Functioning
The stage picks up if the negative-sequence system current exceeds the set threshold value and the parame-
terized direction agrees with the measured direction. The pickup drops out if the negative-sequence system
current falls below 95 % of the set threshold.
Stabilization with Phase Current
Unbalance in operation and unbalanced transformer ratios can lead to spurious pickups and incorrect tripping.
In order to avoid this, the directional negative-sequence system stage is stabilized with the phase currents.
The threshold value increases with rising phase currents (see next image).
You can change the stabilization factor (= gradient) via the Stabiliz. w. phase current parameter.
[dwstabil-300112-01.tif, 1, en_US]
Figure 6-159 Stabilization with Phase Currents
Direction Determination
The direction determination takes place with the negative-sequence system measurements I2 and V2.
The forward and reverse region is defined through the parameters Angle forward α and Angle forward
β (see next figure). The reference for the 2 angles that must be set is the positive real axis. The angles are
positively defined in a mathematical sense (counter-clockwise). The region between the limit angle α and the
limit angle β - counted from the former in a positive direction - is the forward region. The remaining region is
the reverse region.
For determining of the direction, the function places the measuring current I2 on the real axis. If the phasor of
the negative-sequence system voltage V2 is located within the defined forward region, the function deter-
mines the direction as
forward
. In the other case, the function determines the direction as
reverse
.
The requirement for determining the direction is that the adjustable minimum variables have been exceeded
for the negative-sequence system current and negative-sequence system voltage (parameters Min. neg.-
seq. current I2 and Min. neg.-seq. voltage V2).
Protection and Automation Functions
6.18 Directional Negative-Sequence Protection
SIPROTEC 5, Overcurrent Protection, Manual 645
C53000-G5040-C017-8, Edition 07.2017
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