Dielectric properties of service aged medium voltage XLPE cable joints

Generally, the failure statistics of medium voltage XLPE cable accessories in Norway show that the overall failure rate is relatively low. However, for some types of joints and especially those installed in the 1980´s, the insulation resistance is becoming very low and such joints have also started to fail. The mechanisms for these failures are yet not fully determined. This work includes both on-site and laboratory characterisation of actual joints. During on-site condition assessment of the cable link, low resistivity values and partial discharges were detected. Then a selected three phase cable was removed from service, shipped to and further analysed in the laboratory. The joints were characterised by dielectric spectroscopy in frequency domain, DC resistance and measurements of partial discharges. The results show that the low resistivity of the joints as well as the partial discharge activity is likely due to the water sorption during service. Drying strongly reduces the dielectric losses and permittivity, as well as extinguishes the partial discharges.