Lightning protection of distribution networks. Part II: Application to MV networks

The basic information for designers of lightning protection of MV and LV networks is given in Part I of the present publication. This Part II gives an application of this basic information to the lightning protection of MV networks containing overhead lines. As the overvoltages due to lightning reach important values, an economical and safe network design calls for extensive lightning protection. This general statement applies to high voltage as well as medium and low voltage networks. Overvoltage protection can be basically achieved in two ways: - reducing the amplitude and rate of occurrence of lightning overvoltages at the point of origin (e.g. through shielding the line conductors or improving the footing resistance of towers); - limiting the overvoltage at the equipment location (e.g. through surge arresters). In high voltage networks, both methods of protection are common. In MV networks shielding the conductors is generally not very effective. Due to the small clearance between the earth wire and the conductors, a direct lightning stroke will usually hit the conductors as well. In addition, induced overvoltages can be reduced only to a low extent by shield wires, as reported in [1]. For these reasons, the most effective protection against overvoltages in such networks is the use of surge arresters or spark gaps in the vicinity of the equipment. The paper gives guidance in the selection of rating and position of such protective devices and gives some indication on the effect of their use on the quality of the supply.