2.2 Distance protection
67
7SA522 Manual
C53000-G1176-C155-3
Figure 2-14 Logic of the phase-earth measuring system
Unfaulted Loops The above considerations apply to the relevant short-circuited loop. All six loops are
however equated in case of impedance pickup; the impedances of the unfaulted loops
are also influenced by the short-circuit currents and voltages in the short-circuited
phases. During an L1-E fault for example, the short-circuit current in phase L1 also
appears in the measuring loops L1-L2 and L3-L1, the earth current is also measured
in the loops L2-E and L3-E. Combined with load currents which may flow, the unfaulted
loops produce the so-called “apparent impedances”, which have nothing to do with the
actual fault distance.
These “apparent impedances” in the unfaulted loops are usually larger than the short-
circuit impedance of the faulted loop because the unfaulted loop only carries a part of
the fault current and always has a larger voltage than the faulted loop. For the selec-
tivity of the zones, they are usually of no consequence.
Apart from the zone selectivity, the phase selectivity is also important to achieve
correct identification of the faulted phases, required to alarm the faulted phase and es-
pecially to enable single-pole automatic reclosure. Depending on the infeed condi-
tions, 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 trip-
ping 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 pickup 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.
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