Computing Transient Voltages on Grounding Grids Buried in Homogeneous and Stratified Soils

Grounding grids (GGs) are essential to guarantee protection to personnel and equipment in the electrical substation. In this context, GGs provide low impedance path to high currents during faults or lightning strikes to minimize the step and touch voltages. Several models to compute the impedance of the GG are presented in the literature combining distributed and lumped approach. In this paper, a lumped approach is developed to represent a GG composed by vertical (VEs) and horizontal electrodes (HEs). Electrodes are modelled individually by lumped models and implemented at the ATP-EMTP software. The grounding impedances of the VEs buried in a stratified soil are calculated by the quadripole approach and an electric circuit is obtained by the Vector Fitting technique. The HE representation is based on the electromagnetic radiation theory, where each segment of the conductor can be seen as a current-carrying conductor. Simulation results compared with the traditional models in the literature indicate a good performance of the lumped representation of the HV and VE for a frequency range that varies from 100 Hz up 10 MHz. Additionally, results have shown that when VES are inserted in GGs and another layer of soil is considered, the GPR and node voltages are significantly reduced.