How to calculate Total harmonic distortion using FFT analysis tool of power gui block

Hello All, I have used FFT analysis tool in power gui block to calculate THD of single phase input current. The value of THD is coming more than 10% however is required one is less than 5%. Please suggest

1. How to select the start time and the number of cycles so as to get reduced value THD?

Answer 1

[ALBERT ALEXANDER STONIER]

The normal cycle which need to be considered for the FFT analysis is 20 as much dealt in various IEEE literature. In order to reduce the THD to less than 5%, we need to adopt the various switching techniques than the conventional one for different loading conditions.

Answer 2

[mohamed elbesealy]

you can select the start time at the beginning of steady state of the signal. you can reduce the THD by using Passive filter or active power filter.

[Mohsen Khalaf]

This answer works with me

Answer 3

[PRABHU OMER]

you should select that start time where your results are in steady state. Make sure that your fundamental frequency of your model and base frequency of FFT tool in Powergui block is same. Two number of cycles are sufficient for THD analysis. And to see reduced THD you should change the maximum frequency. However THD also depends on your results.

[AMIT KUMAR]

sir u told about chnaging the maximum frequency can you elaborate more about this point. This question i am asking because In MATLAB 2014 a there is two options are coming for selection one is with nyquist frequency which is default and other option is set as maximum frequency. and while setting the second option the THD is reducing very much But why i am unable to understand and also which one is correct i have doubt. Please help me regarding this. Thank you.

[Mohammed SETTI]

It simply means the maximum frequency used to compute the THD. By default it is set to the Nyquist freqency (fs/2) but you can change it to your desired max freq (e.g. grid code: 2kHz in the case of f_1=50Hz)

Answer 4

[Rakesh Patel]

Adding to Prabhu's answer, I would suggest number of cycles for FFT window should be about 10 cycles.

Answer 5

[vishal charan]

I have also implemented predictive current control of two level VSI i get THD as 8.39%. Is this acceptable.