Abstract
If a lightning back-flashover occurs on a transmission line tower, the lightning surge will be transmitted by the line conductor and penetrate into a substation. Overvoltage protection methods for substations are already published in several papers(1_??_3). In the present analytical method, however, it is assumed that the back-flashover takes place at only one of six line conductors installd on one transmission tower. It is known that when a lightning surge voltage across six insulator strings in one line tower exceeds 1.2_??_1.3 times the lightning impulse 50% flashover voltage of the insulators, flashovers occur on plural strings simultaneously (4_??_7). Authors analized the lightning overvoltage in the substation using EMTP. Considering occurrence of multi-phase simultaneous flashover on a lightning stricken tower, the permissible lightning current, which does not cause the insulation damage in the substation, is more than 25% large than those given by the presently adopted analysis method. This analysis leads to the conclusion that the permissible lightning current by the presently adopted analysis practice has an allowance of more than 25%.
