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Unfaulted Loops
The above considerations apply to the relevant short-circuited loop. A pickup with the current-based fault de-
tection modes (I>, U/I, U/I/ϕ) guarantees that only the faulty loop(s) is/are released for the distance calculation.
All six loops are calculated for the impedance pickup; the impedances of the unfaulted loops are also influenced
by the short-circuit currents and voltages in the short-circuited phases. During a 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 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 selectivity of the zones, they are usually of no consequence.
Apart from the zone selectivity, the phase selectivity is also important to achieve a correct identification of
the faulted phases, to alarm the faulted phases and especially to enable single-pole automatic reclosure. De-
pending on the infeed conditions, 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 7SD5 this is avoided by the implementation of a „loop verification" function which operates in two steps:
Initially, the calculated loop impedance and its components (phase 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 desig-
nated 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 de-
clared to be a valid loop. Furthermore, all loops with an impedance that does not exceed the smallest loop im-
pedance by more than 50 % are declared as being valid. Loops with larger impedance are eliminated. Those
loops which were declared 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, un-
symmetrical multi-phase faults and multiple short-circuits are recognized 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 Earthed Systems
In systems with an effectively or low-resistant earthed starpoint, each connection of a phase with earth results
in a short-circuit condition which must be isolated immediately by the closest protection systems. Fault detec-
tion occurs in the faulted loop associated with the faulted phase.
With double earth faults, fault detection is generally in two phase-to-earth loops. If both earth loops are in the
same direction, a phase-to-phase loop may also pick up. It is possible to restrict the fault detection to particular
loops in this case. It is often desirable to block the phase-to-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
1521 2Ph-E faults = Block leading Ø). Alternatively, it is also possible to block the lagging phase-to-
earth loop (Parameter 2Ph-E faults = Block lagging Ø). All the affected loops can also be evaluated
(Parameter 2Ph-E faults = All loops), or only the phase-to-phase loop (Parameter 2Ph-E faults =
Ø-Ø loops only) or only the phase-to-earth loops (Parameter 2Ph-E faults = Ø-E loops only). All
these restrictions presuppose that the affected loops have the same direction.
In Table 2-9 the measured values used for the distance measurement in earthed systems during double earth
faults are shown.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
Functions
2.5 Distance Protection
127
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