Siemens siprotec 7SA6, Manual

Page: 335 / 876

Functions
6-145
7SA6 Manual
C53000-G1176-C156-2
On three terminal lines, the send signal is routed to both opposite line ends. The re-
ceive signals are then combined with a logical AND gate, as all three line ends must
transmit a send signal during an internal fault. Via the setting parameter
Line Con-
fig.
(address
3202
), the device is informed as to whether it has one or two opposite
line ends.
If the unblock frequency f
U
is received without interference — in the case of three ter-
minal 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 sig-
nal, the unblock logic takes effect: neither the unblock signal “
>
EF UB ub 1
” nor the
monitoring signal “
>
EF UB bl 1
” 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 re-
lease 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 ex-
ternal faults or from direction reversal resulting during the clearance of faults on par-
allel lines, is neutralized by the “Transient Blocking” (refer to Sub-section 6.8.1.4).
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 gen-
erate a permissive signal, as fault detection does not take place there. To ensure trip-
ping by the directional comparison even in this case the device has special features.
This “Weak Infeed Function” is referred to in Sub-section 6.8.1.5. 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 6.9.