Frequency Dependence of Multilayer Soil Electrical Parameters: Effects on the Input Impedance of Grounding Systems

This paper presents an analysis of soil frequency dependence effect on input impedance of grounding system. It focuses on vertical grounding electrodes buried in multilayer soil structure. The proposed analytical formulae obtained from experimental results is used to predict the frequency dependence impacts of multilayer soil electrical parameters.To this aim, the precise full-wave method based upon the method of moment (MoM) is employed. The input impedance is calculated for different soil conditions described by the top layer height and the electrical parameters of soil for all layers. The contributions of this paper are: 1) considering frequency dependent of multilayer soil; 2) using the MoM-based approach, which is accurate and less time consuming, to evaluate transient behavior of grounding system. The effect of frequency dependence is more demonstrated for input impedance of electrodes that are buried in high resistivity soils in opposition with those buried in low resistivity soils and especially when these systems are subjected to high-frequency contents of lightning. These findings demonstrate that the frequency content of the impulse current, the soil electrical parameters and the grounding electrode length are the main factors affecting the lightning performance of grounding systems.