Inclusion of Field-Solver-Based Tower Footing Grounding Models in Electromagnetic Transient Programs

This paper presents a new approach to wideband modeling of tower footing grounding impedances. Appropriate representation of transmission tower footing impedances is essential in electromagnetic (EM) transient simulations for proper evaluation of overvoltages and flashovers in the system during fast-front transient conditions such as lightning strikes. In this paper, a field solver is used to precisely model the most popular types of tower footing grounding systems, i.e., rods, counterpoises, and ring electrodes, for a variety of dimensions and soil parameters. Results are stored in a lookup table and used for generating frequency-dependent models that are exported to EM transient simulators. The table and model generation tool can be used by utility companies to model tower footings while taking into account local variations along the right-of-way, such as local variations in soil resistivity. The presented examples show close agreement of results of lightning studies using developed electrode models included in EM tools, e.g., PSCAD, EMTP-RV, and ATP-EMTP. A sensitivity study shows that the maximum voltage at the tower top during a direct lightning strike can be up to 19% higher when using the adopted modeling procedure, as compared to a standard approach of using a fixed resistor equal to the power frequency resistance.