Laboratory Ageing Thermal Cycles of MV Cable Joints: results of different diagnostic techniques

The article reports the results of a multi-year experimentation in progress at the RSE laboratories and aimed at an in-depth diagnostic analysis of the causes of failure responsible for the out-of-services which affect MV cable joints of most of the urban network in the summer periods. To this aim an experimental set-up consisting of twelve cables equipped with different types of joints were connected together to form a ring in short-circuit. The ring was energized at rated voltage and current cycles were applied by means of external coils in order to reproduce the daily load variation typical of a MV cable network. Felts were wrapped around the cables to simulate the reduced thermal conductivity of the soil during summer periods. Continuous monitoring of the degradation of the joints was carried out by Partial Discharge (PD, on-line and off-line) measurements, together with Frequency Dielectric Spectroscopy (FDS) and Dissipation Factor (DF) - measurements. Results of the experimentation are presented and discussed.