Calculate the THD in percent

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Meikel Vollmers
Meikel Vollmers on 16 Jun 2021
Answered: Arthi Sathyamurthi on 27 Jul 2021
Hey Community,
i try to calculate the THD of a current signal in percent. This is my code
thd_db = thd(ac_current_a.data);
thd_percent = 100*(10^(thd_db/20));
The result of the attached data is 0.88%. But when i analyze the signal with the FFT analysis tool of the powergui, the THD is about 12%, which is more realistic.
I think i have to expand my code with some information, i.e. the fundamental frequency, the number of cycles and so on as it must be done in the FFT Tool.
How can i modify my code to get the correct THD?
  1 Comment
Mathieu NOE
Mathieu NOE on 30 Jun 2021
tried to open the zip but it failed (??)

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Answers (1)

Arthi Sathyamurthi
Arthi Sathyamurthi on 27 Jul 2021
Hello Meikel,Your attached zip file looks corrupted. Try attaching it again for a more precise support. From the other information provided, thd_db = thd(ac_current_a.data) returns the total harmonic distortion (THD) in dBc of the real-valued sinusoidal signal ac_current_a.data. Hence be clear that the thd_db is in dBc and confirm that the ac_current_a.data is a real-valued sinusoidal signal. Also try the calculation of total harmonic distortion as thd(pxx,f,'psd') where the input pxx is the one-sided power spectral density (PSD) estimate othat can be obtained from the periodogram function. f is a vector of frequencies corresponding to the PSD estimates in pxx. You can look at the other ways of calculating the total harmonic distortion here.

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