The Influence of Thermal-Oxidative Ageing on Electrical Properties of Polypropylene

Polypropylene (PP) is a potential recyclable insulation material for high voltage direct current (HVDC) cable application. During operation, cables are exposed to various stresses which can accelerate the ageing reactions in insulators in terms of physical and chemical changes, and subsequently affect electrical performance. In present work, PP samples have been thermally aged at 150 °c in a fan-assisted oven for various amounts of times. The chemical changes were characterised using Fourier-Transform Infrared (FTIR) spectroscopy to show the extent of ageing. Space charge dynamics were measured by the pulse electroacoustic (PEA) technique and the DC breakdown strength was analysed by Weibull statistics. It has been found that the charge injection near the cathode decreased corresponding with the concentration of carbonyl groups during ageing periods, while slightly increased injection appears in the bulk observed in severely aged samples. The DC breakdown strength shows an increase for the auto-oxidation period and reduced when it comes to the further ageing period.