Measures For Weak Or Zero Infeed - Siemens SIPROTEC 7SA522 Manual

2.8.6

Measures for Weak or Zero Infeed

Echo Function
7SA522 Manual
C53000-G1176-C155-3
2.8 Teleprotection for earth fault overcurrent protection (optional)
On lines where there is only a single sided infeed or where the star-point 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
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, 7SA522 provides a weak
infeed tripping supplement. Since this is a separate protection function with a dedicat-
ed trip command, it is described separately in Section 2.9.1.
Figure 2-85 shows the method of operation of the echo function. At address 2501 )&7
:HDN ,QIHHG (Weak Infeed FunCTion) can be activated ((&+2 RQO\) or deactivat-
ed (2))). By means of this "switch" the weak infeed tripping function can also be ac-
tivated ((&+2 DQG 75,3, refer also to Section 2.9.1). This setting 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 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
,R0LQ 7HOHSURW) with simultaneous receive signal from the teleprotection scheme
logic, as shown in the corresponding logic diagrams (Figure 2-76 or 2-80).
To prevent the generation of an echo signal after the line has been tripped and the
earth current stage ,R0LQ 7HOHSURW reset, it is no longer possible to generate an
echo if a fault detection by the earth current stage had already been present (RS flip-
flop in Figure 2-85). The echo can in any event be blocked via the binary input ´!()
%ON(FKRµ.
If the conditions for an echo signal are met, a short delay 7ULS(FKR '(/$< is ini-
tially activated. This delay is necessary to avoid transmission of the echo if the protec-
tion at the weak line end has a longer fault detection time during reverse faults or if it
picks up a little later due to unfavourable short-circuit current distribution. If however
the circuit breaker at the non-feeding line end is open, this delay of the echo signal is
not required. The echo delay time may then be bypassed. The circuit breaker position
is provided by the central information control functions. (refer to Section 2.20.1).
The echo impulse is then transmitted (alarm output ´(&+2 6,*1$/µ), the duration of
which can be set with the parameter 7ULS (;7(16,21. The ´(&+2 6,*1$/µ must
be allocated separately to the output relay for transmission, as it is not contained in the
transmission signal ´() 7HOH 6(1'µ.
After output of the echo pulse or during the transmission signal of the earth fault pro-
tection, a new echo can not be sent for at least 50 ms (default setting). This prevents
echo repetition after the line has been switched off.
The echo function is not required for the blocking scheme, and is therefore ineffective.
189