How can I set the magnitude correctly for the FFT?

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Chonlasit
Chonlasit 2024-9-25
回答： Shivam Gothi 2024-10-7
Hello I want to find the value of each harmonic order for the three-phase current. phase a (dimension 1) as shown in the figure is the correct graph, but why are the magnitudes of phases b(dimension 2) and c(dimension 2) like that? I have also attached the circuit diagram.Shouldn't %THD be approximately the same for each phase? The differences seem larger than usual.
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David Goodmanson
David Goodmanson 2024-9-25
编辑：David Goodmanson 2024-9-25
Hi Chonlasit,
the three phases appear to be close to the same amplitude, approximately 5. It's just that phases 2 and 3 have a large spike at t=0 that affects the scaling of the plot. And none of the plots look anything like a simple sine wave, so the THD is going to be large.

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Shivam Gothi
Shivam Gothi 2024-10-7
Hello @Chonlasit,
Based on my understanding, You are expecting similar FFT analysis for all the phase currents, but not getting the expected output.
Cause of issue:
The observed differences in the FFT analysis of the phase currents appear larger because the FFT is being computed for only one cycle, beginning at time 't=0'. This approach also captures the transient components. If you examine the waveform for each phase, you'll notice that the first cycle (starting at t=0) has varying magnitudes across the phase currents. The 'FFT analyzer' identifies the frequency components present in this single cycle. Consequently, since the initial cycle differs for the 'a', 'b', and 'c' phase currents, you are seeing significant variations in the FFT analysis.
Solution:
If you want to perform FFT analysis after the system has achieved steady state, place the FFT window at the steady state part of the phase currents. I performed the FFT analysis using the Simulink model attached by you and I made the following changes in the “FFT analyser” window:
  • Changed the start time to 0.05 (because we have to consider only steady state part)
  • Changed the “number of cycles” to 20.
These changes are reflected in the below figure:
After making the above mentioned changes, I re-computed the FFT for all the 3 phase currents. I got the following results:
Observe that the “FFT window” (highlighted by red color) is capturing only the steady state part of the waveform.
Conclusions:
As you expected, the FFT analysis of all the phase currents shows a remarkable similarity, as illustrated in the attached results.
I hope this helps !

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