2 Functions
62
7SA522 Manual
C53000-G1176-C155-3
2.2 Distance protection
Distance protection is the main function of the device. It is characterized by high mea-
suring accuracy and the ability to adapt to the given system conditions. It is supple-
mented by a number of additional functions.
2.2.1 Distance protection, general settings
2.2.1.1 Earth Fault Detection
Functional Descrip-
tion
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 7SA522 has a stabilised earth current measure-
ment, 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 of the summated numerically filtered
phase currents is monitored to detect if it exceeds the set value (parameter ,!
7KUHVKROG). It is stabilized against over-operation resulting from unsymmetrical op-
erating 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 pickup threshold automatically increases as the phase current in-
creases (Figure 2-7). The dropout threshold is approximately 95% of the pickup
threshold.
Figure 2-7 Earth current stage: pickup characteristic
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