Instruction manual
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AQ G3x7 Generator protection IED
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compared to the consumption by the load connected to the power system, then the system
frequency is below the rated value.
The under-frequency protection function is usually applied to increase generation or for load
shedding 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 low frequency can be an indication of island operation. Accurate frequency
measurement is also the criterion for the synchro-check and synchro-switch functions.
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 under-frequency protection function generates
a start signal if at least five measured frequency values are below the setting value.
Table 3-36: Setting parameters of the under-frequency 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
frequency
40.00…60.00 Hz by
step of 0.01 Hz
Pick up setting of the function. When the measured frequency
value exceeds the sett
ing value function initiates “Start” signal.
Default setting is 49 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.
3.2.18
R
ATE OF CHANGE OF FREQUENCY DF
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DT
>,
DF
/
DT
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(81R)
The deviation of the frequency from the rated system frequency indicates unbalance between
the generated power and the load demand. If the available generation is small compared to
