Wide-Band Modeling of Tower-Footing Grounding Systems for the Evaluation of Lightning Performance of Transmission Lines

This paper presents the evaluation of lightning performance of transmission lines considering the wide-band modeling of their associated tower-footing grounding systems. To this aim, first, the grounding system admittance matrix over the frequency range of interest is obtained by making use of the method of moments solution to Maxwell´s equations. In the next stage, the matrix pencil method is used to infer the pole-residue identification of the obtained admittance matrix. Finally, the obtained pole-residue model is incorporated into the EMTP-RV in the form of a state-space block. The back-flashover analysis is done for a typical 132-kV transmission line, while the tower-footing grounding system is modeled in three different ways namely: 1) static-model, 2) wide-band model assuming constant electrical parameters for the soil, and 3) wide-band model assuming frequency-dependent electrical parameters for the soil. From the simulations, it is found that the inclusion of the wide-band model of the tower-footing grounding system can markedly affect the total probability of back-flashover in transmission lines. This effect is more pronounced when the frequency dependence of soil electrical parameters is taken into account. It is shown that considering the wide-band modeling of tower-footing grounding system can affect (either decrease or increase) the total back-flashover probability of transmission lines.