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Determination of power swing locus (contd..) |
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There is a geometrical interpretation of above equation. The vector component  in equation (15) is a constant in R – X plane. The component  lies on a straight line, perpendicular to line segment  . Thus, the trajectory of the impedance measured by relay during the power swing is a straight line as shown in fig 24.3. The angle subtended by a point in the locus on S and R end points is angle  . For simplicity, angle of
 ,
 and
 are considered identical. The swing intersects the line AB, when  . |
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The corresponding point of intersection of swing impedance trajectory on the impedance line is known as electrical center of the swing. (fig 24.4(a)). The angle, between two sources can be mapped graphically as the angle subtended by source points ES and ER on the swing trajectory. At the electrical center, angle between two sources is  . The existence of the electrical center is an indication of system instability, the two generators now being out of step.
If the power swing is stable, i.e. if the post fault system is stable, then  will be less than
 . In such an event, the power swing retraces its path at .
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