Siemens SIPROTEC 7SA522 Manual page 183

7SA522 Manual
C53000-G1176-C155-3
2.8 Teleprotection for earth fault overcurrent protection (optional)
possible. On three terminal lines, the unblock logic can be controlled via both receive
channels.
If none of the signals is received for a period of more than 10 s the alarm ´() 7HOH8%
)DLOµ is generated.
The occurrence of erroneous signals resulting from transients during clearance of ex-
ternal faults or from direction reversal resulting during the clearance of faults on par-
allel lines, is neutralized by the "Transient Blocking".
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. To also ensure
tripping by the directional comparison in this case the device has special features. This
"Weak-infeed tripping" (echo function) is referred to in Subsection "Measures for Weak
and Zero Infeed". It is activated when a signal is received from the opposite line end
— on three terminal lines, from at least one of the opposite ends — without the device
recognizing an earth fault.
The circuit breaker can also be tripped at the line end with no or only weak infeed. This
"Weak-infeed tripping" is referred to in Section 2.9.1.
183