ABB RELION REX640 Technical Manual page 1170

Section 5
Protection related functions
1164
Permissive underreaching transfer trip PUTT
In the permissive underreach scheme, a permission to trip is obtained locally from the
underreaching 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 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 external faults.
Inadequate speed or dependability causes delayed tripping for internal faults.
The CS_PRM signal is issued based on the underreaching residual overcurrent
protection function.
The permissive scheme is not applicable on short feeder lengths because the residual
overcurrent protection cannot easily distinguish between the internal and external
faults in such cases.
The residual overcurrent function may start only 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 output is activated. The function also
generates a CS_PRM signal by combining the carrier received CR_PRM signal from a
remote end and also the local activation of CACC, giving the permission to a remote
function to trip instantaneously.
The CR_PRM signal must be received when the overreaching residual overcurrent
protection is still active to achieve an instantaneous trip. In some cases, due to the fault
current distribution, the overreaching protection 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 the instantaneous underreaching function, the CS_PRM
signal that it issues resets before the overreaching function of the local end terminal is
operated. To ensure a sufficient duration of the CR_PRM signal, the CS_PRM signal
can be prolonged by time Carrier Min Dur. The recommended value of the Carrier
Min Dur setting is "0.1" s.
The received communication signal is combined with the output from the local
overreaching function. Therefore, there is less concern about a false signal causing an
incorrect trip. Therefore, the timer Coordination time can be set to "0". A
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
1MRS759142 C
REX640
Technical Manual