Thus the longer length of the filter the better capability it has to reject the disturbing signals close
to the required frequency component and any other noise present in the input signal waveform.
For example if 46 Hz signal wants to be extracted in 50Hz power system, then from Table 420 it
can be concluded that “
FilterLength
=1,0 s” shall be selected as a minimum value. However if
frequency deviation of the fundamental frequency signal in the power system are taken into
account it may be advisable to select “
FilterLength
=2,0 s” for such application.
Note that in case when no clear magnitude peak exist in the set pass frequency band the filter will
return zero values for the phasor magnitude and angle while the signal frequency will have value
minus one. Finally the set value for parameter
FilterLength
also defines the response time of the
filter after a step change of the measured signal. The filter will correctly estimate the new signal
magnitude once 75% of the filter length has been filed with the new signal value (i.e. after the
change).
If for any reason this natural frequency band shall be extended (e.g. to get accurate but wider
filter) it is possible to increase the pass band by entering the value different from zero for
parameter
FreqBandWidth
. In such case the total filter pass band can be defined as:
± (value given in Table 420 + one-half of the set
FreqBandWidth
value)
Example if in 60Hz system the selected values are “
FilterLength
=1.0 s” and “
FreqBandWidth
= 5.0”
the total filter pass band will be ±(3.6+5.0/2)= ± 6.1 Hz.
It shall be noted that the phasor calculation is relatively computation demanding (required certain
amount of the CPU processing time). In order to control the CPU usage for this filter, the setting
parameter
OverLap
is used. This setting parameter defines how often the new phasor value is
calculated during time period defined by the set value for the parameter
FilterLength
(see Table
419). The following list gives some examples how this parameter influence the calculation rate for
the extracted phasor:
• when
OverLap
=0% the new phasor value is calculated only once per
FilterLength
• when
OverLap
=50% the new phasor value is calculated two times per
FilterLength
• when
OverLap
=75% the new phasor value is calculated four times per
FilterLength
• when
OverLap
=90% the new phasor value is calculated ten times per
FilterLength
12.1.7 Filter calculation example
GUID-F5828B0A-CED3-483E-B5B1-F389C3C2AEFB v3
In the following Figure an example from an installation of this filter on a large, 50 Hz turbo
generator with a rating in excess of 1000 MVA is presented. In this installation filter is used to
measure the stator sub-synchronous resonance currents. For this particular installation the
following settings were used for the filter:
•
SetFrequency
= 31.0 Hz
•
FilterLength
= 1.0 s
•
OverLap
= 75%
•
FreqBandWidth
= 0.0 Hz
1MRK 502 066-UUS B Section 12
System protection and control
725
Technical manual
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