Lightning currents and electromagnetic fields associated with return strokes to elevated strike objects

Lightning currents and electromagnetic fields associated with return strokes to elevated strike objects The aim of this thesis is the modeling of lightning return strokes impacting elevated strike objects such as towers. The theoretical and experimental work done led to the evaluation of the effect of the presence of the strike object on the spatial and temporal distribution of the current along the channel and along the strike object, as well as on the radiated electromagnetic fields associated with that current distribution. The first three chapters of the thesis contain a brief description of the lightning discharge, a review of the relevant experimental data available and an overview of the existing return strokes models for lightning initiated at ground level. The main original contributions of this thesis are presented in Chapters 4 through 6. They consist of experimental and theoretical work as follows. For the purpose of validating our theoretical models versus measurements, we participated, during the summers of 2000 and 2001, in experimental campaigns in Toronto, Canada, where we measured currents and electromagnetic fields associated with lightning strikes to the CN Tower in collaboration with the lightning research group of the University of Toronto. The CN Tower is today's tallest free-standing structure in the world (553 m). The collected data constitute the first simultaneous measurements of lightning current, electric and magnetic fields at two distances from the lightning channel, as well as optical measurements using a fast-speed camera system. The set of measurements obtained in Canada was complemented, in the framework of this thesis, with (a) experimental data of lightning return stroke currents measured simultaneously at two locations at the Peissenberg tower in Germany, provided by Prof. Fridolin Heidler, and (b) measurement results obtained using a reduced-scale model also designed, constructed, and tested in the framework of this thesis. The cumulated data allowed us to characterize the elevated strike object and to validate various theoretical expressions developed in this thesis. We generalized in Chapter 4 the so-called engineering models to include the presence of an elevated strike object. The generalization is based on a distributed-source representation of the return stroke channel, which allowed more general and straightforward formulations of these models, including a self-consistent treatment of the impedance discontinuity at the tower top, as opposed to previous representations implying a lumped current source at the bottom of the channel. We modeled the strike object as a vertically-extended, lossless uniform transmission line, characterized by reflection coefficients at its extremities. Special expressions were also derived for the case of electrically short structures. These expressions can be used to quantify the effect of grounding conditions on the current distribution along the strike object and along th