2.18 Rate of change of frequency
(ROCOF) protection df/dt (81R)
2 Protection functions
Technical description
112
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Recorded values of the latest eight faults
There are detailed information available of the eight latest
faults: Time stamp, frequency during fault, elapsed delay and
setting group.
Recorded values of the over & under frequency stages (8
latest faults) f><, f><><, f<, f<< (81H/81L)
Parameter
Value
Unit Description
yyyy-mm-dd
Time stamp of the recording, date
hh:mm:ss.ms
Time stamp, time of day
Flt
Hz
Faulty frequency
EDly
%
Elapsed time of the operating time
setting. 100% = trip
SetGrp
1
2
Active setting group during fault
2.18.
Rate of change of frequency (ROCOF)
protection df/dt (81R)
Rate of change of frequency (ROCOF or df/dt) function is used
for fast load shedding, to speed up operation time in over- and
under-frequency situations and to detect loss of grid. For
example a centralized dedicated load shedding relay can be
omitted and replaced with distributed load shedding, if all
outgoing feeders are equipped with VAMP devices.
A special application for ROCOF is to detect loss of grid (loss of
mains, islanding). The more the remaining load differs from
the load before the loss of grid, the better the ROCOF function
detects the situation.
Frequency behaviour during load switching
Load switching and fault situations may generate change in
frequency. A load drop may increase the frequency and
increasing load may decrease the frequency, at least for a
while. The frequency may also oscillate after the initial
change. After a while the control system of any local generator
may drive the frequency back to the original value. However,
in case of a heavy short circuit fault or in case the new load
exceeds the generating capacity, the average frequency keeps
on decreasing.