Phase comparison carrier relaying

THE ADDITION OF a third terminal or a transformer bank to a 2-terminal line section has economic advantages, but presents problems when using phase comparison carrier relaying on such a line. In this type of relaying, the phase angles of the currents at the line terminals are compared over a carrier channel. The 3-phase line currents feed a sequence network, the output of which energizes two fault detectors, FD-1 and FD-2, and an electronic circuit to control the transmission of carrier. Operation of the low-set fault detector FD-1 starts the transmission of a carrier blocking signal during a fault. The highset fault detector FD-2 supervises tripping and generally is set to operate at a current value 125 per cent of the FD-1 setting on a 2-terminal line. This assures correct relay operation for remote external faults with current magnitudes near the pickup values of the relay.