Lightning Surge Efficiency of Grounding Grids

Two phenomena are important in the dynamic behaviors of grounding systems: soil ionization and inductive behavior. This paper solely focuses on the inductive effects that might impair dynamic performance. This is in agreement with conclusions of recent investigations that the effect of soil ionization can be ignored for grounding grids in high-voltage substations. The inductive effects are enhanced by fast front current pulses that have high-frequency content. We improve the analysis by applying a rigorous electromagnetic model and a realistic waveform of lightning current pulses. The new computer simulation results suggest that values of the grounding grid impulse coefficient are nearly linearly dependent on the side length of square grids. We derive new empirical formulas that approximate the impulse impedance, impulse coefficient, and effective area of grounding grids. These new formulas enable identification of parameters for which the surge performance of grounding grids might be significantly impaired in comparison to low-frequency performance. We verify the simulation method used for deriving the new formulas by comparing our data with published experimental results.