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In medium-voltage distribution lines, lightning outages associated with indirect strokes are less frequent if these lines are equipped with a grounded shield wire and surge arresters are installed at close intervals. Thus, for this case, the lightning performance is mainly associated with the line response due to direct events. Recent analysis concerning medium voltage line equipped with a shield wire shows that the electromagnetic pulse radiated from the channel may enhance the overvoltage across insulators during direct strikes. This paper extends the analysis to lines over lossy soils, assuming different values of ground conductivity. The results indicate that the effect of the electromagnetic pulse needs indeed to be considered for an accurate lightning performance assessment of distribution lines with shield wire. On the one hand, the voltages across insulators increase as the soil conductivity decreases; on the other hand, it is shown that for direct strikes the difference between the voltage amplitudes calculated considering or not the LEMP contribution increases with the ground conductivity. Thus, appropriate modelling and relevant calculation tools availability is fundamental for the problem of interest.

The paper applies the Monte Carlo procedure proposed by the Authors to evaluate the indirect lightning performance of a real medium-voltage distribution network, which includes feeders and laterals, poles, secondary substations and surge protection devices. Such a procedure allows inferring the amplitude of lightning-induced voltages at any point and any phase of the network. The analysis presented in this paper aims at assessing the expected mean time between failures of each MV/LV transformer caused by indirect lightning strikes. A heuristic technique has been specifically developed to reduce the computational effort despite the non-linear response of the network enhanced by the presence of surge arresters.

The papers addresses the problem of locating surge protective devices in a medium voltage distribution network by means of an accurate computer model for the calculation of lightning induced overvoltages and a Monte Carlo statistical procedure. The indirect lightning performance of small network cells of different configuration are evaluated and used as a benchmark for the validation of the surge arrester locations provided by a genetic algorithm procedure. The obtained results appear useful for the lightning protection design of networks with complex topology.