Interfacing electromagnetic model of tower-footing impedance with the EMTP software package

This paper presents a new approach for modeling of time domain multi-port grounding systems, considering the frequency-dependent behavior of the soil. The presented method provides accurate results for the calculation of the lightning-related overvoltages. To this aim, first, the method of moment is used for solving the Maxwell equations, which leads to the extraction of the impedance matrix of the grounding system over the frequency range of interest. In the next step, a logical approximation of the impedance matrix is made using the Vector Fitting (VF) method. The used VF method has led to the fitting of a set of poles-residues for all the arrays of the impedance matrix of the grounding system. Finally, considering that all the electromagnetic transient solvers perform the simulation based on the admittance matrix, the proper model of the time domain single-port and multi-port grounding system is executed by state-space equations. From the simulation results, it is found that the modeling type of the grounding system is significantly effective on the lightning overvoltages, the outage rates of transmission lines (TLs), unavailability of the TLs and the lifetime of surge arresters (SAs). The advantage of the proposed model consists of higher accuracy and lower complexity, as it covers all the frequency range of interest and does not need any matrix inversion. Also, it can be implemented directly in a transient analysis solver, e.g. EMTP.