Porous Soil Effect on Backflashover Analysis of Power Transmission Lines

This paper investigates the influence of the porosity level on soil resistivity and its impact on the overvoltages generated, including the backflashover rate (BFR), on overhead transmission lines (OHTL) subjected to lightning strikes. For this analysis, the general Archie's form was adopted to model the soil where the power system is located. The lightning currents representing first (FRS) and subsequent return strokes (SRS) strike at the tip of the central tower. Four values of porosity level are employed in this analysis: 2,50, 5,0, 12.5, and 25%. Results indicated that the harmonic grounding impedance (HGI) strongly depends on the porosity level. The peak values of ground potential rise (GPR) increase with the increasing porosity level. The critical current to provoke a backflashover is also determined for each porosity value. Finally, the over-voltages across the upper string of insulators and the BFR are assessed. As the porosity level increases, the critical current grows, which leads to higher overvoltages across the string of insulators and higher BFR, showing the importance of considering adequate soil modeling in the transient analysis of OHTL.