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3-60
Protection Functions
Note: At 0.6 s, which is the delay
setting, the average slope exceeds
the setting 0.5 Hz/s and a trip signal
is generated.
857-UM001A-EN-P – July 2009
Figure 3.28 – Example of definite time df/dt operation time
Description of ROCOF implementation
The ROCOF function is sensitive to the absolute average value of the
time derivate of the measured frequency |df/dt|. Whenever the
measured frequency slope |df/dt| exceeds the setting value for 80 ms
time, the ROCOF stage picks up and issues a start signal after an
additional 60 ms delay. If the average |df/dt|, since the pick-up
moment, still exceeds the setting, when the operation delay time has
elapsed, a trip signal is issued. In this definite time mode the second
delay parameter "minimum delay, t
operation delay parameter "t".
If the frequency is stable for about 80 ms and the time t has already
elapsed without a trip, the stage will release.
ROCOF and frequency over and under stages
One difference between over-/under-frequency and df/dt function is
the speed. In many cases a df/dt function can predict an
overfrequency or underfrequency situation and is thus faster than a
simple overfrequency or underfrequency function. However, in most
cases a standard overfrequency and underfrequency stages must be
used together with ROCOF to ensure tripping also in case the
frequency drift is slower than the slope setting of ROCOF.
Definite operation time characteristics
Figure 3.28 shows an example where the df/dt pick-up value is 0.5
Hz/s and the delay settings are t=0.60 s and t
t == t
will give a definite time delay characteristics. Although the
Min
frequency slope fluctuates the stage will not release but continues to
calculate the average 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.
" must be equal to the
Min
=0.60 s. Equal times
Min
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