Dielectric response analysis for cable joint problems in medium voltage underground cable system

This paper represents the investigation of dielectric response analysis of a cable system composed of a medium voltage XLPE underground cable, a heat-shrink cable joint and a heat-shrink cable termination. A 12/20(24) kV single core XLPE underground cable with the nominal cross-section area of 240 mm², three pieces of 22 kV cable joints, and eight pieces of 22 kV cable termination were prepared for the experiment. Then, polarization and depolarization current (PDCs) measurement were performed. The experiments were divided into two parts. The first part was composed series of three tests, A — C, and the second part consisted series of two tests, D — E, as follows: Test A examined dielectric response of a 6 m healthy XLPE underground cable, Test B examined dielectric response of a 6 m healthy XLPE underground cable with two healthy cable terminations, Test C examined dielectric response of a two of 3 m healthy XLPE underground cable with healthy cable terminations connected by a healthy cable joint, Test D used the test object similarly as case C but the cable joint was contaminated with copper particles, and Test E used the test object similarly as case C but the cable joint was contaminated with tiny pieces of water blocking. From the experiment, the PDCs and the derived dielectric parameters such as geometrical capacitance, insulation resistance, polarization index, depolarization index, charge difference curve and charge ratio curve were analyzed. The PDCs and the derived parameters were found different for each case of study as details in this paper. Besides, the charge ratio proposed by authors is a valuable parameter for the diagnosis of insulation system of high voltage equipment.