L1L2A1A2B1B2IEC99000044-2.vsdIEC99000044 V3 EN-USFigure 123: Current distribution for a fault close to B side when breaker B1 hasopenedTo handle this the send signal CS or CSLn from B2 is held back until the reversezone IRVLn has reset and the tDelayRev time has been elapsed. To achieve this thereverse zone on the distance protection shall be connected to input IRV and theoutput IRVL shall be connected to input BLKCS on the communication functionblock ZCPSCH.The function can be blocked by activating the input IRVBLK or the generalBLOCK input.13.2.2.2 Weak-end infeed logic M13895-16 v8Permissive communication schemes can only operate when the protection in theremote IED can detect the fault. The detection requires a sufficient minimum faultcurrent, normally >20% of Ir. The fault current can be too low due to an openbreaker or low short-circuit power of the source. To overcome these conditions,weak-end infeed (WEI) echo logic is used. The fault current can also be initiallytoo low due to the fault current distribution. Here, the fault current increases whenthe breaker opens at the strong terminal, and a sequential tripping is achieved. Thisrequires a detection of the fault by an independent tripping zone 1. To avoidsequential tripping as described, and when zone 1 is not available, weak-end infeedtripping logic is used. The weak end infeed function only works together withpermissive overreach communication schemes as the carrier send signal must coverthe complete line length.The WEI function sends back (echoes) the received signal under the condition thatno fault has been detected on the weak-end by different fault detection elements(distance protection in forward and reverse direction).Also, the WEI function can be additionally extended to trip the breaker in the weakside. The trip is achieved when one or more phase voltages are low during an echofunction.In case of single-pole tripping, the phase voltages are used as phase selectorstogether with the received signal CRLn.When used with the blocking teleprotection scheme some limitations apply:• Only the trip part of the function can be used together with the blockingscheme. It is not possible to use the echo function to send the echo signal tothe remote line IED. The echo signal would block the operation of the distanceSection 13 1MRK 505 393-UEN BScheme communication286 Line differential protection RED650 2.2 IECApplication manual
