Why is the THD% increasing when changing the number of cycles or time interval in FFT Analysis tool Powergui?

Hi,

I am doing a harmonic analysis and want to look at the individual (IHD) and total (THD) harmonic distortion at the point of common coupling PCC of a power system. When I adjust the start time (s) or number of cycles in the FFT analyzer via the powergui, the THD% increases rapidly. Can someone perhaps explain to me what I am doing wrong? Thank you in advance.

Answer 1

[Sabin]

By changing the start time or the number of cycles you change the FFT window. This will consequently give different results because the data you’re analysing is different.

[Rahul Moozhikkal]

This explanation answers how start time changes the FFT results. However, in my system I have seen that at STEADY STATE increasing the number of cycles for example from 1 to 10 drastically reduce the fundamental component and increase the THD. I expect there to be some change but like 20-30% reduction considering the variation in the steady state for the signal under investigation is very well under thos limits (<1%)?

Could you please provide an explanation for this?