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2 Functions
Figure 2-121
Selectivity before
In order for the automatic reclosure to be successful, all faults on the entire overhead
Reclosure
line must be cleared at all line ends simultaneously — as fast as possible.
This is the usual case in differential protection schemes because the strict selective
zone definition of the protected object by the current transformer sets always allows
non-delayed tripping.
In the distance protection, for example, the overreach zone Z1B may be released
before the first reclosure. This implies that faults up to the zone reach limit of Z1B are
tripped without delay for the first cycle (Figure 2-122). A limited unselectivity in favour
of fast simultaneous tripping is accepted here because a reclosure will be performed
in any case. The normal stages of the distance protection (Z1, Z2, etc.) and the normal
grading of the other short-circuit functions are independent of the automatic reclosure
function.
Figure 2-122
If the distance protection is operated with one of the signal transmission methods de-
scribed in Section 2.7, the signal transmission logic controls the overreaching zone,
i.e. it determines whether a non-delayed trip (or delayed with T1B) is permitted in the
event of faults in the overreaching zone (i.e. up to the reach limit of zone Z1B) at both
line ends simultaneously. Whether the automatic reclosure device is ready for reclo-
sure or not is irrelevant, because the teleprotection function ensures the selectivity
over 100% of the line length and fast, simultaneous tripping. The same applies for the
earth fault-direction comparison protection (Section 2.9).
286
Activation and deactivation of the auto-reclosure function
Reach control before first reclosure, using distance protection
7SD5 Manual
C53000-G1176-C169-1
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