13.1.2.3 Permissive schemes
M16866-33 v4
In permissive schemes, the permission to trip is sent from the local end to the
remote end(s), when the protection at the local end has 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 has detected a fault.
Either end may send a permissive (or command) signal to trip to the other end(s),
and the teleprotection equipment needs to be able to receive while transmitting.
A general requirement on permissive schemes is that it shall be fast and secure.
If the sending signal(s) is issued by underreaching or overreaching zone, it is
divided into a permissive underreach or permissive overreach scheme.
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. 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.
Section 13 1MRK 505 393-UEN B
Scheme communication
280 Line differential protection RED650 2.2 IEC
Application manual
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