Role of Semiconducting Screens on Water Treeing in Medium Voltage XLPE Cables

Abstract

Water treeing is a predominant mechanism of premature failure of underground XLPE distribution cables. Recent studies show that insulation degradation is due to injection of hydrophilic ionic species into the insulation and its rupture under the action of electromechanical and electro chemical processes operating at the microscopic level in the presence of strong field enhancement. Therefore, the ionic contamination in semicon screens as well as in the matrix of insulation play dominant role. But, most of these findings are based on the studies carried out on molded insulation plaques. The present study focuses on the role of contaminants in the semiconducting screens of medium voltage XLPE insulated cables commonly used in electric power utilities. Effect of two purposely selected ionic aqueous species on the treeing parameters evolved in these cables as a result of accelerated stress aging in the presence or absence of outer semiconducting screens, has been studied. Results show that ionic impurities present in semicon screens that belong to transitional metallic group, do catalyze the treeing process. If the semiconducting screens are produced and extruded over insulation using extra clean carbon then these will act as a barrier toward the degradation of cable’s insulation. Based on the results and their analysis it is postulated that if some suitable ion trapping materials are blended in the base matrix of semiconducting screens, the water tree growth in polymeric insulation can be effectively retarded.