Inadequate speed or dependability can cause spurious tripping for external faults. Inadequatesecurity can cause delayed tripping for internal faults.Since the blocking signal is initiated by the delta based detection which is very fast the timedelaytCoord can be set to zero seconds, except in cases where the transmission channel isslow.The timertSendMin for prolonging the send signal is proposed to set to zero.A BORBDeltaBasedDetection (deltaA)deltaA CS TRIPB = ORB+ tCoord+ CRIEC11000252-1-en.vsdIEC11000252 V1 EN-USFigure 128: 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 faults13.1.2.3 Permissive schemes M16866-33 v3In 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) iscombined with an overreaching zone and gives an instantaneous trip if the received signal ispresent 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, it isdivided into Permissive underreach or Permissive overreach scheme.Permissive underreaching scheme M16866-53 v3Permissive underreaching scheme is not suitable to use on short line length due to difficultiesfor distance protection measurement in general to distinguish between internal and externalfaults in those applications.The underreaching zones at local and remote end(s) must overlap in reach to prevent a gapbetween the protection zones where faults would not be detected. If the underreaching zonedo not meet required sensitivity due to for instance fault infeed from remote end blocking orpermissive overreaching scheme should be considered.The received signal (CR) must be received when the overreaching zone is still activated toachieve an instantaneous trip. In some cases, due to the fault current distribution, theoverreaching zone can operate only after the fault has been cleared at the terminal nearest to1MRK 505 363-UEN A Section 13Scheme communication253Application manual
