High Frequency Characteristics of Medium Voltage XLPE Power Cables

The response of a cable can be used to analyze the variation of the material characteristics along its length. For diagnosis of possible ageing, it is necessary to know how cable design, material properties and cable insulation ageing affects the wave propagation. A cable model has therefore been worked out based upon the high frequency properties of the cable insulation and conductor systems. The high frequency characteristics of the semi-conducting screens, new and water-tree aged cross-linked polyethylene (XLPE) insulation have been determined by two measuring techniques. Semi-conducting screens have a great influence on the attenuation of cables at high frequencies. A dielectric function based on an empirical formula is fitted to the measured characteristics of the semi-conducting screens. The dielectric function is used in the cable model set-up to predict the propagation constant of the cables. The high frequency characteristics of the water-tree degraded XLPE insulation obtained from measurements are related to the water content of the water trees of the samples. An effective materials model is developed to estimate the water content of the water trees and the conductivity of the water. The propagation constant from the cable model is compared to the propagation constant obtained from measurements made on cables. Using approximate high frequency cable models, the relative contributions of the conductor and insulation systems to the attenuation and phase velocity of the cables are analyzed.