Functions
2.9 Teleprotection for Earth Fault Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
253
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 comparison scheme in that trip-
ping 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 reception at the
other line cannot necessarily be guaranteed.
The scheme functionality is shown in Figure 2-121.
Two signal frequencies which are keyed by the transmit output of the 7SD5 are required 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 frequency). When the protection recognizes an earth fault in the forward di-
rection, it initiates 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 a release signal is also received from the opposite end, the trip signal is forwarded to the command relay. A
pre-condition for fast fault clearance is therefore that the earth fault is recognized 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 op-
posite line end even if the earth fault is very rapidly cleared by a different independent protection.
Figure 2-121 Operation scheme of the directional unblocking method
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