Since the blocking signal is initiated by the delta based detection which is very fast thetime delay tCoord can be set to zero seconds, except in cases where the transmissionchannel is slow.The timer tSendMin for prolonging the send signal is proposed to set to zero.A BORBDeltaBasedDetection (deltaA)deltaA CS TRIPB = ORB+ tCoord+ CRIEC11000252-1-en.vsdIEC11000252 V1 ENFigure 192: Principle of delta blocking schemeOR: OverreachingCR: Communication signal receivedCS: Communication signal senddeltaA: Delta based fault inception detection on A side that gets inhibited for forward faults12.1.2.2 Permissive schemesIn permissive scheme permission to trip is sent from local end to remote end(s), that isprotection at local end have detected a fault on the protected object. The received signal(s)is combined with an overreaching zone and gives an instantaneous trip if the receivedsignal is present during the time the chosen zone is detected a fault in forward direction.Either end may send a permissive (or command) signal to trip to the other end(s), and theteleprotection equipment need to be able to receive while transmitting.A general requirement on permissive schemes is that it shall be fast and secure.Depending on if the sending signal(s) is issued by underreaching or overreaching zone, itis divided into Permissive underreach or Permissive overreach scheme.Permissive underreaching schemePermissive underreaching scheme is not suitable to use on short line length due todifficulties for distance protection measurement in general to distinguish between internaland external faults in those applications.Section 12 1MRK 506 334-UUS AScheme communication394Application manual
