Influence of frequency dependent soil electrical parameters on the evaluation of lightning electromagnetic fields in air and underground

摘要

This paper is aimed at analyzing the influence of the frequency-dependent behavior ofthe ground electrical parameters (conductivity and ground permittivity) on theelectromagnetic field radiated by a cloud-to-ground lightning return stroke. Both radiationin air (over the conducting ground plane) and underground are considered in theanalysis. The adopted method is based on the classical Sommerfeld’s theory and takesadvantage of an efficient ad hoc numerical procedure to face with the slow convergingSommerfeld’s integrals. This feature allows the electromagnetic field to be computedwithout any sort of mathematical approximation and, since it is carried out in thefrequency domain, can be used either if the ground permittivity and conductivity areconsidered constant or if they vary with the working frequency with any functional law.Simulations have been performed to identify the cases in which the approximation ofconstant ground permittivity and conductivity leads to satisfactory results. It is shown thatfor soils with water contents of 2% to 10% (ground conductivities in the order of 0.001 to0.01 S/m), the assumption of constant electrical parameters appears to be reasonable.However, for either very poorly conducting soils (104 S/m or so) or highly conductingsoils (101 S/m), the electromagnetic field components appear to be significantly affectedby the frequency dependence of the ground electrical parameters.