Analytical expressions for evaluating screening effect of transmission line earth wire

When an unbalanced fault occurs, the fault current in the transmission lines induces an electromotive force in nearby parallel telecommunication circuits, as well as in different metal pipelines. The existence of an earth wire in these cases exerts a favourable influence known as the `screening effect´ which is considered in the calculation of the induced electromotive force through the screening factor. An original procedure for obtaining relatively simple analytical expressions for determining the screening factors is presented. The procedure incorporates the fact that the earth wire is galvanically connected to the structures along the line and, at the terminals of the line, to the station frame. The procedure is based on Kirchoff´s equations, the principle of superposition and the summing of geometrical series