Siemens siprotec 7SA522 - Page 145

2.6 Teleprotection for distance protection
145
7SA522 Manual
C53000-G1176-C155-3
In case of single- or two-pole pickup of the distance protection, it is nevertheless pos-
sible to send an echo if measurement of the phases that have not picked up recogniz-
es a weak-infeed condition.
To avoid an incorrect echo following switching off of the line and reset of the fault de-
tection, the RS flip-flop in Figure 2-61 latches the fault detection condition until the
signal receive condition resets, thereby barring the release of an echo. The echo can
in any event be blocked via the binary input ´!'LV7%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$/µ must
be allocated separately to the output relay(s) for transmission, as it is not contained in
the transmit signals ´'LV76(1'µ or “Dis.T.SEND L*”.
Note
The ´(&+26,*1$/µ (FNo 4246) must be allocated separately to the output relays
for the transmitter actuation, as it is not contained in the transmit signals of the trans-
mission functions. On the digital protection data interface with permissive overreach
transfer trip mode, the echo is transmitted as a separate signal without taking any
special measures (Figure 2-53).
After output of the echo pulse or during the transmission signal of the distance protec-
tion, 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.
In the case of the blocking scheme and the underreach transfer trip scheme, the echo
function is not required and therefore ineffective.