Effects of dishomogeneities on the dielectric strength of real dielectrics

The local field set up by the inhomogeneities present in the dielectric matrix of power cable insulation is examined. A pure polymeric matrix is considered in AC regime, and the real local field set up by the presence of fillers or impurities is calculated by using a suitable mathematical model. On the hypothesis that the AC breakdown stress is closely related to the local electric field, the model is checked against some experimental data taken from the literature. Various types of inclusions are considered: inert fillers composed of dielectric particles of different shapes; voids (filled with water); trees; and metallic inclusions. The hypothesis of dependence on the dielectric strength on volume indicates that it is important to limit not only the dimensions of the inhomogeneities but also and especially their concentration. Therefore it is not sufficient to use filters to limit the dimension of impurities; it is also necessary to limit the number of impurities