The Study Of Anti-aliasing Harmonic Detection Method Based On Wavelet Transform

Power system harmonic issue is a challenge topic in the world. As core of harmonic issues, harmonic detection and suppression is the main basis and final goal of harmonic studies. Owing to the inherent nonlinearity, randomness, distributing, nonstationarity and complexity of influencing factor on power system harmonics, many conventional detection techniques can't acquire accurate harmonic identification, so it is very valuable in theory and practice to investigate new detection and suppression methods. Because of excellent time-frequency localization characteristics, wavelet transform (WT) can be used to detect harmonics. However, owing to aliasing within almost all the existed wavelets, even though it is shown that wavelet-based methods are superior to the traditional detection techniques, they can't satisfy the accurateness and robustness requirement.This paper deeply studies harmonics detection based on wavelet transform. In view of aliasing within wavelets, aliasing cause is clarified in ways of physical essence and magnitude-frequency characteristics. In addition, aliasing phenomenon and aliasing harm are illustrated. On this basis, a new anti-aliasing wavelet based harmonics detection method is proposed. The method consists of two parts. One is change the elements on odd number coordinates into its negative and get half of value of the array elements , the other is extracted array elements on the even number coordinates and made interpolation. Added together the two resultes. The new algorithm gives both attention between signal preservation and aliasing reduction, it overcomes the inherent bug of traditional wavelet filters rooted from just only concerning perfect reconstruction and total anti-aliasing.Under the new algorithm, two examples are illustrated: One is detection the total harmonic when gives multi-integer harmonics; the other one is detection inter-harmonics and compare it with other algorithms. After simulation, it proves that the new algorithm can effectively eliminate aliasing with high precision which provides a vigorous mean for uncontrolled harmonics detection in power system.