A 3-D nonlinear thermal circuit model of underground MV power cables and their joints

A 3D thermal model of underground power cable is presented in this paper. A detailed representation of the cable and its joints, the laying condition, the soil and the heat exchanges between soil upper surface and air, considering different weather conditions, is given. The model solves the thermal problem by means of an equivalent electric circuit and has been validated against field measurements, showing its applicability both for steady state and transient studies. In order to verify if any correlation between the occurrence of overheatings and the anomalous increase of failures, observed by Italian Distribution System Operators (DSOs) during summer 2015, exists, the model has been applied to simulate one-year operation of an existing underground cable. The cable is located in the MV distribution network owned by A.S.M. Terni S.p.A., the DSO in Terni, a city placed in the Central Italy. Ambient temperature and relative humidity trends have been taken from registrations made by a weather station, whereas the current profile along the cable has been assumed equal to that measured in 2015 by A.S.M., sampled every ten minutes. A second current profile has been also used, derived from the measured one in order to have the cable loaded at ampacity for a considerable time span during the year. In all cases, results show that no overheating occurred and, moreover, that the joint is the coldest part of the underground power cable.