On the Interconnections of HV-MV Stations to Global Grounding Systems

The interconnection of grounding systems of HV-MV stations via the armors of medium voltage cables, is herein analyzed to verify the effects on touch voltages in ground-fault conditions. The major contributions of this paper are two: the analysis of the impact of an HV ground-fault on a global grounding system (GGS), and the analysis of the parameters that may affect safety due to the interconnection between HV-MV stations and the GGS. The authors have analyzed cases when the connection of an HV-MV station to a GGS improves safety, and then may introduce hazards under ground-fault conditions. Two main issues are herein discussed: 1) the transfer of dangerous voltages to substations, due to ground-faults occurring at the HV-MV station; and 2) the reduction in the magnitude of the ground potential rise caused by ground-fault conditions at substations, due to the connection of their ground grids to the HV-MV station's grounding system. This paper, by examining various grid configurations, demonstrates that in some instances the inclusion of HV-MV stations in the GGS may reduce the level of protection against touch voltages, and that this depends on the following elements: the number of MV lines fed by the faulted station, the number of MV-LV substations per line, the value of the ground resistance of the substations, and the distance between the substations. This paper has practical relevance for both utilities distribution systems and industrial facilities supplied by the MV power grid.