Distribution of Ground Fault Current in Substations Fed by a Combined Overhead-Cable Line

When a substation is fed by a combined overhead-cable transmission line, in case of a fault to ground within the substation a consistent part of the fault current flows through the cable sheaths toward the remote source and discharges into the earth at the transition station, where cables are connected to the overhead line. This phenomenon, named "fault application transfer", may result in high ground potentials at the transition station which may cause shocks and equipment damage. In this paper a simple but accurate method on modelling a combined overhead-cable line is presented, allowing the computation of the fault current transferred at the transition station. Numerical analysis shows that, in practical cases, a significant portion of the fault current could involve the transition station due to fault application transfer. In this situation the local ground electrode, typically of high ground resistance with a small grid, could be inadequate in maintaining the ground potential rise within safety limits.