Insulation ageing diagnosis of XLPE power cables under service conditions

Forty percent of transmission and distribution power grids are composed of underground cables in Shanghai, a city with abundance surface water. The cables are subjected to a variety of stress factors due to the complicated underground environment and the ageing process is accelerated consequently. Lots of cables have been in service for over 20 years and there tends to be frequent accidents due to cable failures. So it´s necessary to research the insulation ageing state of cables under unique service conditions. Six physical and chemical analysis had been performed on samples of about 300 cable lines in Shanghai, including water tree investigation, tensile test, dielectric spectroscopy test, TGA, FTIR and electrical ageing test. By statistical analysis and fuzzy clustering, the parameters for identifying and quantifying the XLPE cable insulation aging state were presented and the influence of laying method and sampling section on the degradation of cable insulation was also studied. Based on fuzzy theory, a cable insulation ageing diagnosis model was established, which was proved to be applicable by case study. A database was also developed for cable maintenance according to the multi-parameter test data and diagnosis model.