Lightning Electromagnetic Fields and Their Induced Voltages on Overhead Lines: The Effect of a Horizontally Stratified Ground

The effect of a horizontally stratified two-layer ground on the aboveground lightning electromagnetic (EM) fields at close, moderate, and far distances from the lightning channel base and their induced voltages on overhead transmission lines is discussed. The analysis is performed by making use of a full-wave approach based on the finite-element method solution to Maxwell´s equations. It is shown that the radial electric field is the only component which is significantly affected by ground stratification at any observation point from the lightning channel base. The peak value of the radial electric field over a horizontally stratified two-layer ground, regardless of the conductivities of the soil layers, takes a value in between the peak values of the electric fields corresponding to one-layer homogeneous grounds. For close observation points and when one of the soil layers is highly conductive (i.e., conductivities well above 0.1 S/m or so), this component can be computed assuming a homogeneous ground with the characteristics of the more conductive layer. The vertical electric field and the azimuthal magnetic field are slightly affected by ground stratification only at distant observation points. The induced voltages along the line when the upper soil layer is more conductive and narrow (i.e., 2 m or so), take a value in between those corresponding to the one-layer grounds. However, when the upper soil layer is less conductive and narrow, the induced voltages become identical to those of the homogeneous ground with the same properties of the lower soil layer. When the upper soil layer is more conductive and relatively thick (i.e., 10 m or so), the induced voltages are similar to those associated with the one layer ground with the same properties of the upper soil layer. However, when the upper soil layer is less conductive and relatively thick, the induced voltage on the midpoint of the overhead line is in between those of the homogenous grounds, while at the lin- terminations, the induced voltages take a larger value than those of the homogenous grounds. For the ocean-side overhead lines located above a horizontally stratified two-layer ground, the ocean shows the same additional influence on the induced voltages as the horizontal two-layer ground.