Redundant bus protection using high-impedance differential relays

Minnesota Power is assessing ways to improve redundancy of protection systems for compliance with North American Electric Reliability Corporation (NERC) Standard TPL-002 - System Performance Following Loss of a Single BES Element. Historically, buses rarely included dual differential systems and relied on time-delayed remote backup to cover a failure of the bus protection system. Today's highly stressed power system is less tolerant of delayed fault clearing with loss of multiple branch circuits for a single-contingency failure. Therefore, determining ways to achieve dual high-speed protection systems for buses has become important. Historically, Minnesota Power has used high-impedance bus differential (87Z) protection systems. This principle has many advantages, including high performance, virtually no limit to the number of branch circuits, simple current transformer (CT) wiring, and simple settings calculations. This paper examines various options for obtaining redundancy. The paper includes an emphasis on examining various methods of applying dual 87Z relays in an existing bus differential CT circuit.