Materials, Methods & Technologies, Volume 4, 2010

Abstract: One of the most important characteristic of polyethylene used for power cables insulation is the low water permeability. Experimentally, it was shown that, under the action of a time variable electric field, water can penetrate inside the polyethylene insulation, causing the appereance the so called water trees. In this paper, an experimental study regarding the modification of dielectric properties of polyethylene subjected to the electric field action in the presence of water is presented. Experiments on two sample types (needle-plane and flat samples), made of low density polyethylene were made. In order to reduce the test duration, electric fields (over 4 kV/mm) and frequency (3-5 kHz) superior to those from service were used. For some ageing time values the water trees dimensions, electrical strength in power frequencies electric fields (50 Hz) and permittivity and conductivity components (for frequencies between 1 mHz and 1 MHz) were determined. The results show that, water trees development leads to the electrical strength decrease and permittivity, conductivity and loss factor increase. Finally, it is shown that the dielectric properties variation are most important at lower frequencies and the water penetration under the electric field action facilitates the premature breakdown of cable insulations.