Second feature which is determined by the selected value for parameter FilterLength
is the capability of the filter to separate the desired signal from the other disturbing
signals which may have similar frequency value. Note that the filter output will be the
phasor with the highest magnitude within certain “pass frequency band” around the
SetFrequency.
Table 342 defines the natural size of this pass frequency band for the
filter, depending on the selected value for parameter FilterLength.
Table 342: Pass frequency band due to FilterLength
Value for parameter FilterLength For 50Hz power system For 60Hz power system
0.1 s ±22.5 Hz ±27.0 Hz
0.2 s ±11.5 Hz ±14.0 Hz
0.5 s ±6.0 Hz ±7.2 Hz
1.0 s ±3.0 Hz ±3.6 Hz
2.0 s ±1.5 Hz ±1.8 Hz
4.0 s ±0.8 Hz ±1.0 Hz
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 342 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 342 + 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 341). The following list gives
1MRK502052-UEN B Section 12
System protection and control
599
Technical manual
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