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Scheme communication logic (ZCOM)
ZCOM-CACC
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ZCOM-CR
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ZCOM-CACC
Forward overreaching zone used for the communication scheme.
ZCOM-CR
Carrier receive signal.
ZCOM-TRIP
Trip from the communication scheme.
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In a permissive scheme, the received signal (ZCOM-CR) carries information from the
protection terminal at the opposite end of the line. It indicates detected faults in the for-
ward direction out on the line. The received information is used to allow an overreach-
ing zone to trip almost instantaneously for faults on the protected line.
ZCOM-CACC
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ZCOM-CR
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The permissive scheme principle is further subdivided into two types, underreaching
and overreaching, where the names indicate that the send signal (ZCOM-CS) is issued
by an underreaching or an overreaching zone, respectively.
The signal (ZCOM-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 near-
est 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 (ZCOM-CS) resets before the overreach-
ing zone has operated at the remote terminal. To assure a sufficient duration of the re-
ceived signal (ZCOM-CR), the send signal (ZCOM-CS), can be prolonged by a
tSendMin reset timer. The recommended setting of tSendMin is 100 ms. A ZCOM-CS
signal from an underreaching zone can be prolonged during all circumstances without
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