protected line fault. With power line carrier, for example, the communication signal maybe attenuated by the fault, especially when the fault is close to the line end, therebydisabling the communication channel.To overcome the lower dependability in permissive schemes, an unblocking function canbe 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 aguard signal CR_GUARD, which must always be present, even when no CR signal isreceived. The absence of the CR_GUARD signal during the security time is used as a CRsignal. This also enables a permissive scheme to operate when the line fault blocks thesignal transmission. Set the tSecurity to 35 ms.12.1.2.3 Intertrip schemeIn 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.In intertrip scheme, the send signal is initiated by an underreaching zone or from anexternal protection (transformer or reactor protection). At remote end, the received signalsinitiate a trip without any further protection criteria. To limit the risk for unwanted trip dueto spurious sending of signals, the timer tCoord should be set to 10-30 ms dependant ontype of communication channel.12.1.3 Setting guidelinesThe parameters for the scheme communication logic function are set via the local HMI orPCM600.Configure the zones used for the CS send and for scheme communication tripping byusing the ACT configuration tool.The recommended settings of tCoord timer are based on maximal recommendedtransmission time for analogue channels according to IEC 60834-1. It is recommended tocoordinate the proposed settings with actual performance for the teleprotection equipmentto get optimized settings.12.1.3.1 Blocking schemeSetOperation =EnabledSetSchemeType =BlockingSettCoord 25 ms (10 ms + maximal transmission time)Table continues on next page1MRK 506 334-UUS A Section 12Scheme communication397Application manual
