Siemens siprotec 7SA6 User Manual page 269

Sequence
7SA6 Manual
C53000-G1176-C133-1
For all zones except Z1B, tripping results without release from the opposite line end,
allowing the protection to function with the usual grading characteristic independent of
the signal transmission.
Figure 6-52 shows the logic diagram of the unblock scheme for one line end.
The unblock scheme only functions for faults in the forward direction. Accordingly, the
overreaching zone Z1B of the distance protection must definitely be set to )RUZDUG:
address  2S PRGH =%, refer also to Subsubsection 6.2.4.2 under margin
heading "Controlled Zone Z1B".
On two terminal lines, the signal transmission may be phase segregated. Send and
receive circuits in this case are built up for each phase. On three terminal lines the
send signal is transmitted to both opposite ends. The receive signals are then
combined with the logical AND function, as all three line ends must transmit during an
internal fault. Via the setting /LQH &RQILJ (address ) the device is informed
as to whether it has one or two opposite line ends.
An unblock logic is inserted before the receive logic, which latter in essence
corresponds to that of the permissive overreach transfer scheme. The unblock logic is
shown in Figure 6-53. If an interference free unblock signal is received, a receive
signal e.g. "!'LV78% XE ", appears and the blocking signal e.g. "!'LV78% EO
" disappears. The internal signal "Unblock 1" is passed on to the receive logic, where
it initiates the release of the overreaching zone Z1B of the distance protection (when
all remaining conditions have been fulfilled).
If the transmitted signal is not received at the other line end because the short-circuit
on the line causes too severe an attenuation or reflection of the signal, neither the
unblock signal e.g. "!'LV78% XE ", nor the block signal "!'LV78% EO " is
received at the receiving end. In this case, the release "">Unblock 1" is issued after a
security delay time of 20 ms and passed onto the receive logic. This release is
however removed after a further 100 ms via the timer stage 100/100 ms. When the
transmission is functional again, one of the two receive signals must appear again,
either "!'LV78% XE " or "!'LV78% EO "; after a further 100 ms (drop-off
delay of the timer stage 100/100 ms) the quiescent state is reached again i.e. the
direct release path to the signal "Unblock L1" and thereby the usual release is
possible.
If none of the signals is received for a period of more than 10 s the alarm (address
2107) "'LV7&DUU)DLO" (F.No. 4055) is generated.
In the case of faults in the transmission path, the overreaching zone Z1B can be
activated by an automatic reclose device, via the binary input "!(QDEOH $5]RQHV"
and address  "VW $5 ! =%" set to "Yes" (refer to Figure 6-38 bottom).
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 6.4.1.10).
On feeders with single-sided infeed, the line end with no infeed cannot generate a
release signal, as no fault detection occurs there. To achieve tripping by the
permissive overreach transfer scheme even in this case, the device contains a special
function. This "Weak Infeed Function" (echo function) is referred to in Subsubsection
6.4.1.11. It is activated when a signal is received from the opposite line end — in the
case of three terminal lines from at least one of the opposite line ends — without the
device having detected a fault.
The circuit breaker can also be tripped at the line end that has only weak or no infeed.
This "Weak-Infeed Tripping" is referred to in Section 6.7.
Functions
6-87