on a received CR signal for a fast trip, so its dependability is lower than that of ablocking scheme.13.1.2.1 Blocking schemes M16866-24 v5In a blocking scheme a reverse looking zone is used to send a block signal to theremote end to block an overreaching zone.Since the scheme is sending the blocking signal during conditions where theprotected line is healthy, it is common to use the line itself as communicationmedia (PLC). The scheme can be used on all line lengths.The blocking scheme is very dependable because it will operate for faultsanywhere on the protected line if the communication channel is out of service. Onthe other hand, it is less secure than permissive schemes because it will trip forexternal faults within the reach of the tripping function if the communicationchannel is out of service.Inadequate speed or dependability can cause spurious tripping for external faults.Inadequate security can cause delayed tripping for internal faults.To secure that the send signal will arrive before the zone used in thecommunication scheme will trip, the trip is released first after the time delaytCoord has elapsed. The setting of tCoord must be set longer than the maximaltransmission time of the channel. A security margin of at least 10 ms should beconsidered.The timer tSendMin for prolonging the send signal is proposed to set to zero.A BORBZ revAZ revA CSA TRIPB = ORB+ tCoord+ CRIEC09000015_2_en.vsdIEC09000015 V2 EN-USFigure 118: Principle of blocking schemeOR: OverreachingCR: Communication signal receivedCS: Communication signal sendZ revA: Reverse zoneSection 13 1MRK 505 393-UEN BScheme communication278 Line differential protection RED650 2.2 IECApplication manual
