Analysis of Overvoltages in Power Systems Due to Lightning Strikes: On the Effects of the Line Modeling Approach and Frequency-Dependent Soil Properties

This article presents a comparison considering a more conservative modeling approach (classical) and a more realistic modeling approach (enhanced) for the transient analysis of the overvoltages resulting from lightning strikes in a power system. The classical modeling approach considers JMarti's line model where the ground-return impedance is calculated with Carson's approach assuming frequency-constant (FC) soil conductivity and neglecting the relative permittivity of the soil and the ground-return admittance. For the improved modeling approach, the universal line model (ULM) is used as a line model, incorporating the frequency-dependent (FD) soil parameters and the ground-return impedance and admittance calculated with Nakagawa's equations. This study considers the main components of the power system, such as transmission lines and tower structures with their tower-footing grounding systems. For this analysis, a 138-kV transmission line is subjected to a lightning strike, assuming a homogeneous ground with five different values of soil resistivity (300, 600, 1,000, 2,000, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4,000 \ \Omega.\mathrm{m}$</tex>). The simulation results demonstrate a significant difference in the overvoltage waveforms obtained with the enhanced modeling approach compared to the classical modeling approach, mainly in their peak values. These differences in the overvoltage waveforms are more pronounced for high-resistivity soils. The correct computation of the lightning overvoltage is crucial for designing the adequate insulation level of several components for OHTL and substations, providing safety to operating personnel as well as people in the vicinity of these installations, and computing the backflashover rate.