ABB RELION REX640 Technical Manual page 1247

1MRS759142 F
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
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 (
Min Dur = "0" s). There is no need to delay the tripping while receiving the carrier
signal, so the timer
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 is less secure than a permissive scheme because it trips for the
external faults within the reach of the overreaching local zone if the communication
channel is out of service and no block signal is received.
Inadequate speed or low dependability of the communication channel can cause
spurious tripping for external faults.
To secure that the blocking signal arrives before the local overreaching zone trips,
the trip is delayed by the
than the sum of the maximal transmission time of the channel and the start and
response times of the protection relays sending and receiving the CS_BLOCKING
REX640
Technical Manual
Coordination Time is set to "0".
Coordination Time setting. This setting must be set longer
Protection related functions
Coordination Time .
Carrier
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