Weak-end infeed logicPermissive communication schemes can basically operate only when the protectionin the remote IED can detect the fault. The detection requires a sufficient minimumfault current, 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 in 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 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).The WEI function can be extended to trip also the breaker in the weak side. Thetrip is achieved when one or more phase voltages are low during an echo function.Together 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 distanceprotection at the remote line end and in this way prevents the correct operationof a complete protection scheme.• A separate direct intertrip channel must be arranged from remote end when atrip or accelerated trip is given there. The intertrip receive signal is connectedto input CRL.• The WEI function shall be set to OperationWEI=Echo&Trip. The WEIfunction block will then give phase selection and trip the local breaker.16.4.3 Setting guidelines SEMOD156555-1 v2SEMOD156562-5 v4The parameters for the current reversal and weak-end infeed logic for phasesegregated communication function (ZC1WPSCH) are set via the local HMI orPCM600.Common base IED values for primary current (IBase), primary voltage (UBase)and primary power (SBase) are set in Global base values for settings functionGBASVAL.GlobalBaseSel: It is used to select a GBASVAL function for reference of basevalues.Current reversal logicSet OperCurrRev to On to activate the function.1MRK 506 369-UEN B Section 16Scheme communicationLine distance protection REL670 2.2 IEC 719Application manual
