Blocking Schemes - ABB REC670 Applications Manual

Section 15
Scheme communication
15.1.2.1
466
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.

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.
1MRK 511 401-UUS A
M16866-24 v5
Bay control REC670 2.2 ANSI
Application manual