ABB Relion 670 series, Applications Manual

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15.2.2.2 Permissive schemes
SEMOD141790-30 v2
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.
Depending on if the sending signal(s) is issued by underreaching or overreaching zone, it is
divided into Permissive underreach (PUR) or Permissive overreach (POR) scheme.
Permissive underreach scheme
SEMOD141790-35 v2
Permissive underreach 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 overreach scheme should be considered.
The carrier 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 IED nearest to the
fault.
There is a certain risk that in case of a trip from an independent tripping zone, the zone issuing
the carrier send signal (CS) resets before the overreaching zone has operated at the remote
IED. 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
tCoord to zero. Failure of the communication channel does not affect the selectivity, but
delays tripping at one end(s) for certain fault locations.
Permissive overreach scheme
SEMOD141790-41 v1
In permissive overreach scheme there is an overreaching zone that issue the carrier 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 scheme must be activated at the same time as the received signal is present.
The scheme can be used for all type line lengths.
In permissive overreach schemes, the communication channel plays an essential roll to
obtaining fast tripping at both ends. Failure of the communication channel may affect the
selectivity and delay tripping at one end at least, for faults anywhere along the protected
circuit. Teleprotection operating in permissive overreach scheme must beside the general
requirement of fast and secure operation also requirement on dependability must be
considered. Inadequate security can cause unwanted tripping for external faults. Inadequate
speed or dependability can cause delayed tripping for internal faults or even unwanted
operations.
This scheme may use virtually any communication media that is not adversely affected by
electrical interference from fault generated noise or by electrical phenomena, such as
lightning, that cause faults. Communication media that uses metallic path are particularly
subjected to this type of interference, therefore, they must be properly shielded or otherwise
designed to provide an adequate communication signal during power system faults. At the
permissive overreaching scheme, the carrier send signal (CS) might be issued in parallel both
from an overreaching zone and an underreaching, independent tripping zone. The CS signal
from the overreaching zone must not be prolonged while the CS signal from zone1 can be
Section 15 1MRK 505 343-UEN B
Scheme communication
608
Application manual