2.8 Teleprotection for earth fault overcurrent protection (optional)
181
7SA522 Manual
C53000-G1176-C155-3
Figure 2-78 Logic diagram of the directional comparison scheme with protection data interface (for one device) — con-
tinued
2.8.3 Directional Unblocking Scheme
The following scheme is suited for conventional transmission media.
Principle The unblocking method is a permissive scheme. It differs from the directional compar-
ison scheme in that tripping is possible also when no release signal is received from
the opposite line end. It is therefore mainly used for long lines when the signal must
be transmitted across the protected feeder by means of power line carrier (PLC) and
the attenuation of the transmitted signal at the fault location may be so severe that re-
ception at the other line cannot necessarily be guaranteed.
The scheme functionality is shown in Figure 2-79.
Two signal frequencies which are keyed by the transmit output of the 7SA522 are re-
quired for the transmission. If the transmission device has a channel monitoring, then
the monitoring frequency f
0
is keyed over to the working frequency f
U
(unblocking fre-
quency). When the protection recognizes an earth fault in the forward direction, it ini-
tiates the transmission of the unblock frequency f
U
. During the quiescent state or
during an earth fault in the reverse direction, the monitoring frequency f
0
is transmitted.
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