Functions
2.9 Teleprotection for Earth Fault Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
260
2.9.6
Measures for Weak or Zero Infeed
On lines where there is only a single-sided infeed or where the starpoint is only earthed behind one line end,
the line end without zero sequence current cannot generate a permissive signal, as fault detection does not
take place there. With the comparison 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 infeed 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 infeed.
To enable even the line end with the weak infeed to trip, 7SD5 provides a weak infeed tripping supplement. As
this is a separate protection function with a dedicated trip command, it is described separately in Section 2.11.2.
Echo Function
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 common echo signal (see Figure , Section 2.11.1) is triggered by both the earth fault protection and the
distance protection. Figure 2-127 shows the generation of the echo release by the earth fault protection.
The detection of the weak infeed condition and accordingly the requirement for an echo are combined in a
central AND gate. 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
simultaneous receive signal from the teleprotection scheme logic, as shown in the corresponding logic dia-
grams (Figure 2-120, or 2-122).
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 2-127). The echo can in any event be blocked via the binary input
„>EF BlkEcho“
.
Figure 2-127 shows the generation of the echo release signal. Since this function is also associated with the
weak infeed tripping, it is described separately (see Section 2.11.1).
Figure 2-127
Generation of the echo release signal
