A Novel Generator Distance Backup Protection Element

Generator protective relays usually provide a backup distance protection element to protect the machine in the event of an uncleared system fault. The zone of coverage of the backup distance element begins at the generator terminals and extends into the power system through the delta-wye generator step-up transformer (GSU). The element is designed to account for transformer winding connections to provide an accurate reach for phase-to-phase and three-phase faults. However, due to the open circuit that the GSU introduces in the zero-sequence network, a generator backup distance element cannot accurately detect system ground faults by using the generator terminal voltages and currents. This paper describes a novel distance element that provides accurate reach for both phase and ground faults on the system side of the transformer. The element uses currents and voltages measured at the generator terminals along with the current from the grounded neutral connection of the GSU system-side winding. The presented element used to compensate the measured voltages and currents works for any implementation of a distance element.