Siemens SIPROTEC 7SA522 Manual page 110

2 Functions
Trajectory
Continuity and
Monotony
Trajectory Stability
110
Figure 2-37 Pickup characteristic for the power swing detection for the MHO circle
Figure 2-38 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. This is shown in Figure 2-38. 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 detec-
tion function to pick up.
When the impedance vector enters the impedance characteristic during a power
swing this is at a point of the elliptical curve that corresponds to a steady state insta-
7SA522 Manual
C53000-G1176-C155-3