Computing Grounding Impedance of Rods Buried in Frequency-Dependent Stratified Soils

Grounding conductors play a fundamental role when transmission towers are subjected to lightning strikes in power systems. In this context, several studies have been carried out in the literature to compute the impedance of grounding conductors buried in homogeneous soils. However, for an accurate transient analysis, the assessment of grounding impedance must include the frequency-dependence of the soil electrical parameters (resistivity and dielectric permittivity) for conductors buried in a stratified ground. This paper investigates the performance ABCD-matrix representation of deep rods buried in frequency-dependent stratified soils based on an equivalent quadripole method, where each electrode segment is modelled as a short transmission line. Results demonstrated that the grounding impedance is significantly reduced when frequency-dependent soil electrical parameters are used in the frequency-domain simulations. Additionally, the time-domain Grounding Potential Rise (GPR) waveforms are computed for the lightning current represented by the first and subsequent return strokes. The transient responses show that the peak of the GPR waveforms is highly reduced when frequency-dependent stratified soils are considered. Simulation results show that adding another layer of frequency-dependent soil cause a pronounced reduction on the grounding impedance waveforms above a given frequency and the voltage peaks of GPR waveforms are highly decreased, especially for subsequent strokes.