One of the main targets of the research in the field of polymer nanocomposite dielectrics is to obtain new materials with improved dielectric properties (resistivity, dielectric strength, permittivity and dielectric losses). In this paper the variation of the real part of the permittivity and of the loss tangent with the frequency are investigated for three formulations of nanocomposites obtained from polyethylene filled with nanoparticles of SiO₂, TiO₂ and Al₂O₃, respectively. The influence of the filler concentration (between 2 and 10 wt%) on the dielectric behavior of the nanocomposite is analyzed as well. To simulate the electrical behavior of the polymer-filler interface which might explain the experimental results a 3D electrostatic model proposed on the basis of Tanaka's multi-core model is discussed. This model allows one to study the influence of several parameters such as the nanoparticle diameter, thickness of the interface layers, concentration and permittivity of the nanoparticles or the permittivities of the interface layers, on the effective permittivity of a plane nanodielectric sample.

The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees.