Discharge Inception Voltages Due to Voids in Power Cables

Expressions are derived for the inception voltage at which discharges occur in gaseous voids entrained within the insulation of single and three-core, screened as well as belted, power cables. The inception voltage Vi depends upon the conductor diameter, the insulation thickness, the dielectric permittivity, the location, shape and length L of the void as well as the pressure P of the gas inside the void. It is shown that for a given cavity, Vi depends upon the product PL as well as P. For a cavity of known shape, size, and location, a minimum voltage Vmin exists below which partial discharges do not oc cur. Vmin decreases with increasing void size and is lower for cylindrical voids when compared to the spherical ones. For a given cable of known voltage rating, a "critical" cavity size Lcrit is defined. Any cavity smaller than this critical size can be tolerated in the cable insulation without the risk of partial discharges.. Expressions to compute Vmin and L, crit are given for coaxial as well as belted power cables.