12.3.6.2 Weak-end infeed logicThe weak-end infeed logic (WEI) function trip and sends back (echoes) the receivedsignal under the condition that no fault has been detected on the weak-end by differentfault detection elements (distance protection in forward and reverse direction).The WEI function returns the received signal, see figure 236, when:• No active signal present on the input BLOCK.• The functional input CRL is active. This input is usually connected to the CRL outputon the scheme communication logic ZCPSCH.• The WEI function is not blocked by the active signal connected to the WEIBLK1functional input or to the BLKZ functional input. The later is usually configured tothe BLOCK functional output of the fuse-failure function.• No active signal has been present for at least 200 ms on the WEIBLK2 functionalinput. An OR combination of all fault detection functions (not undervoltage) aspresent within the IED is usually used for this purpose.BLOCKCRLWEIBLK2ECHOECHO - cont.ANDBLKZANDWEIBLK1OR-00tWEI0200ms050ms200ms0ANSI10000260-1-en.vsdANSI10000260 V1 ENFigure 240: Echo of a received signal by the WEI functionWhen an echo function is used in both IEDs (should generally be avoided), a spurioussignal can be looped round by the echo logics. To avoid a continuous lock-up of thesystem, the duration of the echoed signal is limited to 200 ms.An undervoltage criteria is used as an additional tripping criteria, when the tripping of thelocal breaker is selected, setting WEI = Echo & Trip, together with the WEI function andECHO signal has been issued by the echo logic, see figure 237.Information from the open circuit breaker can be connected to the input CBOPEN to avoidWEI-trip if the circuit breaker already was opened.1MRK 506 335-UUS A Section 12Scheme communication511Technical manual
