Ground potential rise characteristics of urban step-down substations fed by power cables-a practical example

Some special ground potential rise characteristics of substations fed by power cables are shown. These characteristics were detected during a study of the interconnected ground system of 14 step-down urban substations fed by the 138 kV underground cable network serving the south zone of Rio de Janeiro, Brazil. It was verified that when a fault occurs at a substation fed exclusively by power cables, almost no ground potential effects were detected at the faulted substation or at the other such substations of the 138 kV network. However, high values of ground potential occurred at the transition substations, i.e., those in which the power cables are connected to overhead 138 kV transmission lines with steel ground-wires. That ground potential was enough to produce shocks and equipment damage in certain transition substations. It is shown that the utilization of overhead lines with aluminium-conductor steel-reinforced ground-wires on the initial spans closer to the transition substation would alleviate the problem and also help reduce the amount of the copper necessary to control step, touch, and mesh potentials. Additional mitigation procedures are examined