2.18 Rate of change of frequency
(ROCOF) protection df/dt (81R)
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slope since the initial pick-up. At the defined operation time, t
= 0.6 s, the average slope is 0.75 Hz/s. This exceeds the setting,
and the stage will trip.
At slope settings less than 0.7 Hz/s the fastest possible
operation time is limited according the Figure 2.18-2
Figure 2.18-2 At very sensitive slope settings the fastest possible operation
time is limited according the figure.
Inverse operation time characteristics
By setting the second delay parameter t
Min
smaller than the
operational delay t, an inverse type of operation time
characteristics is achieved (Figure 2.18-3 ).
Figure 2.18-4 shows an example, where the frequency
behaviour is the same as in the first figure, but the t
Min
setting
is 0.15 s instead of being equal with t. The operation time
depends of the measured average slope according the following
equation.
Equation 2.18-1
where,
t
TRIP
= Resulting operation time (seconds).
s
SET
= df/dt i.e. slope setting (hertz/seconds).
t
SET
= Operation time setting t (seconds).
s = Measured average frequency slope (hertz/seconds).
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