A permissive scheme is inherently faster and has better security against false tripping than
a blocking scheme. On the other hand, a permissive scheme depend on a received CR
signal for a fast trip, so its dependability is lower than that of a blocking scheme.
16.1.2.1 Blocking schemes
In a blocking scheme a reverse looking zone is used to send a block signal to the 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 line lengths.
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. On the other hand, 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 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 maximal transmission time of the channel.
A security margin of at least 10 ms should be considered.
The timer tSendMin for prolonging the send signal is proposed to set to zero.
Section 16 1MRK 506 369-UUS -
Scheme communication
748 Line distance protection REL670 2.2 ANSI
Application manual
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