• If Wei mode = "Off", the WEI logic is disabled.• If Wei mode = "Echo", the WEI logic is enabled without Level detector.• If Wei mode = "Echo and Operate", the WEI logic is enabled with Level detector.Level detectorLevel detector sends an enabling signal to Weak-end trip logic when the residualvoltage is above the Residual voltage Val setting.Echo logicEcho logic returns the received carrier signal with the ECHO output when it receivesa carrier receive signal (CR) if no fault has been detected by the forward and reversedirectional residual overcurrent functions and the duration of CR is longer than thecoordination time delay (the setting Wei Crd time). The received carrier receive signalis also given as the CR output.When the WEI logic is enabled at both feeder ends, a spurious signalcan be looped round by Echo logic. To avoid a continuous lockup ofthe system, the duration of the echoed signal is limited to 200 ms.The ECHO and CR outputs can be blocked by activating the BLOCK input. The FF,BLK_WEI1 and BLK_WEI2 inputs can be used to block the ECHO output. The FFinput is usually connected to the functional output of the fuse failure function, and theBLK_WEI1 input is typically connected to the start signals of the forward and reversedirectional residual overcurrent functions in the terminal. The BLK_WEI2 signal hasan in-built 200 ms drop-off delay and is usually connected to all the fault detectionfunctions in the terminal except the residual overvoltage function.Weak-end trip logicWeak-end trip logic is activated if it receives an enable signal from Level detector andECHO output of Echo logic is activated. Then a general trip signal OPR_WEI isactivated. The activation of the CB_OPEN input blocks the tripping.The OPR_WEI output can be blocked by activating the BLOCK input.5.13.6 ApplicationCurrent reversal logicWhen the overreaching permissive communication schemes are used in parallelfeeder applications, the current reversal can result in unselective tripping. Such acondition can occur in the healthy feeder when the fault is cleared in the faulty feeder.The tripping of the healthy parallel feeder can lead to losing the connection betweenthe two buses. To avoid this situation, the current reversal logic (transient blockinglogic) can be used.Section 5 1MRS759142 CProtection related functions1180 REX640Technical Manual
