The underreaching zones at the local and remote end(s) must overlap in reach to prevent
a gap between the protection zones where faults would not be detected. If the
underreaching zone do not meet the required sensitivity due to for instance fault infeed
from the remote end, a blocking or permissive overreaching scheme should be considered.
The received signal (CR) must be received when the overreaching zone is activated to
achieve an instantaneous trip. In some cases, due to the fault current distribution, the
overreaching zone can operate only after the fault has been cleared at the terminal nearest
to the fault. There is a certain risk that in case of a trip from an independent tripping zone,
the zone issuing the send signal (CS) resets before the overreaching zone has started at the
remote terminal. To assure a sufficient duration of the received signal (CR), the send
signal (CS) can be prolonged by a tSendMin reset timer. The recommended setting of
tSendMin is 100 ms.
Since the received communication signal is combined with the output from an
overreaching zone, there is less concern about a false signal causing an incorrect trip.
Therefore set the timer tCoord to zero.
Failure of the communication channel does not affect the selectivity, but delays tripping
at one end(s) for certain fault locations.
A B
OR
A
OR
B
UR
B
CS
B
UR
A
CS
A
TRIP: UR or OR+CR
IEC09000013-2-en.vsd
IEC09000013 V2 EN
Figure 369: Principle of Permissive underreaching scheme
UR:
Underreaching
OR: Overreaching
CR: Communication signal received
CS: Communication signal send
1MRK 506 369-UUS - Section 16
Scheme communication
Line distance protection REL670 2.2 ANSI 751
Application manual
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