Research Of Active Power Filter In Three-phase Four-wire System

Due to more and more single phase non-linear loads being put into use, people begin to pay attentions to harmonic problem in three-phase four-wire system. The 3-phase 4-wire tri-level three-leg neutral-point split APF not only can effectively compensate the harmonic current in loads, but also has the characteristics of fast response and dynamic continuous real-time compensation, so this article mainly focuses on harmonic suppression technology of 3-phase 4-wire tri-level three-leg neutral-point split APF, mathematical models, the current loop controller design and modulation strategies design are studied in detail.In this thesis the system mathematical model is established in the a-b-c coordinate when APF converter is expressed as the equivalent of controlled APF converter voltage source and controlled current source, and analyze the similarities and differences between the mathematical models as the DC bus voltage under balanced and unbalanced , reveal the reason why modulation strategy can be designed flexibly from solution space of linear equationsIn this thesis the traditional deadbeat control affected by one-beat-delay implementation is analyzed in detail. Based on the principle of the traditional deadbeat control, a improved deadbeat control is proposed. If the precise model parameters have been given, the improved deadbeat control will completely eliminate the affect of voltage reference by one-beat-delay implementation.In this thesis the objectives of system modulation strategy with respect to load harmonic current compensation, mid-point potential control and switching power losses decrease, are analyzed in detail. The precise mathematical expressions about the objectives of system modulation strategy are also given, and then a optimal modulation strategy in the a-b-c coordinate is proposed based on functions of the objectives of system modulation strategy. The optimal modulation strategy furthest realize the load harmonic current compensation, the mid-point potential control and the switching power losses decrease, so it can be used as an practical modulation strategy.In this thesis an prototype of APF on floating-point DSP TMS320F28335 is established to verify the improved deadbeat control and the new modulation strategy. The results show that the deadbeat controller can well calculate the appropriate amount of voltage control, the new modulation strategy can also realize the objectives about load harmonic current compensation, mid-point potential control and switching power losses decrease, which verify that the improved deadbeat and anew modulation strategy are correct and feasible.