Functions
2.7 Teleprotection for Distance Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
194
Figure 2-87
Function diagram of the direct underreach transfer trip scheme
2.7.6
Permissive Overreach Transfer Trip (POTT)
Principle
The permissive overreach transfer mode uses a permissive release principle. The overreaching zone Z1B set
beyond the opposite station is decisive. This mode can also be used on extremely short lines where a setting
of 85% of line length for zone Z1 is not possible and accordingly selective non-delayed tripping could not be
achieved. In this case, however, zone Z1 must be delayed by T1, to avoid non-selective tripping by zone Z1
(Figure 2-88).
If the distance protection recognizes a fault inside the overreaching zone Z1B, it initially sends a release signal
to the opposite line end. If a release signal is also received from the opposite end, the trip signal is forwarded
to the command relay. A prerequisite for fast tripping is therefore that the fault is recognised inside Z1B in
forward direction at both line ends. The distance protection is set in such a way that overreaching zone Z1B
reaches beyond the next station (approximately 120% of the 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. approximately 85% of the line length; on three
terminal lines at least beyond the tee point.
The transmit signal can be prolonged by T
S
(settable under address
2103
Send Prolong.
). The prolongation
of the send signal only comes into effect if the protection 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.
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.
