Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
115
2.5 Distance Protection
Distance protection is the second main function of the device. It can operate as a fully-fledged redundant
second protection function (Main2) in parallel to differential protection, or be configured as the only main pro-
tection function of the device (Main only). The distance protection distinguishes itself by high measuring accu-
racy and the ability to adapt to the given system conditions. It is supplemented by a number of additional func-
tions.
2.5.1 Distance Protection, General Settings
2.5.1.1 Earth Fault Detection
Functional Description
Recognition of an earth fault is an important element in identifying the type of fault, as the determination of the
valid loops for measurement of the fault distance and the shape of the distance zone characteristics substan-
tially depend on whether the fault at hand is an earth fault or not. The 7SD5 has a stabilized earth current mea-
surement, a zero sequence current/negative sequence current comparison as well as a displacement voltage
measurement.
Furthermore, special measures are taken to avoid a pickup for single earth faults in an isolated or resonant-
earthed system.
Earth Current 3I
0
For earth current measurement, the fundamental component of the sum of the numerically filtered phase cur-
rents is supervised to detect if it exceeds the set value (parameter 3I0> Threshold). It is stabilized against
spurious operation resulting from unsymmetrical operating currents and error currents in the secondary circuits
of the current transformer due to different degrees of current transformer saturation during short-circuits without
earth: the actual pick-up threshold automatically increases as the phase current increases (Figure 2-35). The
dropout threshold is approximately 95 % of the pickup threshold.
Figure 2-35 Earth current stage: pickup characteristic
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