Evaluation of Lightning Electromagnetic Fields and Their Induced Voltages on Overhead Lines Considering the Frequency Dependence of Soil Electrical Parameters

This paper presents a comprehensive study on the effect of the frequency dependence of soil electrical parameters on the lightning radiated electromagnetic fields as well as their associated induced voltages on overhead lines. To this aim, a full-wave approach based upon the finite-element method (FEM) is utilized. In the analyses, frequency dependence of soil conductivity and relative permittivity is introduced, using available analytical formulae that is obtained from experimental data. It is shown that the radial electric field is the only component which is significantly affected by the frequency dependence of soil electrical parameters at observation points as close as some tens of meters from the lightning channel. The vertical component of the electric field and the azimuthal component of the magnetic field are not much influenced by this property of soil at moderate distances (up to several hundred meters) from the lightning channel. For distant observation points and for poorly conducting grounds, however, these components are also affected. It is also shown that for soils characterized by relatively moderate and lowresistivity values (less than 1000 Omega.m), lightning-induced voltages are not significantly affected by the frequency dependence of soil electrical parameters. For poorly conducting soils, instead, the frequency dependence of soil electrical parameters results in a decrease of lightning-induced voltages.