ABB RELION REX640 Technical Manual page 1246

Protection related functions
communication channel operating in direct intertripping applications should be
secure and dependable.
Setting guidelines for the intertrip scheme DUTT
Scheme type = "Intertrip"
•
• Coordination Time = "0.050" s (10 ms + maximal transmission time)
Carrier Min Dur = "0.1" s
•
Permissive underreaching transfer trip PUTT
In the permissive underreach scheme, the permission to trip is obtained locally
from the underreaching zone and sent to the remote end. The received signal is
combined with the local fault indication from the overreaching zone resulting in a
trip command after a settable pick-up delay
The protection relay of either feeder end sends a permissive signal to trip the
other end. The communication channel must be able to receive and transmit at
the same time. In permissive schemes, the communication channel must be fast
and secure, but also dependable. Inadequate security causes unwanted tripping for
the external faults. Inadequate speed or dependability causes delayed tripping for
internal faults.
The CS_PRM signal is issued based on the underreaching zone Z1.
The underreach scheme is not applicable on short feeder lengths because the
impedance measurement cannot easily distinguish between the internal and
external faults in such cases.
The underreaching zones Z1 at the local and remote ends must overlap to prevent
a gap between the protection zones, where the faults are not detected. If the
underreaching zone does not meet the required sensitivity, for example, the
fault infeed from a remote end, the blocking or permissive overreach scheme is
considered.
To achieve an instantaneous trip, the carrier received signal CR_PRM must be
received when the overreaching zone Z2 is still starting. In some cases, due to a fault
in current distribution, the overreaching zone can operate only after the fault has
been cleared by the remote terminal. In this case, there is a risk that if the remote
terminal is tripped directly by, for example, zone Z1, the CR_PRM signal issued by the
zone resets before the overreaching zone of the local end terminal has operated.
To assure a sufficient duration of the CR_PRM signal, the CR_PRM signal can be
prolonged by the time
Dur setting is "0.1" s.
The received communication signal is combined with the output from the local
overreaching zone. Therefore, there is less concern about a false signal causing
an incorrect trip, and therefore the timer
communication channel failure does not affect the selectivity, but delays the
tripping at the other end for certain fault locations.
Setting guidelines for the permissive underreach scheme PUTT
Scheme type = "Permissive Underreach"
•
Coordination Time = "0" s
•
Carrier Min Dur = "0.1" s
•
1246
Carrier Min Dur . The recommended value of the Carrier Min
Coordination Time is set to "0". A
Coordination Time .
1MRS759142 F
REX640
Technical Manual