Functions
6-30 7SA522 Manual
C53000-G1176-C119-2
6.2 Distance Protection
Distance protection is the main function of the device. It distinguishes itself by high
measuring accuracy and the ability to adapt to the given system conditions. It is sup-
plemented by a number of additional functions.
6.2.1 Earth Fault Recognition
6.2.1.1 Method of Operation
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.
Earth Current 3I
0
For earth current measurement, the fundamental sum of the numerically filtered phase
currents 3·I
0
is monitored to detect if it exceeds the set value (parameter ,!
7KUHVKROG, address ). It is stabilized against over-operation resulting from un-
symmetrical operating currents and error currents in the secondary circuits of the cur-
rent 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 6-15). The reset value is approximately 95 % relative
to the pick-up value.
Figure 6-15 Earth current stage: pick-up characteristic
10 x 3I0
I
N
3I0
I
N
I
Ph max
slope
0,1
0,95·
,
3I0>
block
release
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