How can I calculate THD using MATLAB?

[EE86]

Hello,

I'm a novice in using MATLAB. I want to calculate THD value from my experimental data using MATLAB. I have 10,001 data points, sampling frequency is 100kHz and fundamental frequency is 50 Hz (I'm not in USA). Below is my code. Could you please check that my code is correct or not ?


>> data = importdata('sample_1.xlsx');
% 10,001 data points, sampling rate = 100kHz

>> spect = fft(data)/10001;
% frequency of each element = 0, 10, 20, 30,...Hz (100kHz/10,001 data)

>> harmonic = abs(spect(11:5:201));
% here, fundamental = 50 Hz so, frequency 100, 150, 200,...,2000 Hz correspond to 11th, 16th, 21st,...,201st element of spect
% now, harmonic has 39 elements

>> harmonic_sqr = harmonic.^2;
% square each element

>> THD = sqrt(sum(harmonic_sqr(1:39)))/abs(spect(6))*100
% 50 Hz corresponds to 6th element of spect

 

[the_emi_guy]

Greetings,

I would create xls files with known THD to test the algorithm.

Pure 50Hz and pure 50Hz plus a single harmonic would only take a minute to write and give you a quick sanity check.

You could spend a little more time and create an xls that added predetermined power at each harmonic, and even have the amount of THD as a parameter so that you could verify the dynamic range.