Synergistic Effect of Polarity Reversals and Load Current on Life of HVDC Cable Insulation

The paper presents a mathematical model for estimating the degradation/aging of a HVDC cable considering the combined effect of polarity reversal of applied voltage and load current. The load cycle and the polarity reversal cycle are imposed on a 320 kV XLPE insulated DC cable. Electric field and thermal distribution across the XLPE insulation have been obtained using Finite Element Method (FEM). Electric field distribution associated with DC applied voltage on a HVDC cable has been shown and resistive field distribution is reported. A significant rise in electric field across the cable insulation is observed after polarity reversal apparently one of the causes for cable damages. Investigations based on electric field and thermal distribution have been carried out using a standard life estimation model based on Inverse Power Model (IPM) and lifetime of cable insulation has been computed. However, it is observed that lifetime of cable insulation varies significantly across several points of study used for the analysis. The ageing parameter does not represent the predictable trend due to field distribution associated with polarity reversal and temperature distribution associated with load cycle.