Relay device and corresponding method

A computer program product comprising:

• a program storage device readable by a relay device, tangibly embodying a program of instructions executable by the relay device to perform method steps for operating the relay device, the method steps comprising:
• a) providing a an electrical circuit comprising first and second main circuit breakers, and a bus tie breaker connecting the first and second main circuit breakers;
• b) computing positive sequence first current phasor and second current phasor for the first main circuit breaker and the second main circuit breaker, respectively, and computing currents from phasor values, and the bus tie breaker;
• c) computing positive sequence voltage for the bus tie breaker;
• d) compute a correction of the positive sequence voltage for the bus tie breaker to a line to neutral phase angle reference, if there is any rotation in a potential transformer connection;
• e) triggering the performance of a method when at least one magnitude of at least one fault current at the first main circuit breaker exceeds a predetermined threshold, without the presence of a fault on at least one of the feeders;
• f) triggering the performance of a method when at least one magnitude of at least one fault current at the tie breaker exceeds a predetermined threshold, without the presence of a fault on at least one of the feeders wherein if a trigger is performed, a bus fault or a reverse fault is backfed through the bus tie breaker;
• g) determining whether the fault is an internal fault or an external fault by comparing the phase angle of the positive sequence current through the first main circuit breaker with the positive sequence current through the second main circuit breaker, wherein the fault is internal if the positive sequence current through the first main circuit breaker is in phase with the positive sequence current through the second main circuit breaker, and wherein if the positive sequence current through the first main circuit breaker is not in phase with the positive sequence current through the second main circuit breaker, the fault is external to the bus;
• h) determining whether the fault is on a first side of the tie circuit breaker or a second side of the tie circuit breaker by comparing the phase angle of the positive sequence current through the tie breaker with the positive sequence current through one of the first and second main circuit breakers, if the fault is internal;
• and
• i) locating the fault by comparing the direction of a voltage drop due to the positive sequence current through the bus tie circuit breaker with a pre-fault positive sequence voltage on the bus, if the fault is external to the bus.