Siemens siprotec 7SD5 User Manual page 188

2 Functions
Figure 2-76
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.
Sequence Figure 2-77 shows the logic diagram of the unblocking 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
Forward in Address 1651 Op. mode Z1B, see also Subsection 2.5.1 at 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 com-
bined with a logical AND gate, as all three line ends must transmit a send signal during
an internal fault. With the setting parameter Type of Line (address 2102) 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 in essence corresponds to
that of the permissive overreach transfer scheme, see Figure 2-78. If an interference
free unblock signal is received, a receive signal, e.g. „>Dis.T.UB ub 1", appears
and the blocking signal, e.g. „>Dis.T.UB bl 1" disappears. The internal signal „Un-
block 1" is passed on to the receive logic, where it initiates the release of the over-
reaching zone Z1B of the distance protection (when all remaining conditions have
been fulfilled).
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, neither the unblocking signal e.g., „>Dis.T.UB ub 1", nor the blocking
signal „>Dis.T.UB bl 1" will appear on the receiving side. 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
100/100ms. When the transmission is functional again, one of the two receive signals
must appear again, either „>Dis.T.UB ub 1"or „>Dis.T.UB bl 1"; after a further
188
Function diagram of the directional unblocking method
7SD5 Manual
C53000-G1176-C169-1