Validity of RLC Equivalent Circuit of Grounding Electrodes in Combination with Equivalent Resistivity in Two-layer Soils and Its Application in Transient Analysis of Arrester-Connected Overhead Lines under Lightning Strike

In this paper, approximate and exact equivalent circuits for grounding electrodes buried in horizontally two-layered soils are introduced. In the approximate one,  two-layer soil is approximated with equivalent resistivity and then the grounding electrode is modelled with RLC equivalent circuit, while in the exact one, the input impedance of the grounding electrode is first computed in the frequency domain via numerical solution of Maxwell’s equations. Then the input impedance is replaced with rational functions and finally the equivalent circuit in time domain is achieved. In order to extract the validity range of the approximate circuit in two-layer soils, transient analysis of overhead line terminated to arrester in the presence of two-layer soils is carried out where the grounding electrodes are modelled with approximate and exact circuits. The simulation results show that when the thickness of the first layer is less than 1 meter or greater than 40 meter, the approximate circuit yields acceptable results. In addition, sensitivity analysis is carried out on the thickness of the first layer with respect to the single-layer soil. The simulation results show that when the thickness is greater than 40 meter, the two-layer and single-layer soils have the same behavior.