Parameters of lightning strokes and their effects on power systems

A lightning stroke can generate transient overvoltage on an overhead line by (i) directly striking a phase conductor, (ii) directly striking a tower or shield wire causing backflash, and (iii) striking a nearby ground. All three types of transient voltages are directly proportional to the return-stroke peak current. In addition, voltages by (ii) depend upon the current front time, and those by (iii) depend upon the front time and velocity of the return-stroke current and on the cloud height. The energy of the lightning flash may exceed the thermal limit of the struck object causing thermal failure. The statistical variations of these lightning parameters are discussed.