ABB RELION REX640 Technical Manual page 1156

Section 5
Protection related functions
1150
Permissive overreaching transfer trip POTT
In the permissive overreach scheme, the permission to trip is obtained locally from an
overreaching zone and sent to the remote end. The received signal is then combined
with the local fault indication from the overreaching zone, 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. The communication on permissive schemes should be fast and
secure.
The CS_PRM signal is issued based on the overreaching zone Z2.
The permissive overreach scheme can be used for all feeder types and lengths.
In the permissive overreach scheme, the communication channel plays an essential
role in obtaining a fast tripping at both ends. Typically, a lost communication channel
does not cause false tripping. However, it delays the operation of the protection for the
faults anywhere 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 zones. The CS_PRM signal from the
overreaching zone must not be prolonged, while the CS_PRM signal from the
underreaching zone must typically be prolonged. In parallel feeder applications, the
scheme typically needs to be complemented by the current reversal logic. To ensure
the 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 while receiving the carrier
signal, so the timer Coordination Time is set to "0".
Setting guidelines for the permissive overreaching transfer trip scheme POTT
Scheme type = "Permissive Overreach"
Coordination Time = "0" s
Carrier Min Dur = "0.1" s
Carrier Min Dur = "0" s (in case of parallel lines and current reversal logic)
Directional comparison blocking scheme DCB
In the blocking scheme, the reverse-looking zone Z3 is used for sending a block signal
to the remote end to block the overreaching zone.
The blocking scheme is very dependable because it operates on faults anywhere on the
protected feeder, even if the communication channel is out of service. Conversely, it
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REX640
Technical Manual