ABB Relion 670 Series Applications Manual page 390

Section 14
Scheme communication
Depending on if the sending signal(s) is issued by underreaching or overreaching zone, it is
divided into Permissive underreach or Permissive overreach scheme.
Permissive underreaching scheme
Permissive underreaching scheme is not suitable to use on short line length due to difficulties for
distance protection measurement in general to distinguish between internal and external faults in
those applications.
The underreaching zones at local and remote end(s) must overlap in reach to prevent a gap
between the protection zones where faults would not be detected. If the underreaching zone do
not meet required sensitivity due to for instance fault infeed from remote end blocking or
permissive overreaching scheme should be considered.
The received signal (CR) must be received when the overreaching zone is still activated to achieve
an instantaneous trip. In some cases, due to the fault current distribution, the overreaching zone
can operate only after the fault has been cleared at the terminal nearest to the fault. There is a
certain risk that in case of a trip from an independent tripping zone, the zone issuing the send
signal (CS) resets before the overreaching zone has operated at the remote terminal. To assure a
sufficient duration of the received signal (CR), the send signal (CS), can be prolonged by a
tSendMin reset timer. The recommended setting of tSendMin is 100 ms.
Since the received communication signal is combined with the output from an overreaching zone,
there is less concern about false signal causing an incorrect trip. Therefore set the timer
zero.
Failure of the communication channel does not affect the selectivity, but delays tripping at one
end(s) for certain fault locations.
OR
B
IEC09000013 V1 EN-US
Figure 185: Principle of Permissive underreaching scheme
UR: Underreaching
OR: Overreaching
CR: Communication signal received
CS: Communication signal send
Permissive overreaching scheme
In permissive overreaching scheme there is an overreaching zone that issues the send signal. At
remote end the received signal together with activating of an overreaching zone gives
instantaneous trip of the protected object. The overreaching zone used in the teleprotection
384
A
UR
B
CS
B
TRIP: UR, OR+CR
UR
A
CS
A
B
IEC09000013-1-en.vsd
1MRK 511 358-UUS A
M16866-53 v3
tCoord to
OR
A
M16866-41 v3
Application manual