Underground MV power cable joints: A nonlinear thermal circuit model and its experimental validation

The analysis of the thermal stress of underground MV power cable joints under different environmental conditions, both in normal operation and during faults, is crucial to assess any correlation between the occurrence of overheatings and the anomalous increase of failures (two to three times compared to previous years) which have been observed in the Italian distribution network during summer 2015 (the hottest year ever since 1880). In order to perform this analysis, we have developed a nonlinear circuit model that is able to estimate, both in stationary conditions and in transient ones, temperature profiles inside MV joints, referring to different environmental conditions and with current flowing both in the core conductor and in the metallic shield. The model has been implemented in the Simulink environment and then it has been validated with a set of experimental tests, specifically set up in order to check its performance in steady state and during transients. The numerical results were always in good agreement with the measurements. Subsequently, the circuit model has been employed to perform a predictive analysis of the joint internal temperatures, which may be achieved in operation and during faults by varying the ambient temperature. In all cases, the temperature limits tolerated by insulations have never been exceeded.