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Functions
Trajectory
Continuity and
Monotony
Trajectory Stability
6-74
A power swing is detected, if during the last eight measuring cycles (corresponding to
two periods), the continuity of the changing impedance vector is confirmed. In this
way, slip frequencies of up to at least 7 Hz are detected.
X
Fault
impedance
Fault entry
dR
dX
(k)
Figure 6-41
Impedance vector during power swing
The rate of change of the impedance vector is very important for the differentiation
between faults and power swing conditions. In Figure 6-41 this is shown. During the
power swing the measured impedance from one sample to the next has a defined
change in R and X, referred to as dR(k) and dX(k). Important is also the fact that from
one sample to the next the difference is small: i.e. |dR(k) - dR(k+1)|< threshold.
During a fault entry there is a rapid change that will not cause the power swing function
to pick up.
When the impedance vector enters the impedance characteristic during a
power swing this is on a point of the elliptical curve that corresponds to steady
state instability. For release of the power swing detection a further criterion is
therefor used. In Figure 6-42 the range for steady state instability is shown.
This range is detected in the distance relay by calculating the center of the
ellipse and checking if the actual measured X value is less than this value.
X
0°
-90°
X
0
-180°
+180°
Figure 6-42
Steady state instability range
Power swing
dR
(k-n)
dX
(k-n)
(k)
+90°
Steady state
instability range
Load
impedance
R
R
C53000-G1176-C156-2
7SA6 Manual
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