ABB REL670 2.2 IEC, Applications Manual

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When only one channel is available in each direction, use the
optionally available three phase communication scheme logic
ZCPSCH . Note that this logic can issue an unwanted three-pole
trip at the described simultaneous faults close to one line end.
16.2.2.1 Blocking scheme
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In blocking scheme a reverse looking zone is used to send a block signal to remote
end to block an overreaching zone. Since the scheme is sending the blocking signal
during conditions where the protected line is healthy, it is common to use the line
itself as communication 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 the protected line if the communication channel is out of service.
Conversely, it is less secure than permissive schemes because it will trip for
external faults within the reach of the tripping function if the communication
channel 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 carrier send signal will arrive before the zone
used in the communication scheme will trip, the trip is released first after the time
delay
tCoord has elapsed. The setting of tCoord must be set longer than the
maximum transmission time of the channel. A security margin of at least 10 ms
should be considered.
The timer tSendMin for prolonging the carrier send signal is proposed to set to zero
in blocking schemes.
16.2.2.2 Permissive schemes
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In 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 received signal 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 the teleprotection equipment need
to be able to receive while transmitting.
Depending on if the sending signal(s) is issued by underreaching or overreaching
zone, it is divided into Permissive underreach (PUR) or Permissive overreach
(POR) scheme.
Permissive underreach scheme
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Permissive underreach scheme is not suitable to use on short line length due to
difficulties for distance protection measurement in general to distinguish between
internal and external faults in those applications.
1MRK 506 369-UEN B
Section 16
Scheme communication
Line distance protection REL670 2.2 IEC 711
Application manual