Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
The power swing detector bases its operation on the analysis of the transfer speed of the impedance
point through the R-X diagram. In case of a fault, the transfer between the non-faulted condition and
the fault condition presents a very high transfer speed of the impedance point (since this involves an
electromagnetic phenomenon). The transfer of this same point in case of a power swing involves a
much lower speed (given that this is an electromechanical phenomenon), which depends on the initial
load, the difference between generation and demand, generator inertia, etc.
The principle of operation is based on time measurement that the impedance locus takes to travel
between the two quadrilateral zones, outer and middle. If this time is longer than a threshold (set by the
power swing detection time setting) there is no system fault but rather a power swing.
Once a power swing has been detected, if the Power swing Trip function has been enabled, it is
determined if the swing is stable or unstable. If the measured impedance reaches an inner quadrilateral
zone, the swing is considered unstable and allowed to generate a trip.
The power swing detector has three units of phase-to-phase impedance measurement per zone. When
the three poles of the breaker are closed, it is sufficient to verify one of these measuring units, for
example, AB, given the symmetry of the power swing phenomenon. (For single pole trip applications,
the opening of a pole disables the measuring units related to the open phase.)
The power swing detector is based on three quadrilateral impedance zones; outer, middle, and inner.
These are all formed by resistive blinders and reactance blinders. The ‘inner’ zone is only used for
power swing trip. In case this function is not used, the setting is ignored.
Figure 9-32. Power swing Unit
August 1, 2009
9-34
973.334.3100
