Lightning protection of a multi-circuit HV-MV overhead line

This paper deals with the assessment of the lightning performance of a medium voltage (MV) power line sharing the same poles of a high voltage (HV) line. We focus on the case of a 15-kV line with compact configuration that is supported by the concrete poles of a 69-kV or a 138-kV transmission line. Due to the taller HV pylons and conductors, the number of direct events to the multi-circuit line is higher than for a MV line alone. The presence of the HV conductors and of the overhead ground wire reduces the amplitudes of the induced overvoltages in the MV conductors due to indirect lightning events. For the considered configurations, with a compact MV line, the number of flashovers per year expected in the double-circuit line due to both direct and indirect flashes is lower than in a MV line alone. It is shown that such a number depends mainly on the line withstand voltage, on the grounding resistance and on the number of surge arresters. The dependence is quantitatively discussed in the paper. The analysis of the overvoltages transferred on the LV side of the distribution transformers is also performed.