Ground effects on induced voltages from nearby lightning

The lightning induced voltage on overhead lines from return strokes and it´s dependency of a lossy ground is analyzed using a new, analytical vector potential formulation. Norton´s (1937) approximation and the surface impedance approach are used to take loss effects into account. The surface impedance method predicts in general induced voltages in good agreement with Norton´s approximation, but the accuracy of the method is dependent on the variation of the current along the lightning channel. Norton´s method is compared with the exact Sommerfeld solution, showing a deviation <10% even for low conducting grounds and distances from 100-1000 m. The effect of stroke location and line termination is also analyzed, showing that a line terminated by it´s characteristic impedance and excited by a return stroke at the prolongation of the line is especially sensitive to lossy ground effects. Strokes near the mid-point of an overhead line gives less loss effect than strokes at the end of the line. The surface impedance approximation is derived from Norton´s method and the necessary assumptions are outlined