The use of non linear permittivity fillers for the purposes of stress grading within cables

High degrees of electrical stress are behind many of the aging mechanisms in the insulation of electrical power cables. It has been widely known that the use of multiple insulation layers of differing permittivity can be used to reduce the levels of electrical stress at the centre of the cable. However this theory relies on the use of a discrete method of stress grading. By having a ´non linear´ insulation layer, through the incorporation of ferroelectric fillers into the linear dielectric, a dielectric is created which allows the permittivity to increase with electrical field strength, allowing a method for continuous stress grading to take place within the material. In this paper, several finite element models of the electrical stress distribution within cables with the non linear permittivity insulation are proposed and suggestions on reductions in insulation thickness are made, compared with conventional linear dielectric cables. The data, on which the non linear dielectric insulation models are based, has previously been experimentally determined from barium titanate filled acrylic resin at a variety of filler concentration levels.