Protection of indoor SF6 substations against lightning overvoltages in hybrid overhead/underground medium voltage networks

The scope of this work was to define criteria for locating the surge arresters required in order to protect medium voltage indoor gas insulated substations, in hybrid overhead lines and underground cables networks, against lightning overvoltages. In order to examine the influence of lightning strokes hitting overhead lines, computerized EMTP simulations were carried out using a detailed model. Parametric analysis was performed in order to identify the influence of various parameters on the lightning overvoltages. The calculated overvoltages were compared with the substation equipment impulse insulation level in order to define the amount and the location of the required surge arresters. The contribution of this work is that instead of dealing with radial distribution feeders in a fixed configuration network, it deals with modern open loop distribution networks, which, by using indoor SF₆ transformation substations having multiple possible feeding directions, permits easy operational topological changes. The uncertainties related to the flexible network configuration yield difficulties in estimating the best protective solution against lightning overvoltages.