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.

Abstract: A protective relay system for opening local and remote circuit breakers of a protected line segment when a fault occurs thereon. The protective relay system includes local and remote fault detectors for producing local and remote blocking carrier signals, respectively, for transmission to the other terminal when a fault is in the forward direction from the terminal. Reception of a blocking signal blocks tripping of the circuit breaker at the receiving terminal. When the fault is internal, the blocking signals are transmitted for only a short duration, sufficient to allow the protective relays at both terminals to detect the internal fault. When transmission of the blocking signals stops, the trip coils at both terminals are energized because the relays at both terminals see the fault in the forward direction and do not receive the blocking signal. For an external fault the relays at the terminal from which the fault is seen in the forward direction transmit a blocking signal to the other terminal.