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Trajectory
Symmetry
Power Swing
Detection
7SA6 Manual
C53000-G1176-C156-2
In addition to these measures, a comparison of the three phases is done to
ensure that they are symmetrical. During a power swing condition in the single
pole open condition only 2 of the three phases will have an impedance
trajectory. In this case only these 2 remaining phase trajectories are checked
to ensure that they are symmetrical.
To ensure stable and secure operation of the power swing detection without
risking unwanted power swing blocking during power system faults, a logical
combination of a number of measuring criteria are used
No trip output
present
Impedance in PPOL
Calculation of
R, X
the R und X
3
values
Figure 6-43
Logic diagram of power swing
In Figure 6-43 a simplified logic diagram for the power swing function is given. This
measurement is done on a per phase basis although Figure 6-43 only shows the logic
for one phase. Before a power swing detected signal is generated, the measured
impedance must be inside the power swing polygon (PPOL). A further 4 measuring
criteria must be fulfilled.
G
Trajectory continuity
The measured R and X values must describe a steady path without a jump from one
measured value to the next. Refer to Figure 6-41.
G
Trajectory monotony
The impedance trajectory must initially not change R-direction. Refer to
Figure 6-41.
G
Trajectory symmetry
The trajectory of each phase is evaluated. If no fault is present these 3 trajectories
must be symmetrical. During single pole open conditions the remaining 2
trajectories must be symmetrical.
Trajectory continuity
No jump of R-values
and X-values
Trajectory monotony
No change in R-direction
Trajectory symmetry
Check symmetry of
trajectories that may
be swinging
Trajectory stability
Calculate centre of
trajectory
Trajectory check OST
Check sign of R when fault
enters and exits zone
.
Power swing
&
detected
&
S Q
&
R
Change
of sign
Out of step
protection trip
Functions
6-75
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