Permissive underreaching scheme
M16866-53 v4
Permissive underreaching 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.
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. T
o 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.
1MRK 511 401-UUS A Section 15
Scheme communication
Bay control REC670 2.2 ANSI 469
Application manual
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