2 Functions
110
7SD5 Manual
C53000-G1176-C169-1
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 protec-
tion, or be configured as the only main protection function of the device (Main only).
The distance protection distinguishes itself by high measuring accuracy and the ability
to adapt to the given system conditions. It is supplemented by a number of additional
functions.
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 substantially depend on whether the fault
at hand is an earth fault or not. The 7SD5 has a stabilized earth current measurement,
a zero sequence current/negative sequence current comparison as well as a displace-
ment voltage measurement.
Furthermore, special measures are taken to avoid a pickup for single earth faults in an
isolated or resonant-earthed system.
Earth Current 3
I
0
For earth current measurement, the fundamental sum of the numerically filtered phase
currents is supervised to detect if it exceeds the set value (parameter
3I0>
Threshold
). Restraint is provided against spurious operation resulting from unsym-
metrical operating currents and error currents in the secondary circuits of the current
transformer due to different degrees of current transformer saturation during short-cir-
cuits without earth: the actual pick-up threshold automatically increases as the phase
current increases (Figure 2-27). The dropout threshold is approximately 95 % of the
pickup threshold.
Figure 2-27
Earth current stage: pickup characteristic
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