Digital Control And Its Implementation Of Modular Three-phase Four-wire Shunt Active Power Filter

With the growing maturity of power electronics technology and widely used of electric power automation equipment, the development of industrial productivity is promoted and the level of electrical automation is improved. Meanwhile, serious grid harmonic pollution problem was raised. Active power filters (APFs), known as harmonic and reactive power dynamic compensation equipment, can effectively solve the problem of utility grid harmonic pollution. In this paper, the modular three-phase four-wire shunt active power filter with split dc capacitor topology is used as a carrier. Several issues such as selective harmonic detection algorithms and current limiting control strategy, the digital control strategy for compensation current and the implementation and control of multi-module parallel combination system are focused on.From the point view of seeking selective harmonic detection method which is suitable to the industrial application, study and analysis of currently commonly used selective harmonic detection algorithms which are separately based on the synchronous rotating coordinate transformation, the digital band-pass filter and the Discrete Fourier Transform (DFT) methods. A DFT based optimized selective harmonic current detection aigorithm and its implementation method in program is proposed. In ensuring the high steady-state detection accuracy, the occupation of program space is sharply reduced. The reason and situation of overcurrent in APF system are analyzed. An optimized DFT algorithm based current limiting control strategy is proposed and the APF operated in limiting capacity can be assured.The tracking accuracy of command current depends on the selection of control strategy and the design of controller parameters. In this paper, from the point view of improving the accuracy of harmonic compensation, dynamic performance and adaption of complex industrial conditions, a control strategy based on ABC frame is introduced. It achieves the closed-loop control of DC-bus voltage and compensating current in time domain. This paper completed the detailed design of the inner current loop and outer voltage loop controller, while the former is composed by the Proportional Integral (PI) controller and repetitive controller, and the latter is composed by the stabilization voltage loop and equalization voltage loop of DC-bus. This control strategy greatly improved the steady-state compensation accuracy and dynamic response speed. Experimental results in kinds of working conditions verify its efficiency and feasibility.In order to adapt to the industrial applications with large capacity harmonic load, the system with parallel combination of multi-module APFs is studied. The structure of multi-module parallel combination system is given and its basic operating principle is elaborated. The load current sampling method and sharing current control strategy in the combined system are studied. The control system in the combined system is detailed design and its function implementation in digital methods is elaborated. Finally, the correctness and realizability of the combined system is valitaded through simulation and experiment.