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2 Functions

182

7SD5 Manual

C53000-G1176-C169-1

2.7.6 Permissive Overreach Transfer Trip (POTT)

The following procedure is suited for both conventional and digital transmission media.

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-selec-

tive tripping by zone Z1 (Figure 2-72).

If the distance protection recognizes a fault inside the overreaching zone Z1B, it ini-

tially sends a release signal to the opposite line end. If a release signal is received

from the opposite end, a trip signal is forwarded to the trip logic. A prerequisite for fast

tripping is therefore that the fault is recognized 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 (approximately 120% of line length). On three ter-

minal 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

(settable under address 2103 Send

Prolong.). 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.

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 2-72 Operation scheme of the permissive overreach transfer trip method

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