2 Functions
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7SA522 Manual
C53000-G1176-C155-3
Figure 2-54 Logic diagram of the permissive overreach transfer trip (POTT) scheme with protection data interface -
continued
2.6.6 Directional Unblocking Scheme
The following scheme is suited for conventional transmission media.
Principle The unblocking method is a permissive release scheme. It differs from the permissive
overreach transfer 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 line by means of power line carrier
(PLC) and the attenuation of the transmitted signal at the fault location may be so
severe that reception at the other line cannot necessarily be guaranteed. Here, a
special unblocking logic takes effect.
The scheme functionality is shown in Figure 2-55.
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 a fault inside the overreaching zone Z1B, it
initiates the transmission of the unblock frequency f
U
. During the quiescent state or
during a fault outside Z1B, or in the reverse direction, the monitoring frequency f
0
is
transmitted.
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