ECS-9616216 Chowdhuri The probability of backflashover of a power transmission line is estimated, at present, by computing the insulator-string voltage caused by the traveling voltage waves along the vertical tower. This method of computation neglects the voltage components induced across the insulator string by (I) the electromagnetic fields of the lightning channel, and (ii) the transient magnetic field of the current which flows along the tower. The effects of these two voltage components could be significant because of the close proximity of the sources to the field points. In addition, the insulator-string voltage can also be significantly affected by the front time of the lightning return-stroke current. However, this effect is presently not taken into account. Since the design of the first 345-kV power line, it has been recognized that the electrogeometric model is technically more logical and accurate than the electrical shadow method in estimating the outage rates of overhead power lines. Yet the electrical shadow method is still being used to estimate the backflashover because of its simplicity. The following are proposed for the estimation of outrage rates caused by backflashover for lightning strikes to a tower of the overhead power line: (i) the effects of the electromagnetic fields of lightning channel as well as the transient magnetic field of the tower current on the total insulator-string voltage; (ii) the effects of the front of the lightning return-stroke current on the insulator- string voltage; and (iii) application of the electrogemetric model for the computation of the attractive area of a lightning stroke. The effects of the shape of the insulator-string voltage on its critical flashover voltage, V50, is related but separate area of concern, which will not be covered in this research project.
