Lightning attachment to overhead power lines

Lightning is the main cause of unscheduled interruptions in overhead power lines, affecting reliability of power supply and thus, consequently, resulting in economic losses. Lightning-caused insulation flashover in overhead power lines is associated with the fast-front overvoltages across line insulation, arising due to direct lightning strokes or induced by nearby lightning. Shielding against direct lightning strokes to phase conductors of overhead power lines is provided by shield wires. The latter are metallic elements that are able to, by physical means, launch a connecting upward discharge that intercepts the descending lightning leader from a distance, called striking distance, commonly also called attractive radius or lateral distance. Lightning leaders intercepted by shield wires, increasing the potential of the transmission-line tower, may result in power-line outages due to backflashover, that is, insulation flashover between tower and phase conductors. However, some of the less intense lightning strokes, not being intercepted by shield wires terminating thus to the phase conductors, may cause powerline outages due to shielding failure. In addition, descending lightning leaders which are not intercepted by the line conductors, striking to ground nearby the power line or to adjacent structures may result in power-line outages due to induced voltages on line conductors causing insulation flashover. As the line operation voltage increases, a higher line insulation level is utilized, and the lightning performance of overhead power lines becomes increasingly determined by the direct stroke flashover rate. In this chapter, the physical process of lightning attachment to overhead power lines is presented and discussed. Engineering models of lightning attachment are described in detail. Finally, a general procedure for the estimation of lightning incidence to overhead power lines is presented.