Table. 5.4.15. - 140. Register content.
Date & Time
Event code
f Pretrig (Hz)
f Fault (Hz)
Setting group in use
dd.mm.yyyy
hh:mm:ss.mss
6336-6383
Descr.
Start –20ms
averages
Fault
frequency
Starts used at the triggering
moment
5.4.16. Rate-of-change of frequency protection df/dt (81R)
Rate of change of frequency (df/dt or ROCOF) is used to detect fast drops or increases of frequency. If
the load changes fast df/dt protection will detect and clear the frequency based faults faster than
conventional under- and over frequency protections. One of the most common causes for the
frequency to deviate from nominal value is unbalance between generated power and the load demand.
If the unbalance is big frequency will change rapidly.
Rate of change of frequency protection can also be applied to detect loss of mains situation. Loss of
mains is a situation where part of the network (incorporating generation) loses connection with the rest
of the system (i.e. islanded network). If the generator is not disconnected from the network as well it
can cause safety hazards. Generator can also be automatically reconnected to the network which can
cause damage to the generator and the network.
Figure. 5.4.16. - 86. Operation of df/dt function when the frequency starts but doesn’t trip.
In the gure above is presented an example case of df/dt function operation when the frequency is
decreasing. If the f
limit
is activated df/dt doesn’t trip no matter how fast the measured frequency
changes if it’s over the f
<limit
or under f
>limit
. In the gure above when the frequency decreases under
the f
<limit
tripping of the function is allowed but change of frequency isn’t yet fast enough for the df/dt
function to trip. Later the frequency makes a fast dip and as a result the change of the frequency is
faster than the set pick-up value which causes the relay to operate.
Activation and deactivation of each individual stage can be done inside the protection function menus
info-tab.
Outputs of the function are Start Trip and Blocked signals. Setting parameters are static inputs for the
function which are changed only by user input in the setup phase of the function. Frequency protection
utilizes total of eight separate setting groups which can be selected from one common source.
The function can operate on instant or time delayed mode.
AQ-F255
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
150
