Section 1UnrestrainedLimitSection 2 Section 3RestrainOperateunconditionally5432100 1 2 3 4 5IdMinEndSection1EndSection2Restrain current[ in pu of IBase]Operate current[ in pu of IBase]SlopeSection2SlopeSection3en05000300.vsdOperateconditionallyIdMinHighACBIEC05000300 V1 EN-USFigure 35: Overestimated charging currentIf a high resistive fault is considered, the voltage in the fault location will not be much reduced.Consequently, the value of the pre-fault “false” differential current will be a good estimation ofthe actual charging current.Subtracting the pre-fault charging current from the differential current under fault conditionswill make it possible to setIdmin mainly without considering the charging current in order toachieve maximum sensitivity. The stability at external faults will not be affected.6.2.2.4 Time synchronization M12022-66 v3Time synchronization of sampled current values is a crucial matter in numerical line differentialprotections. The synchronization is made with the so called echo method, which can becomplemented with IRIG-B time synchronization. In applications with symmetricalcommunication delay, that is, send and receive times are equal, the echo method is sufficient.When used in networks with asymmetrical transmission times, the optional IRIG–B timesynchronization is required.6.2.2.5 Analog signal communication for line differential protection M12022-69 v6Line differential protection uses digital 64 kbit/s communication channels to exchangetelegrams between the protection IEDs. These telegrams contain current sample values, timeinformation, trip signals, block signals, alarm signals and eight binary signals, which can beused for any purpose. Each IED can have a maximum of two communication channels.On a two terminal line there is a need of one 64 kbit/s communication channel provided thatthere is only one CT in each line end, as shown in Figure 36.1MRK 505 363-UEN A Section 6Differential protection77Application manual
