Experience from on-site condition assessment of XLPE MV cables

This paper presents results from on-site condition assessment of six MV XLPE cables installed at Norwegian electricity utilities for more than 24 years. The main purpose of this work is to present results from trend analysis and to show that strongly water tree aged cables equipped with joints with high dielectric losses to some extent can be assessed. The water treed cables were assessed on-site using a method based on dielectric spectroscopy, where the complex permittivity can be measured in the frequency range of 100 to 0.001 Hz up to 21 kV_rms. One cable was assessed in the laboratory where also water tree analysis and measurement of the breakdown voltage were carried out. Five cables were re-measured after some years making trend analysis possible for the estimation of the ageing rate of the cable insulation. The results show that the cable assessed in the laboratory had severely water tree degradation (bridging water trees) and a residual breakdown voltage of 4U₀. This cable had more than 100 times higher dielectric losses (epsiv") than the cables equipped with joints. A strong voltage dependence in the real part of the permittivity (epsiv\´) was also measured for this cable at voltages above U₀. The trend analysis show that the dielectric loss tangent increased in the range from 10^-4 to 10^-3/year for the old cables manufactured in the seventies. No such change was detected for the cable manufactured in the eighties with strippable insulation screens