Research On A New Harmonic Detection Method About The Grid-connected Active Power Filter

The operation of the command current and the current tracking module have been the key point of the Active Power Filter research. They are related to whether the APF can achieve the function of compensation. The traditional harmonic current detection method has some problems such as detection delay and slow dynamic response. In this paper,based on the shortage of traditional method, the paper mainly research on the control method of three-phase shunt Active Power Filter, and validates the feasibility of APF’s topology acting as grid-connected inverter. Because of that the harmonic detection and the control of current both need extracting the grid’s synchronization signal, so the paper conduct a deep study on the frequency locked loop.First of all, the ip-iq detection method of instantaneous reactive power theory is analyzed, and it is concluded that the ip-iq method can not be influenced by the fluctuations of the grid and detect the accurate harmonic current. However, due to the digital low pass filter causing detection delay, so on the basis of ip-iq method, and comparing between SOGI-QSG and MCCF structure, the paper proposes the SAI harmonic current detection method that can achieve positive and negative sequence separation.The method do not need the positive and negative sequence separation module, and also the cascade structure is simple, it can achieve the positive and negative sequence separation directly under the unbalanced grid condition, and realize the function of APF, so the detecting time of the ip-iq method is greatly improved. Then for the three-phase balance grid, this paper presents the improved SAI harmonic current detection method, making the detection structure is more simplified.Secondly, because of the SAI structure is a sinusoidal amplitude integrator based on the frequency control, so the FLL can directly control it. Although the traditional SOGI-FLL can quickly track the frequency of the grid, but because it includes a second order low pass filter, when the grid voltage contains DC component, the FLL output will be affected. In this paper, by introducing a notch filter can make the DC component of the input signal filtering out, and making the low pass filter and the band pass filter of SOGIboth turning up into the three order band-pass filter, making the orthogonal signal is no longer subject to perturbations of the DC component bias, and then realizing the frequency locking. We apply the TOGI-FLL structure to three-phase unbalanced power system. By constructing the trigonometric function, making the three orders FLL can provide the phase information of the grid, so the scope of application of FLL is added.Then, the unified control of Active Power Filter and grid-connected inverter is carried out. By judging the threshold range of the input power to conduct the operating mode of the inverter and the synthesis of the compensating current. By adopting the parallel structure between PI and quasi PR regulator, can make the control of grid connected active-power current and harmonic current realizing no static difference control under the coordinated control. And then the design method of regulator parameters is discussed.Finally, on the basis of theoretical and simulation studies, the main circuit of the gridconnected APF is the three-phase inverter platform, and the DSP is used for digital control.The feasibility and correctness of the above theory is verified by experiments.