Siemens siprotec 7SA6 User Manual page 307

7SA6 Manual
C53000-G1176-C133-1
&RQILJ (address ), the device is informed as to whether it has one or two
opposite line ends.
If the unblock frequency f
terminal lines both receive signals combined by AND — it is used to release tripping.
If the transmitted signal does not reach the other line end because the short circuit on
the protected feeder causes too much attenuation or reflection of the transmitted
signal, the unblock logic takes effect: neither the unblock signal ">() 8% XE " nor
the monitoring signal ">() 8% EO " are received. In this event, the release state in
the logic is set after a security margin of 20 ms. With the timer stage 100/100 ms this
release is however removed after a further 100 ms. If the interference signal
disappears again the quiescent state is reached again after a further 100 ms (reset
delay of the timer 100/100 ms). On three terminal lines, the unblock logic can be
controlled via both receive channels.
The occurrence of erroneous signals resulting from transients during clearance of
external faults or from direction reversal resulting during the clearance of faults on
parallel lines, is neutralized by the "Transient Blocking" (refer to Sub-section 6.6.1.4).
On lines where there is only a single sided infeed or where the system 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. To ensure
tripping by the directional comparison even in this case the device has special
features. This "Weak Infeed Function" is referred to in Sub-section 6.6.1.5. 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 6.7.
is received without interference — in the case of three
U
Functions
6-125