Siemens siprotec 7SA6, Manual

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Functions
6-30
7SA6 Manual
C53000-G1176-C156-2
Negative Sequence
Current 3I
2
On long, heavily loaded lines, the earth current measurement could be overstabilized
by large currents (ref. Figure 6-15). To ensure secure detection of earth faults in this
case, a negative sequence comparison stage is additionally provided. In the event of
a single-phase fault, the negative sequence current I
2
has approximately the same
magnitude as the zero sequence current I
0
. When the ratio zero sequence current/
negative sequence current exceeds a preset ratio, this stage picks up. It is also stabi-
lized in the event of large negative sequence currents by a parabolic characteristic.
Figure 6-16 illustrates this relationship. A release by means of the negative sequence
current comparison requires a current of at least 0,2·I
N
for 3I
0
and 3I
2
.
Figure 6-16 Characteristic of the I
0
/I
2
–stage
Neutral
Displacement
Voltage 3U
0
For the neutral displacement voltage recognition the displacement voltage (
3U0>
) is
numerically filtered and the fundamental frequency is monitored to recognize whether
it exceeds the set threshold ((3·U
0
). The reset threshold is approximately 95 % of the
pick-up threshold. For earthed systems, the U0–criterion may be disabled by applying
the
∞
setting.
Logical
Combination for
Earthed Systems
The current and voltage criteria supplement each other, as the displacement voltage
increases when the zero sequence to positive sequence impedance ratio is large,
whereas the earth current increases when the zero sequence to positive sequence im-
pedance ratio is smaller.
For earthed systems the current and voltage criteria are logically combined with an
OR-
function (Figure 6-17). The earth fault recognition on its own does not cause a
general fault detection of the distance protection, but merely controls the further fault
detection modules. It is only alarmed in case of a general fault detection.
0.0
0
3I0>
0,5
1,0
1,5
2,0
2,5
3,0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
release
block
3I
2
I
N
3I
0
I
N