A finite-difference time-domain approach for the evaluation of electromagnetic fields radiated by lightning strikes to tall structures

We present an analysis of the electromagnetic fields at very close range from a tower struck by lightning. The electromagnetic fields are evaluated for observation points above, on the surface and below the ground plane characterized by a finite conductivity. The computations are obtained using the Finite-Difference Time-Domain (FDTD) technique, in which the so-called engineering models are incorporated to represent the spatial–temporal distribution of the current along the channel and along the strike object. The approach is tested using a set of simultaneously recorded data published in the literature consisting of the current measured at the top of the Peissenberg tower and the associated electric and magnetic fields and very good agreement has been found. Simulation results are performed for an observation point located 50 m from the base of the channel (or tower, when present) and for three cases, namely (i) a lightning strike to ground, and (ii) a lightning strike to a 168-m tall tower, and (iii) a lightning strike to a 553-m tall tower. The effect of the presence of the tower and the effect of finite ground conductivity on the generated above-ground and underground electromagnetic fields are illustrated and discussed. It is shown that the underground electric fields are markedly affected by the ground conductivity. The underground electric field is predominantly horizontal with a negative polarity. The vertical electric field component is characterized by a bipolar wave-shape. The ground conductivity affects in a lesser degree the magnetic field penetrating into the ground. Above the ground and on the ground surface, the vertical electric field and the azimuthal magnetic field generated by a lightning return stroke initiated at ground level are nearly insensitive to the height of the observation point above ground. For the considered distance range (50 m), they can be computed assuming the ground as a perfectly conducting plane. The magnetic field above ground at such close distance is virtually not affected by the ground conductivity. The presence of a tower results in a significant decrease of the vertical electric field in the immediate vicinity of the tower. Unlike the case of a ground-initiated return stroke, the above-ground vertical electric field associated with a return stroke to tall tower is very much affected by the ground conductivity. Depending on the value of this latter, this component could exhibit an inversion of polarity.