Resumo:
The high incidence of lightning strikes and the specific characteristics of Brazilian soil contribute to a significant number of transmission line outages, mainly due to backflashovers caused by direct strikes. This study addresses the performance of transmission lines under lightning conditions, evaluating the impact of different grounding representation models on the performance of a 230 kV transmission line, with emphasis on overvoltage estimation and backflashover rates. The analyzed models include: (i) the low-frequency resistance model (STM), commonly used in grounding design; (ii) the impulse impedance model (Z_p), which approximates the frequency-dependent behavior of the grounding system at the instant of the lightning current peak; and (iii) the frequency-dependent models over the entire spectrum — LTAEM, based on transmission line theory with electromagnetic coupling, and HEM, grounded in electromagnetic field theory and widely recognized as a benchmark in the literature. The results show that the grounding system modeling plays a decisive role in evaluating transmission line performance under lightning events. Advanced models that consider the frequency-dependent behavior of the soil and grounding electrodes over the entire frequency spectrum—such as the HEM and the LTAEM—provide responses that are more consistent with physical reality, thereby reducing uncertainties in overvoltage estimation and in the prediction of line outages. On the other hand, the Z_p model yielded satisfactory results regarding the estimation of overvoltage levels across the insulator string and the rate of outages per 100 km of line per year due to backflashover. However, a limitation of this approach lies in the fact that the frequency sweep is performed only at the instants corresponding to the peak surge current (I_p2) and the ground potential rise (GPR) developed by the grounding system. Consequently, it is recommended that the performance assessment of transmission lines under lightning conditions be preferably carried out using the HEM or LTAEM models, with particular emphasis on the computational efficiency of the latter. Nevertheless, the Z_p model remains a technically sound and practically attractive alternative. Finally, the results show that the phase with the highest insulator string overvoltages does not necessarily correspond to the phase with the highest number of backflashover outages, underscoring the importance of assessing all transmission line phases jointly.
