GE Multilin D60 Line Distance Relay 8-278 THEORY OF OPERATION 8.5 SINGLE-POLE TRIPPING88.5.4 PERMISSIVE ECHO SIGNALINGThe “echo” feature can reduce the response time of an over-reaching scheme when a terminal is disconnected from theline. In this condition, a Zone 2 element at the terminal that remains in-service can detect a fault, but cannot trip, as a per-missive signal is not received from the remote terminal. This feature is provided in the permissive over-reaching transfer tripand hybrid permissive over-reaching transfer trip schemes.PERMISSIVE OVER-REACHING TRANSFER TRIP SCHEME:When used this feature will “echo” a reliable received permissive signal back to the originating terminal when a line-end-open condition is identified by the Line Pickup logic. The Permissive Echo is programmed as a one-shot logic. Theecho is sent only once and then the echo logic locks out for a settable period. The duration of the echo pulse does notdepend on the duration or shape of the received RX signals but is settable.The echo is sent back only if none of the overreaching protection elements operates.HYBRID PERMISSIVE OVER-REACHING TRANSFER TRIP PERMISSIVE ECHO:When used this feature will “echo” a reliable received permissive signal back to the originating terminal if the line-end-open condition is recognized by the LINE PICKUP scheme and the fault is not identified as a reverse fault by the zone4 or the ground directional overcurrent function (if used). The Permissive Echo is programmed as a one-shot logic. Theecho is sent only once and then the echo logic locks out for a settable period. The duration of the echo pulse does notdepend on the duration or shape of the received RX signal but is settable as ECHO DURATION.The echo is sent back only if none of the overreaching protection elements operates.PERMISSIVE ECHO OPERANDS AND TRANSMIT CODES:In single-pole tripping, single-bit channel applications the signal received on bit no. 1 (RX1) is echoed back on bit no. 1(TX1). In two- and four-bit applications the following Echo Tables apply.Table 8–23: ECHO TABLE FOR 2-BIT CHANNELSLOCAL DETERMINATION OF FAULT TYPE ECHOED BITSTX1 TX2AG 1 0BG 0 1CG 1 1AB, ABG, BC, BCG, CA, CAG, 3P, Unrecognized Send back as receivedTable 8–24: ECHO TABLE FOR 4-BIT CHANNELSLOCAL DETERMINATION OF FAULT TYPE ECHOED BITSTX1 TX2 TX3 TX4AG 1 0 0 0BG 0 1 0 0CG 0 0 1 0AB, ABG, BC, BCG, CA, CAG, 3P, Unrecognized Send back as received
