The effect of sandy soil porosity on lightning overvoltages of overhead 138 kV transmission line

Proper soil modeling considering its physical properties is essential to design grounding systems and assess its grounding impedance for transient analysis. This paper investigates the improved Archie’s soil model with resistivity varying with porosity and water content. The impact of these two factors on the groundingimpedance, the ground potential rise (GPR) of grounding vertical rods, and the voltages on the cross-arms of a transmission tower subjected to lightning currents of first and subsequent return strokes are evaluated. The grounding impedance of the rods buried in porous soil is calculated using electromagnetic software (FEKO) for a frequency range of 100Hz to 10 MHz. Three levels of soil porosity 5, 15, and 25% and water content of 1% for the porous sandy soil were assumed. Results are compared with those assuming a dry soil with constant resistivity of 2,500 Ω m and relative permittivity of 10. Results demonstrated a significant influence on the grounding impedance and the effective length of the vertical rod is reduced when both factors are included. Therefore, the GPR and transient voltages on the transmission tower show a notable increase in the peaks as the porosity level gets higher showing the importance of considering both physical factors. •The electrical soil resistivity depends on the porosity and water content of ground.•Influence of the porosity on the harmonic impedance and GPR waveforms are analyzed.•The effective length of the vertical rod is reduced as the soil porosity decreases.•Voltage on transmission tower struck by lightning decreases with the soil porosity.•A more realistic soil is used to assess the transient responses accurately.