ABB RELION REX640 Technical Manual page 1171

1MRS759142 C
REX640
Technical Manual
Carrier Min Dur = "0.1" s
Permissive overreaching transfer trip POTT
In the permissive overreach scheme, the permission to trip is obtained locally from an
overreaching function and sent to the remote end. The received signal is then
combined with the local fault indication from the overreaching function, resulting in
a trip command after a settable pick-up delay Coordination time.
The protection relay at either end of the feeder sends a permissive signal to trip the
other end. Therefore the communication channel must be able to receive and transmit
at the same time. A general requirement for the communication on permissive
schemes is that it should be fast and secure.
In the permissive overreach scheme, the CS_PRM signal is issued based on the start of
the overreaching residual overcurrent function.
The residual overcurrent function may start at one end due to the system conditions
such as weak zero-sequence source at one end, or high fault resistance. However, fast
tripping from both ends can be achieved by connecting a general forward start signal
to CACC. So, even if the local overreaching residual overcurrent function has not
picked up, the local OPERATE is activated. The function also generates a CS_PRM
signal by combining the CR_PRM signal from a remote end and local activation of
CACC, giving permission to the remote function to trip instantaneously.
The permissive overreach scheme can be used for all feeder types and lengths.
In the permissive overreach scheme, the communication channel is essential for
obtaining fast tripping at both ends. Typically, a lost communication channel does not
cause false tripping, but it delays the operation of the protection for the faults on the
feeder.
The communication channel for a permissive overreach scheme must be fast and
secure, but also dependable. Inadequate security can cause unwanted tripping for
external faults. Inadequate speed or dependability can cause delayed tripping for
internal faults.
In the permissive overreaching scheme, the CS_PRM signal can be issued in parallel
both from the overreaching and underreaching functions. The CS_PRM signal from
the overreaching function must not be prolonged, while the CS_PRM signal from the
underreaching function must typically be prolonged. In parallel feeder applications,
the scheme typically needs to be complemented by the current reversal logic. To
ensure a correct operation in this case, the CS_PRM signal must not be prolonged
(Carrier Min Dur = "0" s). There is no need to delay the tripping when receiving the
carrier signal, so the time Coordination time is set to "0".
Setting guidelines for the permissive overreaching transfer trip scheme POTT
Scheme type = "Permissive Overreach"
Coordination time = "0" s
Protection related functions
Section 5
1165