protection IED. A correct single-pole trip can be achieved on both lines and at both lineIEDs.ZC1PPSCH (85) requires three individual channels between the protection IEDs on eachline in both directions. In case of single-phase faults, only one channel is activated at atime. But in case of multi-phase faults, two or three channels are activated simultaneously.The following descriptions of the schemes generally presents one of the three identicalphases.When only one channel is available in each direction, use the optionallyavailable three phase communication scheme logic ZCPSCH (85). Notethat this logic can issue an unwanted three-pole trip at the describedsimultaneous faults close to one line end.16.2.2.1 Blocking schemeIn blocking scheme a reverse looking zone is used to send a block signal to remote end toblock an overreaching zone. Since the scheme is sending the blocking signal duringconditions where the protected line is healthy, it is common to use the line itself ascommunication media (PLC). The scheme can be used on all types of line length.The blocking scheme is very dependable because it will operate for faults anywhere on theprotected line if the communication channel is out of service. Conversely, it is less securethan permissive schemes because it will trip for external faults within the reach of thetripping function if the communication channel is out of service. Inadequate speed ordependability can cause spurious tripping for external faults. Inadequate security cancause delayed tripping for internal faults. To secure that the carrier send signal will arrivebefore the zone used in the communication scheme will trip, the trip is released first afterthe time delay tCoord has elapsed. The setting of tCoord must be set longer than themaximum transmission time of the channel. A security margin of at least 10 ms should beconsidered.The timer tSendMin for prolonging the carrier send signal is proposed to set to zero inblocking schemes.16.2.2.2 Permissive schemesIn permissive scheme permission to trip is sent from local end to remote end(s) that is,protection 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.Section 16 1MRK 506 369-UUS -Scheme communication758 Line distance protection REL670 2.2 ANSIApplication manual
