Directional comparison blocking protective relay system

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. At the latter terminal the blocking signal is received and causes transmission of a longer duration blocking signal to the transmitting terminal. The receivers at both terminals are responsive to the longer duration blocking signal to block tripping. The duration of this longer duration blocking signal is established so that the external fault is cleared by other protective relays before transmission of the longer-duration blocking signal terminates, thereby preventing tripping at the terminal where the fault was originally detected in the forward direction.