Grounding System Transient Characteristics of Underground GIS Substations

The transient grounding impedance of a gas-insulated switchgear (GIS) substation was investigated through measurements on a commercial 300-kV underground substation. Based on the results, the macroscopic transient impedance of the grounding grid was clarified to be the inductance component and estimated at approximately 0.2 μ H. In the low-frequency region where the frequency is almost zero or so, the impedance is shown to be lower than the general concrete resistance value by one digit due to the complexity of grounding systems specific to underground substations. Hence, the simple model of the grounding grid can be expressed mainly by inductance. The detailed model was expressed by using a π-type equivalent circuit, where each circuit constant was calculated using the specific low resistivity of concrete. In addition, the validity of both models was confirmed by comparing the measurement results. The proposal models are applicable to the Electromagnetic Transients Program analysis on the surge phenomena within the GIS grounding system.