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15.2
DIGITAL FILTER
The first operation performed by the CPU with the voltage and/or current signal samples is the DFT.
The Discrete Fourier Transformation consists in decomposing a signal into a series of sinusoidal signals with
frequencies that are multiples of the fundamental frequency. If after this operation, we take the fundamental
frequency signal, and we disregard the rest of signals (harmonics), we will get a harmonic filter. This action is
performed by the MIF relay.
The MIF uses a complete cycle recursive DFT, that is, for each sample it calculates the phasor from the
previous sample phasor and the difference between the current sample and the previous cycle sample. This makes
the relay require a complete cycle to obtain the correct measure value.
In the following figure (figure 2), we can see how the measure is established from a signal value that changes
from 0 to 1.
Figure 3 shows the answer from the digital filter with the frequency. The figure shows how all the high level
harmonics are eliminated. This makes the MIF suitable for applications where it is necessary to filter any type of
harmonic, for example, the 2
1,2
1
0,8
0,6
0,4
0,2
0
15-2
ANNEX 5 HARMONIC FILTERING
nd
rd
and 3
, which are the most commonly found in electrical lines.
1 , 2
1
0 , 8
0 , 6
0 , 4
0 , 2
0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
0
15
30
MIF Digital Feeder Protection
Figure 2
9 0
1 0 0
1 1 0
1 2 0
1 3 0
1 4 0
1 5 0
1 6 0
1 7 0
F r e q u e n c y
Figure 3
MEASURE
45
60
SAMPLES
5 0 H z
6 0 H z
1 8 0
1 9 0
2 0 0
2 1 0
2 2 0
75
90
GEK-106273L
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