An analysis methodology to evaluate the contribution to electrical security given by bare buried conductors in a system of intertied earthing grids

The paper presents an analysis methodology for a single-line-to-ground fault, occurring at a secondary MV/LV substation in a power network formed by a HV/MV station, feeding, through a MV tri-phase cable line, N MV/LV substations whose earth electrodes are interconnected by bare buried conductors. In the preliminary explanation of the methodology, earth buried conductors are studied in their double function of earth electrodes and connection elements between earthing grids, both in absence and in presence of other interfering conductors. Subsequently analyzed is, with a distributed parameters approach, the system constituted by an earth buried conductor and a MV tri-phase cable line. The equations that describe the system behaviour are solved with the decoupling method of modal analysis. The proposed methodology allows the determination of the currents injected in the ground by the buried conductor and by the earth electrodes of the substations, as a consequence of an earth fault, whichever is the faulty substation. A numerical example clarifies the presented methodology.