2.7 Teleprotection for Distance Protection (optional)
187
7SD5 Manual
C53000-G1176-C169-1
2.7.8 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 2-76.
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 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.
If the release frequency is received from the opposite end, a trip signal is forwarded to
the trip logic. Accordingly, it is a prerequisite for fast tripping, that the fault is recog-
nized inside Z1B in the forward direction at both line ends. The distance protection is
set such that the overreaching zone Z1B reaches beyond the opposite station (ap-
proximately 120% of line length). On three terminal lines, Z1B must be set to reliably
reach beyond the longer line section, even if there is an additional infeed via the tee
point. The first zone is set in accordance with the usual grading scheme, i.e. approxi-
mately 85% of the line length; on three terminal lines at least beyond the tee point.
The transmit signal can be prolonged by T
S
(settable under address 2103 Send
Prolong.). The prolongation of the send signal only comes into effect if the protec-
tion has already issued a trip command. This ensures release of the opposite line end
even when the short circuit has been switched off rapidly by the independent zone Z1.
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