| Summary: | A model is developed for determining the frequency, the characteristics, and the spatial distribution of the lightning flashes terminating on power lines, both the shielded and the unshielded types. The model takes into consideration the existence of the towers, the sag of the conductors, and the inequality of the striking distances to the different elements of the model (towers, wires, and the ground plane), and assumes the strokes to arrive in the vertical direction. In case of power lines, the striking distances to ground and to the wires are taken equal to the base striking distance, and the striking distance to the towers is taken to be in the range 1.0 to 1.1 times the base striking distance. In case of medium height masts, the striking distance to the mast is taken equal to 1.2 times the base striking distance. The median current to flat ground is taken equal to 24 kA. The model is used in an extensive quantitative analysis of the frequency and the characteristics of the strokes collected by power lines and masts and the spatial distribution of the flashes collected by power lines. The model successfully predicts available observations regarding the effect of height of structure on both the median current of the collected strokes and the protective ratio. The output of the model is used to calculate the outage rate of unshielded steel-structure power lines using a newly developed method. The study covers the voltage classes 230 kV, 500 kV, and 765 kV. A theory is provided explaining the discrepancies between measured and calculated striking distances. A preliminary generalized model for the termination of lightning flashes, both the upward and the downward types, on tall structures is proposed. The termination of lightning flashes at points below the top of a tall structure is explained, and a new method for designing the necessary shielding system is given. Finally, a three-dimensional model for the shielding effect of forests is developed and a quantitative analysis of the effect of height of trees, width of right-of-way, and voltage class of the line is provided. === Applied Science, Faculty of === Electrical and Computer Engineering, Department of === Graduate
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