Qualification of MV Cable Joints: Partial Discharges Innovative Tests

Medium Voltage (MV) underground cable systems are one of the most important and critical assets within urban electrical distribution networks. The weakness of medium voltage cables is due to the fact that they contain joints, which are elements made on-site to connect two cable sections to restore the circuit. Sometimes cable joints introduce unavoidable imperfections such as interface defects, lacking sealing, hot spots or other micro voids where an increase of the localized electric field will appear. In such defects or in such micro voids, partial discharge phenomena could take place leading, sooner or later at the final breakdown. This model has been described by the authors and in which was depicted the role of an increased temperature which will reduce the inception voltage of partial discharges. Consequently, the number of failures was noted to increase significantly during the summer period. The scope of the present paper is to illustrate a possible test able to highlight the quality of the different types of MV cable joints. The test consists in a repetition of applied voltage to a specified joint under test registering the inception voltage of partial discharges at different operational temperature; in this way, is possible to establish a cable joint immune to failures when the applied voltage, at the maximum given temperature, is lower than the inception voltage.