2.8 Earth Fault Protection in Earthed Systems (optional)
223
7SD5 Manual
C53000-G1176-C169-1
Figure 2-97 Curve parameters in the logarithmic–inverse characteristic
Zero Sequence
Voltage Stage with
Inverse Character-
istic
If you have configured the zero sequence voltage controlled stage (address 131
Earth Fault O/C = U0 inverse), the operating mode is initially set: address 3140
Op. mode 3I0p. The stage can be set to operate Forward (usually towards line),
Reverse (usually towards busbar) or Non-Directional (in both directions). If the
stage is not required, set its mode to Inactive.
Address 3141 3I0p PICKUP indicates the minimum current value above which this
stage is required to operate. The value must be exceeded by the minimum earth fault
current value.
The voltage-controlled characteristic is based on the following formula:
U
0
is the actual zero sequence voltage. U
0 min
is the setting value U0inv. minimum
(address 3183). Please take into consideration that the formula is based on the zero
sequence voltage U
0
, not on 3U
0
. The function is illustrated in the Technical Data.
Figure 2-98 shows the most important parameters. U0inv. minimum displaces the
voltage-controlled characteristic in direction of 3U
0
. The set value is the asymptote for
this characteristic (t → ∞). In Figure 2-98, a' shows an asymptote that belongs to the
characteristic a.
The minimum voltage 3U0>(U0 inv) (address 3182) is the lower voltage threshold.
It corresponds to the line c in Figure 2-98. In characteristic b (asymptote not drawn)
the curve is cut by the minimum voltage 3U0>(U0 inv) (line c).
In address 3184, an additional time T forw. (U0inv) that is added to the voltage-
controlled characteristic can be set for directional-controlled tripping.
With the non-directional time T rev. (U0inv) (address 3185) a non-directional
back-up stage can be generated.
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