Siemens siprotec 7SA6, Manual

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Functions
6-149
7SA6 Manual
C53000-G1176-C156-2
Figure 6-85 Transient blocking for a
directional comparison and directional unblocking
schemes
6.8.1.5 Measures for Weak or Zero Infeed
On lines where there is only a single sided infeed or where the system starpoint is only
earthed behind one line end, the line end without zero sequence current cannot gen-
erate a permissive signal, as fault detection does not take place there. With the com-
parison schemes, using a permissive signal, fast tripping could not even be achieved
at the line end with strong infeed without special measures, as the end with weak in-
feed does not transmit a permissive release signal.
To achieve rapid tripping at both line ends under these conditions, the device has a
special supplement for lines with weak zero sequence current infeed.
To enable even the line end with the weak infeed to trip, 7SA6 provides a weak infeed
tripping supplement. As this is a separate protection function with a dedicated trip
command, it is described in a separate section (6.9).
Echo Function Figure 6-86 shows the method of operation of the echo function. It may be switched in
address
2501 FCT Weak Infeed
(Weak Infeed MODE) to be activated (
ECHO
only
) or to be deactivated (
OFF
). By means of this “switch” the weak infeed tripping
function can also be activated (
ECHO and TRIP
, refer also to Section 6.9). This set-
ting is common to the teleprotection function for the distance protection and for the
earth fault protection.
The received signal at the line end that has no earth current is returned to the other
line end as an “echo” by the echo function. The received echo signal at the other line
end enables the release of the trip command.
The detection of the weak infeed condition and accordingly the requirement for an
echo are combined in the central AND gate (Figure 6-86). The earth fault protection
must neither be switched off nor blocked, as it would otherwise always produce an
echo due to the missing fault detection.
The essential condition for an echo is the absence of an earth current (current stage
3IoMin Teleprot
) with the simultaneous reception signal from the teleprotection
scheme logic, as shown in the corresponding logic diagrams (Figure 6-79 or 6-82).
To prevent the generation of an echo signal after the line has been tripped and the
earth current stage
3IoMin Teleprot
has reset, it is not possible to generate an
echo if a fault detection by the earth current stage had already been present (RS flip-
flop in Figure 6-86). In any event, the echo may be blocked at any time via the binary
input „
>EF BlkEcho
“.
EF forward
&
3209 TrBlk Wait Time
3210 TrBlk BlockTime
EF Telep. off
>EF TeleprotBLK
≥
1
&
transient
blocking
Figure 6-79
or 6-82
T
T
EF TeleTransBlk
FNr 1313
FNr 1386
EF Pickup
I
E
3I
0
3105
3IoMin Teleprot
≥
1