Overvoltage protection for MV-substations

In the past, MV substations, which were fed via cable link with the overhead line, were often destroyed due to lightning flashes. These damages occurred although the unit substations are in general protected by a surge arrester at the cable/overhead line conjunction against lightning overvoltages. This protection is in many cases insufficient to avoid an overstress of the substation. For this reason, the installation of an additional surge arrester inside the substation is necessary. In this paper, a protection scheme against overvoltages concerning these MV substations is developed and recommended which takes into account that an overstress due to a lightning flash should be avoided. Due to the increased service of SF₆-insulated metal-enclosed switchgear in the medium voltage range, the overvoltage protection of such equipment will be more important in the future.