Siemens SIPROTEC 7SA522 Manual page 137

Sequence
7SA522 Manual
C53000-G1176-C155-3
If the release frequency is received from the opposite end, a trip signal is forwarded to
the trip logic. Accordingly, it is a prerequisite for fast tripping, that the fault is recog-
nized inside Z1B in the forward direction at both line ends. The distance protection is
set such that the overreaching zone Z1B reaches beyond the opposite station (ap-
proximately 120% of line length). On three terminal lines, Z1B must be set to reliably
reach beyond the longer line section, even if there is an additional infeed via the tee
point. The first zone is set in accordance with the usual grading scheme, i.e. approxi-
mately 85% of the line length; on three terminal lines at least beyond the tee point.
The transmit signal can be prolonged by T
3URORQJ). The prolongation of the send signal only comes into effect if the protec-
tion has already issued a trip command. This ensures release of the opposite line end
even when the short circuit has been switched off rapidly by the independent zone Z1.
Figure 2-55 Function diagram of the directional unblocking method
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 2-56 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
)RUZDUG in Address  2S PRGH =%, see also Subsection 2.2.1 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 7\SH RI /LQH (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 in essence corresponds to
that of the permissive overreach transfer scheme, see Figure 2-57. If an interference
free unblock signal is received, a receive signal e.g. ´!'LV78% XE µ, appears
2.6 Teleprotection for distance protection
(settable under address  6HQG
S
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