Simulation of Structure Height Influences on Electromagnetic Field of Lightning Return Stroke

With the development of economy and society and the continuous acceleration of urbanization, there are more and more tall-objects in urban area. Tall-objects can attract nearby downward lightning and trigger upward lightning. When a thunderstorm occurs, tall-object may be struck multiple times, which lead to a severe lightning electromagnetic environment in the vicinity. Therefore, the study of lightning striking to tall-object is of significance to the lightning protection of nearby equipment and cables.An electromagnetic model of lightning striking to tall-object is developed based on the three-dimensional finite-difference time-domain (FDTD) numerical method. Using FDTD method, the propagation law of lightning vertical electric field, the azimuthal magnetic field and the horizontal electric field along the ground surface, produced by negative cloud-to-ground lightning flash to tall-object, is investigated considering cases of different heights (100-600 m) of tall-objects.The amplitude of lightning short-circuit current waveform is 11 kA, and the corresponding 10%-to-90% rise-time is 0.15 μs. Simulation results show that the height of the tall-object plays an important role on the peak value of lightning electric field. For example, for the case of distance d=100 m, the peak value of vertical electric field decreases from 8.59 kV·m^-1 to 3.41 kV·m^-1 with the height of the tall-object increasing from 100 m to 600 m (decrement:63%). Moreover, for horizontal electric field, the positive peak value increases from 0.7 kV·m^-1 to 1.29 kV·m^-1 (increment:84%), and the amplitude of negative peak increases up to 130%. The peak value of azimuthal magnetic field and horizontal electric field will increase with the tall-object height, when the distance of the observation point is constant. However, for the case of distance d=100 m, 300 m, the vertical electric field peak will decrease with the tall-object height increasing. Furthermore, for the case of d=500 m, the vertical electric field peak increase first and then decrease with the height. In addition, the tall-object height has an effect on the dependence of vertical electric field peak on distance d. For relative lower/taller strike object, the peak value of the corresponding vertical electric field attenuates quickly/slowly with the observation distance d. Results obtained are very useful for lightning protection design of power cables and electronic equipment in the vicinity of tall objects.