DISCHARGE INCEPTION VOLTAGES DUE TO VOIDS IN POWER CABLES.

Abstract

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 V//i depends upon the conductor diameter, the insulation thickness, the dielectric permittivity, the location, shape and length L of the void, and the pressure P of the gas inside the void. It is shown that for a given cavity, V//i depends upon the product PL as well as P. For a cavity of known shape, size, and location, a minimum voltage V//m//i//n exists below which partial discharges do not occur. The voltage V//m//i//n decreases with increasing void size and is lower for cylindrical voids when compared to spherical ones. For a given cable of known voltage rating, a critical cavity size L//c//r//i//t 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 V//m//i//n and L//c//r//i//t are given for coaxial as well as belted power cables.