Electric field distribution distorted by voids inside power cables

The paper reports findings of electric field distribution in power cables with voids entrained in the insulation. The shapes of voids studied are cylindrical and spherical types. Samples studied are oil-impregnated paper-insulated cables and polymer-insulated cables of single-core. The model selected for study is a single void adhered to the conductor surface. Both field distributions inside and outside the void have been studied. Analytical and numerical methods were used for comparison. It was found that the electric field is distorted outside a cable void and enhanced as well as slightly nonuniform inside the void. The nonuniformity of field distribution in a cable void has never been reported in the literature; however, under certain circumstances, it is important in the case of estimating the insulation strength of a cable. It was also found that in comparison with a numerical method, it is sufficiently accurate to use an approximate analytical formula for cable field calculations. Which is accurate for a uniform field, if the formula is point-wise or piece-wise employed. It is still found that in estimating the inception discharge voltage of multiple-void cases, the field distortion effect can be neglected if the voids are circularly distributed around the cable conductor. Moreover, for polymeric distribution cables of small conductors, a single-void model is a reasonable reduction for the multiple-void case in reality.