Research On Key Technology Of Modular Active Power Filter And System Design

With the development of power electronic technology, power harmonic problems have become increasingly prominent. However, as an effective device for harmonic elimination and reactive power compensation, the Active Power Filter(Active Power Filter, APF) is receiving more and more attention, which has important practical significance to the improvement of power quality and the stable operation of the grid. With the development of industry, the demand for large capacity APF becomes increasingly pronounced. The modular APF solves the contradiction between the technical problems of large capacity devices and the demand for large capacity filter effectively. Taking the modular APF as the research object, the key technologies including harmonic detection, split flow and current limiting protection, carrier phase shift modulation, parameter selection and the design of software and hardware are studied in this paper.Two common methods of harmonic detection based on the synchronous rotation coordinates and Fourier transform respectively are introduced in this paper with the discussion of their advantages and disadvantages at first. As for the harmonic detection method based on Fourier transform, the differences between discrete Fourier transform, fast Fourier transform and sliding-window iterative Fourier transform in terms of computation cost and realizing function are analyzed. It can be concluded that the sliding-window iterative Fourier transform algorithm has the characteristics of low computational complexity and fast response speed in harmonic detection. According to the particularity of the modular APF, the distribution principles of the reference current between different modules are analyzed as well. Moreover, considering the operation stability, security and redundancy of the system, the current sharing scheme with updatable shunt coefficient and the over-current protection scheme are designed.In order to reduce the output ripple current of APF, the carrier phase shift method is applied in the modular APF and the control effect of equivalent high switching frequency is achieved at low switching frequency. Firstly, the difference of the output voltage harmonic characteristics between bipolar modulation and unipolar dual-frequency modulation are compared using the double Fourier analysis method. Then, according to the system transfer function and module current superposition principle, the total output current harmonic characteristics of the two modulation methods are compared based on mathematical analysis and simulation verification. It is shown that the unipolar dual-frequency carrier phase shift modulation has advantage to counteract the switching harmonics with better modulation performance.The modular APF system is designed in this paper. As for the LCL-type three-phase H full-bridge main circuit, the parameter design methods of DC bus capacitor, filter inductance capacitance and damping resistance are presented. According to the mathematical model of the system, the control methods of the current inner loop based on feed-forward decoupling and the voltage outer loop based on two-level control are adopted. The static stability and dynamic stability of the designed control system are verified by simulation results. Finally, the software and hardware design methods of the modular APF system are represented, and the theoretical analysis is validated by the experimental results on the prototype.