To secure correct operations of current reversal logic in case of parallel lines thesend signal CS shall not be prolonged. Set the tSendMin to zero in this case.There is no need to delay the trip at receipt of the signal, so set the timer tCoord tozero.A BORAORBORA CSA TRIPB = ORB+ CRB , ORB+ T2IEC09000014-1-en.vsdIEC09000014 V1 EN-USFigure 121: Principle of Permissive overreaching schemeOR: OverreachingCR: Communication signal receivedCS: Communication signal sendT2: Timer step 2Unblocking scheme M16866-67 v5Metallic communication paths adversely affected by fault generated noise may notbe suitable for conventional permissive schemes that rely on a signal transmittedduring a protected line fault. With power line carrier for example, thecommunication signal may be attenuated by the fault, especially when the fault isclose to the line end, thereby disabling the communication channel.To overcome the lower dependability in permissive schemes, an unblockingfunction can be used. Use this function at older, less reliable, power-line carrier(PLC) communication, where the signal has to be sent through the primary fault.The unblocking function uses a guard signal CRG, which must always be present,even when no CR signal is received. The absence of the CRG signal during thesecurity time is used as a CR signal. This also enables a permissive scheme tooperate when the line fault blocks the signal transmission. Set the tSecurity to 35ms.13.1.2.4 Intertrip scheme M16866-71 v4In some power system applications, there is a need to trip the remote end breakerimmediately from local protections. This applies for instance when transformers orreactors are connected to the system without circuit-breakers or for remote trippingfollowing operation of breaker failure protection.Section 13 1MRK 505 393-UEN BScheme communication282 Line differential protection RED650 2.2 IECApplication manual
