Substation and transmission lines earthing system design under substation fault

High-voltage substations are fed by transmission lines with the earthing system solidly bonded to the substation earth grid. Under substation fault, both the substation earth grid and the pole grid resistance are exposed to voltage rise. This voltage rise could reach unacceptable and dangerous levels. Earthing system design ensures safety compliance for both the substation and transmission lines under fault conditions. This article analyses the relation between the substation earth potential rise and the transmission pole earth potential rises. The analysis shows that the poles located within the finite length from the substation form a solid input to the substation earth potential rise. The article reviews the existing literature and develops the formulas to assist the designer to compute the substation earth potential rise from the pole earth potential rise and vice versa. The article proposes modifications to IEEE earthing design block diagram. This modification ensures that the transmission line earthing system is always compliant to allowable safety limits under substation fault. Furthermore, the article shows the method to estimate the substation earth potential rise by measuring the pole earth potential rise with a case study.