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
•
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 residual overcurrent function is used for
sending a block signal to the remote end to block the overreaching forward-looking
residual overcurrent function.
The blocking scheme is very dependable because it operates for 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 function 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.
1MRS759142 F
Protection related functions
REX640
Technical Manual
1261