Static-digital overcurrent relays permit enhanced protection

Multi-function static overcurrent relays employing digital technology have functionality that permits improved and expanded protection in many applications. While a relay is normally selected for the required overcurrent and ground fault protection, effective utilization of additional available functions can provide added security without significant additional cost. This paper describes an application for an oil refinery power system that uses directional overcurrent elements and reverse interlocking between the main, tie and feeder relays for a dual source-normally closed tie system. This scheme provides fast tripping for bus faults, which has several advantages over traditional bus differential relaying. This paper also describes a ground fault protection scheme to detect feeder grounds on a high resistance grounded system.