How many samples in the memory are used for the phasor calculation depends on the setting
parameter
FilterLength
. Table 419 gives overview of the used number of samples for phasor
calculation by the filter. Note that the used number of samples is always a power of number two.
Table 419: Length of the filtering window
Value for parameter
FilterLength
Used No of samples for
calculation (fixed,
independent from rated
frequency)
Corresponding length of
the input waveform in
miliseconds for 50Hz
power system
Corresponding length of
the input waveform in
miliseconds for 60Hz
power system
0.1 s
128 = 2
7
128 ms 107 ms
0.2 s
256 = 2
8
256 ms 213 ms
0.5 s
512 = 2
9
512 ms 427 ms
1.0 s
1024 = 2
10
1024 ms 853 ms
2.0 s
2048 = 2
11
2048 ms 1707 ms
4.0 s
4096 = 2
12
4096 ms 3413 ms
Note that the selected value for the parameter
FilterLength
automatically defines certain filter
properties as described below:
First in order to secure proper filter operation the selected length of the filter shall always be
longer than three complete periods of the signal which shall be extracted. Actually the best results
are obtained if at least five complete periods are available within the filtering window. Thus, this
filter feature will limit which filter lengths can be used to extract low frequency signals. For
example if 16,7 Hz signal shall be extracted the minimum filter length in milliseconds shall be:
EQUATION000028 V1 EN-US (Equation 201)
Thus based on the data from Table 419 the minimum acceptable value for this parameter would be
“
FilterLength
= 0.2 s” but more accurate results will be obtained by using “
FilterLength
= 0.5 s”
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 420 defines the
natural size of this pass frequency band for the filter, depending on the selected value for
parameter
FilterLength
.
Table 420: 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
Section 12 1MRK 502 066-UUS B
System protection and control
724
Technical manual
To Next Page
Loading...
