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76
Scheme communication logic (ZCOM)
drawbacks, but a ZCOM-CS signal from an overreaching zone must never be prolonged
in case of parallel lines, to secure correct operation of current reversal logic, when ap-
plied.
At the permissive overreaching scheme, the carrier send signal (ZCOM-CS) might be
issued in parallel both from an overreaching zone and an underreaching, independent
tripping zone. The ZCOM-CS signal from the overreaching zone must not be prolonged
while the ZCOM-CS signal from zone 1 can be prolonged.
There is no race between the ZCOM-CR signal and the operation of the zone in a per-
missive scheme. So set the tCoord to zero. A permissive scheme is inherently faster and
has better security against false tripping than a blocking scheme. On the other hand, a
permissive scheme depends on a received ZCOM-CR signal for a fast trip, so its de-
pendability is lower than that of a blocking scheme.
To overcome this lower dependability in permissive schemes, an 8QEORFNLQJ function
can be used. Use this function at power-line carrier (PLC) communication, where the
signal has to be sent through the primary fault. The unblocking function uses a carrier
guard signal (ZCOM-CRG), which must always be present, even when no ZCOM-CR
signal is received. The absence of the ZCOM-CRG signal during the security time is
used as a CR signal. See figure 40. This also enables a permissive scheme to operate
when the line fault blocks the signal transmission. Set the tSecurity at 35 ms.
Z C O M -C R
1
Z C O M -C R G
2 0 0 m s
t
)LJXUH &DUULHU JXDUG ORJLF ZLWK XQEORFN ORJLF
The ZCOM-CR signals are always transferred directly to ZCOM-CRL without any de-
lay.
tS e c u rity
t
1 5 0 m s
> 1
t
&
&KDSWHU
/LQH LPSHGDQFH
Z C O M -C R L
> 1
&
Z C O M -L C G
e n 0 0 0 0 0 4 9 1 .v s d
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