Probabilistic determination of the impact of lightning surges on 145kV GIS equipment — A comprehensive ATP/EMTP study

Transient overvoltages due to external disturbances have been a major concern in the protection of Gas Insulated Substation (GIS) switchgear equipment often considered as requiring the installation of surge diverters within the GIS equipment, which has consequently increased cost and space requirements. The differences in surge impedances and transit times of overhead lines (OHL), cables and GIS switchgear together with the combination of circuit lengths and the complexity of the GIS switchgear arrangement are such as to rule against simplified analysis of the GIS substation using lattice diagram techniques. This paper presents the results of a mixed deterministic and statistical model that deals with the probability of a lightning strike reaching the GIS substation and its impact on the switchgear equipment whilst also considering the probability of the stroke current exceeding a certain magnitude coupled with the likelihood of such a strike to a critical section of the overhead line. A comprehensive ATP/EMTP model has been developed to examine a number of different scenarios based on varying the magnitude and location of lightning strikes to structures and towers and associated overhead earth/screen wires as well as varying tower footing impedance and also the presence or otherwise of surge arresters (SA) within the GIS switchgear.