2 Functions
236
7SD5 Manual
C53000-G1176-C169-1
2.9.3 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 2-102.
Two signal frequencies which are keyed by the transmit output of the 7SD5 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.
If the unblock frequency is received from the opposite end, a signal is routed to the trip
logic. A pre-condition for fast fault clearance is therefore that the earth fault is recog-
nized in the forward direction at both line ends.
The send signal can be prolonged by T
S
(settable). The prolongation of the send signal
only comes into effect if the protection has already issued a trip command. This
ensures that the permissive signal releases the opposite line end even if the earth fault
is very rapidly cleared by a different independent protection.
Figure 2-102 Operation scheme of the directional unblocking method
Sequence Figure 2-103 shows the logic diagram of the unblocking scheme for one line end.
The directional unblocking scheme only functions for faults in the „forward“ direction.
Accordingly the overcurrent stage intended for operation in the directional unblocking
scheme must definitely be set to Forward (RICH.3I0...); refer also to Section 2.8
at the margin heading „Teleprotection with Earth Fault Protection“.
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