ABB RELION REX640 Technical Manual page 1051

1MRS759142 F
element. The apparent impedance moves from the pre-fault value to a fault value in
a very short time, a few milliseconds, during a fault condition. However, the rate of
change of the positive sequence impedance is much slower during a power swing
condition than during a fault. It should be noted that the impedance measurement
should not be used by itself to distinguish between a power swing condition from a
phase fault. The fundamental method for discriminating between faults and power
swings is to track the rate of change of the measured impedance.
The function measuring the rate of change of the positive sequence impedance
comprises an inner and an outer impedance characteristic with a timer. If the
positive sequence impedance remains between the inner and outer characteristics
for a predetermined time and enters the inner characteristic, the function declares a
power swing condition and asserts an output that can be used to block the distance
protection. Figure below gives an example of power swing detection.
Figure 587: Example of power swing detection
In the above figure, the power swing detection characteristics and three typical
positive sequence impedance trajectories are plotted. For trajectory A, the
impedance moves into the power swing detection zone and leaves slowly, indicating
the occurrence of a swing that quickly stabilizes. For trajectory C, impedance rapidly
moves into, and remains in, the inner characteristic indicating an actual fault and
not a power swing condition. For trajectory B, the impedance moves slowly into
the power swing detection zone and exits the zone indicating that the network is
becoming unstable. Only in this case power swing blocking functionality should be
activated in order to avoid unwanted/unselective operation of distance protection.
The settings of outer impedance measurement characteristic of DSTRPSB should be
set so that load impedance does not enter the outer characteristic during normal
operation of the power system and covers the inner characteristic entirely. The
settings of inner impedance measurement characteristic should be set to make the
inner characteristic cover the distance protection zone entirely which will be blocked
during power swing.
REX640
Technical Manual
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Protection functions
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