Coupling Studies

study

Two detailed studies of zero sequence mutual coupling to a closed 33 kV ring were undertaken. Unwanted directional trippings of the ring circuit breakers were occurring at the same time as earth faults occurred elsewhere in the system (110 kV or 11 kV faults). DIgSILENT power system models were prepared to simulate these actual real events. These mutual coupling configurations were evaluated to determine the worst case situations, then relay setting changes were identified to either reduce or eliminate the unwanted trippings. A simple calculation method was developed to determine the size of the induced currents as a first step to help identify other potential problem sites.