Toshiba's new line differential protection GRL150 has been successfully applied on a 33kV cable-feeder circuit with UK utility SP Power Systems. GRL150 is an advanced numerical relay which can act as a direct replacement for conventional analogue pilot-wire differential relays, connecting directly to the metallic pilots via integral 5 kV isolation without the need for external modems or other equipment. In addition to phase-segregated differential protection, GRL150 also provides signaling channel supervision, transfer trip, inrush restraint and back-up protection functions, and can also be applied over fiber optic communication links. During the course of a 1 year trial, a number of primary system faults occurred (both internal and external to the protected zone), with
GRL150 performing correctly in each case, its communication system proving highly robust under severe conditions in terms of channel noise..