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
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
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
1MRK 506 369-UUS - Section 16
Scheme communication
Line distance protection REL670 2.2 ANSI 759
Application manual
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