Evaluation of harmonic contribution to unbalance in power systems under non-stationary conditions using wavelet packet transform

Unbalance and harmonic distortion are two significant power quality (PQ) problems in modern power systems. Current research is focused on the contribution to unbalance of only the fundamental frequency component, separating the unbalance from the harmonics and ignoring the unbalance distortion existed in all harmonic components. This paper proposes a new methodology to accurately evaluate the contribution to unbalance of every harmonic component of voltages and currents using wavelet packet transform (WPT). WPT provides the advantage of handling non-stationary disturbances in compare with other signal-processing techniques, such as fast Fourier transform (FFT). The proposed WPT methodology offers an effective generalization to unbalanced distorted three-phase systems by maintaining a sufficient distinction between the effects of unbalance and distortion. The IEEE Standard 1459-2010 definitions are adapted, and the reformulation of the concept of symmetrical harmonic components using WPT is introduced to evaluate the balanced, first unbalanced and second unbalanced components of all harmonic orders. Synthetic waveforms including noise, a simulated three-phase system, and an experimental test are examined, considering stationary and non-stationary disturbances. The results are compared with their true values and those by FFT demonstrating the accuracy of the proposed WPT methodology and its superiority over FFT under non-stationary conditions.