2.18 Rate of change of frequency
(ROCOF) protection df/dt (81R)
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The minimum operation time is always limited by the setting
parameter t
Min
. In the example of the fastest operation time,
0.15 s, is achieved when the slope is 2 Hz/s or more. The
leftmost curve in Figure 2.18-3 shows the inverse
characteristics with the same settings as in Figure 2.18-4.
Figure 2.18-3 Three examples of possible inverse df/dt operation time
characteristics. The slope and operation delay settings define the knee
points on the left. A common setting for t
Min
has been used in these three
examples. This minimum delay parameter defines the knee point positions
on the right.
Figure 2.18-4 An example of inverse df/dt operation time. The time to trip
will be 0.3 s, although the setting is 0.6 s, because the average slope 1 Hz/s
is steeper than the setting value 0.5 Hz/s.
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