Permissive Schemes - ABB Relion REC670 Applications Manual

1MRK 511 358-UEN A
Inadequate speed or dependability can cause spurious tripping for external faults. Inadequate
security can cause delayed tripping for internal faults.
Since the blocking signal is initiated by the delta based detection which is very fast the time
tCoord can be set to zero seconds, except in cases where the transmission channel is
delay
slow.
The timer
DeltaBasedDetection (deltaA)
IEC11000252 V1 EN-US
Figure 171: Principle of delta blocking scheme
OR:
CR:
CS:
deltaA: Delta based fault inception detection on A side that gets inhibited for forward faults
14.1.2.3

Permissive schemes

In permissive scheme permission to trip is sent from local end to remote end(s), that is
protection at local end have detected a fault on the protected object. The received signal(s) is
combined with an overreaching zone and gives an instantaneous trip if the received signal is
present during the time the chosen zone is detected a fault in forward direction.
Either end may send a permissive (or command) signal to trip to the other end(s), and the
teleprotection equipment need to be able to receive while transmitting.
A general requirement on permissive schemes is that it shall be fast and secure.
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
Application manual
tSendMin for prolonging the send signal is proposed to set to zero.
A
OR
B
CS
deltaA
Overreaching
Communication signal received
Communication signal send
Scheme communication
B
TRIP = OR + tCoord+ CR
B B
IEC11000252-1-en.vsd
Section 14
M16866-33 v3
M16866-53 v3
343