Insulation coordination of MV cables against lightning - induced overvoltages generated by LEMP - coupled overhead lines

-The paper deals with the stresses of cable insulations due to lightning induced overvoltages for the case of distribution networks composed by MV overhead lines and buried cables. We are here interested only in the conditional probability of the cable breakdown in absence of line flashovers, which means that flashovers along the overhead line are disregarded. We illustrate a procedure based on the application of the Monte Carlo method and on a simulation tool for the accurate computation of lightning-induced voltages on an overhead line, namely the LIOV code. A frequency dependent parameter cable model is used for the calculation of the overvoltages stressing the cable insulation, which is assumed not to be illuminated by LEMP. We associate to each calculated overvoltage a breakdown probability for the cable, by using experimentally obtained probabilistic distributions of the breakdown voltages of the cable insulation for different families of voltage wave shapes. These probabilities, together with the expected number and characteristics of the lightning events, allow us to infer the yearly number of expected failure events in the above assumptions. The breakdown behavior of extruded cable insulation is represented by a 2-parameter Weibull distribution obtained from experimental tests on cable models, manufactured with the same materials and same technologies of real cables, by applying the well-known statistical “enlargement law”.