Study Of Harmonic Detection And Control Algorithm For APF

With the widely use of power electronic equipment and the rapid increase of non-linear loads, the harmonic pollution problem in power system is becoming more and more serious, and so is concerned by the people. As the most effective method to control harmonic, active power filtering technology has become hotspot.Aim at the active power filter (APF), this thesis discusses the harmonic current detection and compensation current control technologies. The main works and innovation points are as fellows:1. Some commonly used methods in harmonic detection and compensation current control are dicussed, the principle, the advantages and disadvantages of these methods are also studied, which lays the foundation of following chapters.2. The principles of harmonic detection method baesd on synchronous rotating coordinate system (dq method) and Discrete Fourier Transform (DFT) are analyzed, there simulation models are built using Matlab-Simulink, and the harmonic detection simulation are performed respectively. The results indicates, as a kind of full harmonic detection method, dq method is not ideal for the detection of special order harmonic, DFT can precisely detect any order of harmonic, but its accuracy is lower than dq method in full harmonic detection. In addition, Fourier transform deals with the signals of infinite length, in discrete state the infinite length signals are treated as periodic signals, when harmonic sources are non periodic, DFT method cannot detect harmonics accurately because of its one dimensional characteristic.3. Based on wavelet transform theory, the principle of wavelet transform in harmonic detection is discussed. The reason of frequency aliasing, a difficult problem for wavelet transform in harmonic detection, is analysed, and a new method to avoid frequency aliasing is proposed, that is the modified wavelet packet decomposition based on atrous algorithm. The key of this method is to compose a group of wavelet filters. With the calculation and analyse of filter type and parameters, a new filter group is designed to detect the amplitude of fundamental wave. The filter has good frequency selectivity, the amplitude-frequency response can be quantified accurately and pre-corrected in order to improve the accuracy of the fundamental wave amplitude detection. The simulation results show the effectiveness that the reconstructed signal after using the modified wavelet packet decomposition based on atrous algorithm is smoother than the signal of DFT, more similar to the original signal.4. A new type of vector current tracking strategy is studied and proposed. This strategy solves the problems of slow tracking, low accuracy and unfixed switching frequency of traditional current tracking method. Based on voltage and current subarea figure and judgement rules of three bridge APF, combining vector current tracking and 3D space vector, it is applied to three-phase four-wire system in addition to three-phase three-wire system by specific corrections according to the particularity. Calculation and simulation results verify the effectiveness and correctness of the method in both power systems.5. The harmonic detection and current control method dicussed in this thesis are used in the two level three-phase four-wire and three-phase three-wire APFs based on DSP and FPGA. The experiments results verify there effectiveness.