Joints defectiveness of MV underground cable and the effects on the distribution system

MV underground cable systems are one of the main assets in electrical distribution infrastructures in densely populated urban areas. Experience of several distribution operators (DSOs) has shown an increasing number of failures of MV underground cable joints in the summer period. e-distribuzione, the main Italian DSO, and the University of Roma “La Sapienza” have investigated the causes of these joints failures, ran a temperature measurement campaign on cable joints and performed experimental inspections on both failed and not-failed cable joints. Field observations have been compared with the failure statistics of pre-qualification, type testing and testing after installation of power cables and accessories at KEMA Labs. The relatively high number of failures in testing proves the importance of these tests and awards an insight into failure mode. In this paper are reported the main results of a temperature measurement campaign on such MV underground cable joints and an experimental inspection on both failed and not failed cable joints. In this contribution, a discharge mechanism and the ongoing adopted mitigation actions are also presented. The gained information will be useful in re-designing innovative MV underground cable joints for a more resilient distribution system.