Siemens siprotec 7SA6 User Manual page 278

Functions
6-96
If there is no fault detection, the echo function causes the received signal to be sent
back to the other line end as an "echo", where it is used to initiate permissive tripping.
The detection of the weak infeed and accordingly the requirement for an echo are
combined in the central AND gate (Figure 6-59). The distance protection must neither
be switched off nor blocked, as it would otherwise always produce an echo due to the
missing fault detection. If however the time delayed overcurrent protection is used as
an emergency function, an echo is nevertheless possible if the distance protection is
out of service, because the fault detection of the emergency overcurrent protection
replaces the distance protection fault detection. During this mode of operation, the
emergency overcurrent protection must naturally not also be blocked or switched off.
The essential condition for an echo is the absence of distance protection or
overcurrent protection fault detection with the simultaneous reception of a signal from
the teleprotection scheme logic, as shown in the corresponding logic diagrams (Figure
6-48 or 6-52).
To avoid an incorrect echo following switching off of the line and reset of the fault
detection, the RS flip-flop in Figure 6-59 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 „!'LV7%ON(FKR".
If the conditions for an echo signal are met, a short delay 7ULS(FKR '(/$< is
initially activated. This delay is necessary to avoid transmission of the echo if the
protection at the weak line end has a longer fault detection time during reverse faults
or if it picks up a little later due to unfavourable fault current distribution. If however the
circuit breaker at the non-feeding line end is open, this delay of the echo signal is not
required. The echo delay time may then be bypassed. The circuit breaker switching
state is provided by the central information control functions. (refer to Subsection
6.20.2).
The echo impulse is then issued (event output "(&+2 6,*1$/"). It's length is set with
the parameter 7ULS (;7(16,21.
Note:
The "(&+2 6,*1$/" (F.No. 4246) must be separately assigned to the output relay(s)
for signal transmission, as it is not contained in the transmit signals "'LV76(1'" or
"'LV76(1' / ".
After issue of the echo impulse, the transmission of a new echo is prevented for at
least 20 ms. This prevents the repetition of an echo 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.
7SA6 Manual
C53000-G1176-C133-1