Directional comparison blocking system

Abstract

An echo-keyed directional comparison blocking system for protecting an electrical power transmission path in a power system network against internal faults is disclosed. The system includes a terminal unit disposed at each end of the transmission path for measuring characteristics representative of the power at its corresponding end and for activating a corresponding circuit breaker when an internal fault of the transmission path is detected. Each terminal unit includes a transmitter and a receiver for communicating transmit and receive signals with each other through a communication medium and additionally includes at least one forward-looking pilot relay, at least one over-current relay, and a plurality of circuits. The aforementioned combination provides for internal fault preference over an external fault, speeds up tripping of the circuit breakers under the conditions of one relay responding slower to a fault detection than another, eliminates problems which may delay circuit breaker activation caused by electrical noise coupled to the receiver, and improves transient block and unblock responses to fault conditions of the power system network.