Using Discrete Wavelet Transform to analyze quasi-stationary three-phase systems

The paper deals with the Discrete Wavelet Transform (DWT) analysis of quasi-stationary regimes (with small variations of magnitude and harmonic contents) in three-phase systems. The analyzed data, acquired on stand, correspond to a driving system using chopper and DC motor. Unfiltered currents are included in the first data set, unlike those from the second set. The harmonic orders to be filtered (5,7,11,13 etc.) were determined using a FFT analysis. 10 levels decompositions using DWT were made, using Daubechy filters of length 8, implemented by 2 functions. “dwm” assumes that the analyzed signal is periodic, whilst “dwi” performs interpolations to estimate the first 6 values of the decomposed signal beyond its right edge. During analysis, FFT is applied over one period P, whilst DWT operates over 6 consecutive periods starting with P. The analysis concerns instantaneous and global power quality related quantities and provides possible explanations for the cases when differences appear between the results yielded by the various analyzed methods.