Instruction manual
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AQ G3x7 Generator protection IED
94 (211
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the consumption by the load connected to the power system, then the system frequency is
below the rated value. If the unbalance is large, then the frequency changes rapidly. The rate
of change of frequency protection function is usually applied to reset the balance between
generation and consumption to control the system frequency. Another possible application is
the detection of unintended island operation of distributed generation and some consumers.
In the island, there is low probability that the power generated is the same as consumption;
accordingly, the detection of a high rate of change of frequency can be an indication of island
operation. Accurate frequency measurement is also the criterion for the synchro-switch
function.
The source for the rate of change of frequency calculation is an accurate frequency
measurement. The frequency measurement is based on channel No. 1 (line voltage) and
channel No. 4 (busbar voltage) of the voltage input module. In some applications, the
frequency is measured based on the weighted sum of the phase voltages. The accurate
frequency measurement is performed by measuring the time period between two rising edges
at zero crossing of a voltage signal.
For the confirmation of the measured frequency, at least four subsequent identical
measurements are needed. Similarly, four invalid measurements are needed to reset the
measured frequency to zero. The basic criterion is that the evaluated voltage should be above
30% of the rated voltage value. The rate of change of frequency protection function generates
a start signal if the df/dt value is above the setting vale. The rate of change of frequency is
calculated as the difference of the frequency at the present sampling and at three cycles
earlier.
Table 3-37: Setting parameters of the df/dt function
Parameter
Setting value, range
and step
Description
Operation
Off
On
Operating mode selection for the function. Operation can be
either disabled “Off” or enabled “On”.
Default setting is
enabled.
Start signal
only
Activated
Deactivated
Selection if the function issues either “Start” signal alone or
both “Start” and after set time delay “Trip” signal.
Default is
that both signals are generated (=deactivated).
Start df/dt
-
5…5 Hz/s by step of
0.01 Hz
Pick up setting of the function. When the measured
frequency value exceeds the setting value function initiates
“Start” signal.
Default setting is 0.5 Hz
Time delay
100…60000 ms by
step of 1 ms.
Operating time delay setting for the “Trip” signal from the
“Start” signal.
Default setting is 200 ms.
