Out-of-step blocking and selective tripping with impedance relays

WHEN two interconnected power systems pull out of step the relative values of voltages and currents are such as to indicate a three-phase fault somewhere in the interconnection, which if in the zone of high-speed impedance or reactance relays will cause them to trip their associated oil circuit breakers. Although this phenomenon has been recognized for some time, and various blocking schemes suggested,^3,4 it has been generally accepted that when high-speed relays are involved, carrier current or pilot wires giving simultaneous control of the relays at both ends of a protected section are required to discriminate between a fault and an out-of-step condition.^2,7 This paper describes a scheme developed for an important tie circuit of the Gulf States Utilities Company, whereby out-of-step blocking and selective tripping are obtained with existing impedance relays and without the use of carrier current or pilot wires.