Functions
6-36 7SA522 Manual
C53000-G1176-C119-2
close-in short circuits may cause unfaulted loops to “see” the fault further away than
the faulted loop, but still within the tripping zone. This would cause three-pole tripping
and therefore void the possibility of single-pole automatic reclosure. As a result power
transfer via the line would be lost.
In the 7SA522 this is avoided by the implementation of a loop verification function
which operates in two steps:
Initially, the calculated loop impedances and its components (phase and/or earth) are
used to simulate a replica of the line impedance. If this simulation returns a plausible
line image, the corresponding loop pick-up is designated as a definitely valid loop.
If the impedances of more than one loop are now located within the range of the zone,
the smallest is still declared to be a valid loop. Furthermore, all loops that have an im-
pedance which does not exceed the smallest loop impedance by more than 50 % are
declared as being valid. Loops with larger impedance are eliminated. Those loops
which were declared as being valid in the initial stage, cannot be eliminated by this
stage, even if they have larger impedances.
In this manner unfaulted “apparent impedances” are eliminated on the one hand, while
on the other hand, unsymmetrical multi-phase faults and multiple short circuits are rec-
ognized correctly.
The loops that were designated as being valid are converted to phase information so
that the fault detection correctly alarms the faulted phases.
Double Faults in
Effectively Earthed
Systems
In systems with an effectively earthed star-point, each connection of a phase with
earth results in a short-circuit condition which must be isolated immediately by the
closest protection systems. Fault detection occurs in the faulted loop associated with
the faulted phase.
With double earth faults, fault detection is generally in two phase-earth loops. If both
earth loops are in the same direction, a phase-phase loop may also pick-up. It is pos-
sible to restrict the fault detection to particular loops in this case. It is often desirable
to block the phase-earth loop of the leading phase, as this loop tends to overreach
when there is infeed from both ends to a fault with a common earth fault resistance
(Parameter 1221 3K(IDXOWV = %ORFN OHDGLQJ). Alternatively, it is also pos-
sible to block the lagging phase-earth loop (Parameter 3K(IDXOWV = %ORFN
ODJJLQJ). All the affected loops can also be evaluated (Parameter 3K(IDXOWV
= $OOORRSV), or only the phase-phase loop (Parameter 1221 3K(IDXOWV =
ORRSVRQO\) or only the phase-earth loops (Parameter 3K(IDXOWV = (
ORRSVRQO\).
A prerequisite for these restrictions is that the relevant loops indicate fault locations
which are close together and within the reach of the first zone Z1. The loops are con-
sidered to be close together when they have the same direction and do not differ by
more than a factor 1,5 (largest to smallest impedance). This prevents the elimination,
during multiple faults with separate fault location, of the loop relating to the closer fault
location by the set restriction.
In Table 6-2 the measured values used for the distance measurement in earthed sys-
tems during double earth faults are shown.
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