ABB REL670 2.2 IEC, Applications Manual

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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 prolonged. To secure correct operations of current reversal logic in case of
parallel lines, when applied, the carrier send signal CS shall not be prolonged. So
Section 16
1MRK 506 369-UEN B
Scheme communication
712 Line distance protection REL670 2.2 IEC
Application manual