, size and shape of the void, location of the void, dielectric constant of the surrounding medium etc; these parameters have to be taken into account. The computed values are valid in the pressure range of 0.067 kPa to 101.333 kPa. The results obtained have been compared with experimentally obtained values of PDIV for artificial voids of known dimensions in solid sheet insulation.