ABB REG670 Technical Manual page 604

Section 12
System protection and control
598
The filter output can also be connected to the measurement function blocks such as
CVMMXN (Measurements), CMMXU (Phase current measurement), VMMXU
(Phase-phase voltage measurement), etc.
The filter has as well additional capability to report the exact frequency of the
extracted signal. Thus the user can check the actual frequency of some phenomenon
in the power system (e.g. frequency of the sub-synchronous currents) and compare it
with expected value obtained previously by either calculation or simulation. For the
whole three-phase filter group the frequency of the signal connected to the first input
(i.e. phase L1) is reported. This value can be then used either by over-/under-
frequency protections or reported to the built-in HMI or any other external client via
the measurement blocks such is the CVMMXN.
How many samples in the memory are used for the phasor calculation depends on the
setting parameter FilterLength.
samples for phasor calculation by the filter. Note that the used number of samples is
always a power of number two.
Table 341: Length of the filtering window
Value for parameter Used No of samples for
FilterLength calculation (fixed,
independent from rated
frequency)
0.1 s
128 = 2
0.2 s
256 = 2
0.5 s
512 = 2
1.0 s
1024 = 2
2.0 s
2048 = 2
4.0 s
4096 = 2
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:
1000
× =
3 180 ms
16.7
EQUATION000028 V1 EN
Thus based on the data from
parameter would be "FilterLength = 0.2 s" but more accurate results will be obtained
by using "FilterLength = 0.5 s"
Table 341 gives overview of the used number of
Corresponding length of
the input waveform in
miliseconds for 50Hz
power system
128 ms
7
256 ms
8
512 ms
9
1024 ms
10
2048 ms
11
4096 ms
12
Table 341 the minimum acceptable value for this
1MRK502052-UEN B
Corresponding length of
the input waveform in
miliseconds for 60Hz
power system
107 ms
213 ms
427 ms
853 ms
1707 ms
3413 ms
(Equation 136)
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