Sammendrag
Condition assessment of water treeing in medium voltage XLPE cable insulations by tan δ measurements is well established. If a measured cable section has one or more joints with low insulation resistance, the assessment of the complete cable link can be difficult as the tan δ is significantly higher for the joints than for the cable insulation. This paper describes a simple method based on using an insulation tester to reveal if joints with low resistance are present in the cable link. Measurements have been performed on cable samples taken from service after more than 20 years due to high tan δ values and partial discharges located in the cable. The joints were characterized in the laboratory by time domain dielectric response and dielectric spectroscopy. It has been shown that both the polarization and depolarization currents should be used during the assessment to provide robust evaluation criteria to distinguish the response of a low resistance joint from heavily water treed cable insulations avoiding misinterpretation of the cable condition. This method should be used before applying any other diagnostic equipment.