Voltages induced on an overhead wire by lightning strikes to a nearby tall grounded object

The aim of this study was to identify conditions under which the presence of tall strike object can serve to increase or decrease lightning-induced voltages on a nearby overhead wire. We examined the ratios of magnitudes of lightning-induced voltages on the overhead wire for the cases of strikes to a tall object and to flat ground as a function of distance from the lightning channel d, current reflection coefficients at the top of the strike object rho(top) and at the bottom of the strike object rho(bot), the current reflection coefficient at the channel base (in the case of strikes to flat ground) rho(gr), and the return stroke speed v. Lightning-induced voltages were computed using the finite-difference time-domain (FDTD) method. The transmission line (TL) model was used to find the distribution of current along the lightning channel and the strike object. The ratio of magnitudes of lightning-induced voltages for tall-object and flat-ground cases increases with increasing d (ranging from 40-200 in), decreasing rho(bot) (< 1), decreasing rho(top) (< 0, except for the case of rho(bot) = 0), and decreasing v (< c, speed of light). Also, the ratio increases with decreasing the lightning current rise time. Under realistic (expected) conditions such as rho(bot) = 1, rho(top) = -0.5, and v = c/3, the ratio is larger than unity (the tall strike object serves to enhance lightning-induced voltages), but it becomes smaller than unity (the tall object serves to decrease lightning-induced voltages) tinder some special conditions, such as rho(bot) = 1, rho(top) = 0, and v = c.