Investigations on ageing of low density polyethylene under polarity reversals

Abstract

To reverse the power flow, line commutated converter based HVDC transmission systems use polarity reversal. Insulation is an integral part of any power system, therefore it is of importance to analyze the performance of insulating material for better reliability and safety. The paper attempts to address the performance of insulation under polarity reversals and effect of these reversals on age/life of insulating material. The aim of the experiments is to study the ageing of low density polyethylene and to determine the value of power law coefficient of low density polyethylene under the effect of polarity reversals. The authors have conducted a series of experiments under accelerated thermal conditions to quantify the effect of number of reversals, voltage levels and time-step of application of polarities and to determine the ageing coefficient or the power law exponent ‘n’ under the given experimental conditions. The insulation selected for the experiments is low density polyethylene (LDPE), since it is believed that polymeric insulation is most susceptible to the effects of space charge formation. The voltage profile is in the form of square wave. The time-period and amplitude of the wave is varied to observe the effect on the number of polarity reversals occurred before breakdown of LDPE samples. The number of reversals are recorded and fitted in a statistical model to analyze the variation of voltage levels and time-gap between reversals. The number of polarity reversals was recorded before breakdown of LDPE samples at different voltage levels with different pulse duration. The value of power law coefficient is determined for different pulse duration. The discussion includes impact of dc ageing as well as reversals on the life characteristics