LDCMLDCMTelecom. NetworkTelecom. NetworkPrimarychannelSecondary redundantchannelLDCMLDCMen05000289.vsdIEC05000289 V1 EN-USFigure 39: Direct fibre optical connection between two IEDs with LDOM overlonger distances.If communication is lost on the primary channel, switchover to the secondarychannel is made after a settable time delay RedChSwTime. Return of the primarychannel will cause a switchback after another settable time delay RedChRturnTime.For a three-terminals line in a master-master configuration, a loss of onecommunication channel will not cause the line differential protection to beunserviceable. Instead it will automatically revert to a partial master-slave modewith the two IEDs that have an unserviceable communication link between them,will serve as slaves.For more details about the remote communication see section remotecommunication and the application manual.6.1.7.4 Open CT detection feature GUID-5B7ACC4D-4EC9-40F1-B18D-561F13945FE3 v1Line differential protection has a built-in, advanced open CT detection feature.A sudden inadvertently opened CT circuit may cause an unexpected and unwantedoperation of the Line differential protection under normal load conditions. Damageof secondary equipment may occur due to high voltage from open CT circuitoutputs. It is always an advantage, from the point of view of security andreliability, to have the open CT detection function to block the line differentialprotection function in case of an open CT condition, and produce an alarm signal tothe operational personnel to quickly correct the open CT condition.The built-in open CT feature can be enabled or disabled by the setting parameterOpenCTEnable (Off/On). When enabled, this feature tries to prevent mal-operationwhen a loaded main CT connected to line differential protection is by mistake opencircuited on the secondary side. Note that this feature can only detect interruptionof one CT phase current at a time. If two or even all three-phase currents of one setof CTs are accidentally interrupted at precisely the same time, this feature cannotoperate. Line differential protection generates a trip signal if the false differentialcurrent is sufficiently high. An open CT circuit is typically detected in 12–14 ms,Section 6 1MRK 505 394-UEN ADifferential protection104 Line differential protection RED650 2.2 IECTechnical manual
