Impact of concrete-encased grounding systems on lightning overvoltages in transmission lines

This paper addresses the influence of concrete on the overvoltages developed in a 138-kV transmission line (TL) subjected to lightning strikes with variable amplitude. This study considers a tower-footing grounding system consisting of four counterpoise conductors, utilizing both bare and concrete-encased electrodes. The harmonic grounding impedance is initially calculated over a frequency range from 100 Hz to 10 MHz. Subsequently, the ground potential rise (GPR) developed by the tower-footing grounding system under lightning currents with adjustable peak values is analyzed. Furthermore, the impulse impedance is computed to model the tower-footing grounding system within the simulated power network. The TL is modeled using the Universal Line Model (ULM), which incorporates frequency-dependent (FD) soil parameters and ground-return parameters calculated via Nakagawa’s equations. Simulations are performed in ATP-EMTP, where the ULM is implemented in MATLAB and incorporated into the model via a PCH file. Finally, the overvoltage waveforms generated for the 138-kV power system subjected to a lightning strike are analyzed for three different values of low-frequency soil resistivity modeled using Alípio-Visacro’s approach: 700, 1500, and 4000 $\Omega$ m. The simulation results reveal a substantial reduction in harmonic impedance for the concrete-encased grounding system compared to the bare-electrode configuration. This reduction leads to lower impulse impedance and results in GPR waveforms with significantly reduced peak values when concrete is incorporated into the grounding system. However, in high-resistivity soils, the reduction achieved by the concrete-encased system is limited to approximately 33%. In addition, the overvoltages generated by lightning strikes are lower when the concrete-encased grounding system is used. In contrast, the bare grounding system leads to backflashover events, which do not occur when the concrete-encased grounding system is used.