On-line monitoring of relative dielectric losses in cross-bonded cables using sheath currents

Cross-bonded three-phase single-core Cross-linked Polyethylene (XLPE) cables are widely used in power transmission and distribution systems. A great deal has been published on on-line monitoring of HV cable systems to prevent failure. Relative Dielectric Losses (DL) monitoring reveals the general condition of the cable insulation. Generally, DL measurement relies on measuring operating voltage and insulation leakage current. The main challenge to on-line monitoring of DL in Cross-bonded High voltage (CB HV) cables is the interconnectedness of metal sheath. Measured sheath currents are a combination of the leakage currents of each section and the imbalanced circulating current. Differentiating the leakage current of each interconnected section from the measured data is complex. The second challenge is that co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. As a result, any measured value contains currents from the two sheaths connecting at a joint. The third challenge is that it is hard to obtain the reference voltage synchronously and with high accuracy. Consequently, this paper investigates separation of leakage current components and analysis of DL based on leakage currents without reference voltage. A circuit model is proposed to describe the leakage current separation method for CB HV cables to allow on-line monitoring of DL, and the criterion required for assessing relative change of DL among different phases or sections of the cable for insulation deterioration are presented. Operational measurements from a cable system are analyzed and discussed. Results show that there is no significant deterioration in this monitored major section of a CB HV cable system.