Instruction manual
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AQ G3x7 Generator protection IED
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The over-frequency protection function is usually applied to decrease generation 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 high 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 over-frequency protection function generates a
start signal if at least five measured frequency values are above the preset level.
Table 3-35 Setting parameters of the over frequency protection 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 excee
ds the setting value function initiates “Start” signal.
Default setting is 51 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.
3.2.17
U
NDER FREQUENCY F
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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