Modeling of substation grounding for fast front overvoltage studies

When performing insulation coordination studies, grounding electrodes of substations are frequently represented as as lumped resistances, in some cases even when extended grounding grids are dealt with. This paper presents an analysis of the approximations deriving from such a practice when studying fast transient phenomena, for several cases in term of grid geometry and soil electric resistivity. The influences of two models are compared for the grounding system: a very simple model that consists only of a resistor, and a model based on the more rigorous application of Maxwell's equations. The limits of applicability of these models are investigated and discussed by means of a comparative study. We conclude that depending on the type of engineering problem that one has to tackle, the adoption of one model instead of the other can lead to significant differences. For insulation coordination, lightning fast front overvoltages in the substation could be still computed in a first approximation using a simple resistor to model the grounding grid. Regarding EMC studies, the simplest model can lead to a certain underestimation of the potential rise of the grid, which means that, in general, the application of the Maxwell's equations-based model is recommended.