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Rate of change of frequency
(ROCOF) protection df/dt
(81R)
Recorded values of the latest eight faults
Detailed information is available on the eight latest faults: Time
stamp, frequency during fault, elapsed delay and setting group.
Table 3.35 – Recorded Values of the Over and Under Frequency Stages
(8 latest faults) f><, f><><, f<, f<< (81H/81L)
Parameter
Value
yyyy-mm-dd
hh:mm:ss.ms
Flt
EDly
SetGrp
1
2
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 Allen-Bradley 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 behavior 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.
Protection Functions
Unit
Description
Time stamp of the recording, date
Time stamp, time of day
Hz
Faulty frequency
%
Elapsed time of the operating time setting.
100% = trip
Active setting group during fault
857-UM001A-EN-P – July 2009
3-59
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