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Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
147
Figure 2-52 Polygonal characteristic (setting values are marked by dots)
For the first zone Z1, an additional settable tilt
α
exists, which may be used to prevent overreach resulting from
angle variance and/or two ended infeed to short-circuits with fault resistance. For Z1B and the higher zones,
this tilt does not exist.
Determination of direction
For each loop an impedance vector is also used to determine the direction of the short-circuit. Usually similar
to the distance calculation, Z
L
is used. However, depending on the „quality“ of the measured values, different
computation techniques are used. Immediately after fault inception, the short-circuit voltage is disturbed by
transients. The voltage memorised prior to fault inception is therefore used in this situation. If even the steady-
state short-circuit voltage (during a close-up fault) is too small for direction determination, an unfaulted voltage
is used. This voltage is in theory perpendicular to the actual short-circuit voltage for both phase-to-earth loops
as well as for phase-to-phase loops (Figure 2-53). This is taken into account when computing the direction
vector by means of a 90° rotation. Table 2-11 shows the allocation of the measured values to the six fault loops
for the determination of the fault direction.