Study On The Modulation Strategy Of Three-phase Four-wire Active Power Filter

The harmonic and reactive power can be compensated by using active power filter(APF). In the APF system, the compensation effect could be influenced by the modulation strategy. This thesis took the three-phase four-wire APF as the study objective, focused on the modulation strategy for three kinds of main circuit.In the three-phase four-wire APF system, the two-level four-leg converter attract extensive attention because of it can control the neutral line current flexibly. In the tradition modulation strategy for two-level four-leg converter, there are various variables need to compute, and the process is complexity. In order to solve this question, a new three-dimensional space vector pulse width modulation(3D-SVPWM) is presented in this thesis. The characteristic of the modulation strategy is that choice of the vectors and calculate of the duration time could be completed by a group of intermediate variable, and the calculation process has been simplified. The simulation and experiment for the presented strategy have been done, and the results verified the validity and effectiveness of this strategy.In the high voltage level or the situation which need altitude compensation ability, the split-capacitor and the four-leg three-level converter could be to use in the three-phase four-wire APF system. For the three-level split-capacitor converter, by using the reference voltage decomposition, a modulation strategy based on the ghγ coordinate system is given. There are two parts in the strategy, the first one is using the reference voltage decomposition to complete the three-level modulation on the gh plane, the other one is adjust the duration time of the redundant switching state according to the component of the reference vector on the γ axis. The difficulty of the neutral-point voltage control has been increased in the three-level split-capacitor converter. This thesis used the conception of neutral-point control degree. In order to select the redundant switching states and suppress the neutral-point voltage fluctuation, the 16 kinds of state is divided into seven levels. Simulation and experiment results verified the effectiveness of this modulation strategy and the neutral-point voltage control strategy.The advantage of three-level four-leg APF is that the compensation effect is good and its control is flexibly. But as there are so many voltage vectors in the three-level four-leg converter that a simplified modulation strategy will be useful. In this thesis, a new modulation strategy is presented. According to this strategy, the starting vector was determined by the define plane. And then, the three-level four-leg modulate was simplified to two parts: reference vector adjust and two-level four-leg modulate. The three-level four-leg modulation strategy will be much simpler when the two-level four-leg simplified algorithm has been used. Three-level four-leg simplified algorithm was presented in this thesis, the area where the vector is located in can be determined and the function of define plane was confirmed by using the three-phase reference voltage. So, the state transform time of the legs could be determined immediately by the three-phase reference voltage. In order to control the neutral-point voltage, duration time of redundant switching states was adjusted according to the charge balance principle. A neutral-point voltage control strategy apply to the three-level four-leg converter is presented, in this strategy, the state transform time were adjusted in order to control the neutral-point voltage.Three kinds of modulation strategy were applied to three-phase four-wire APF system respectively, and the simulation results verified the validity of these strategy. At the same time, the comparison and analysis job for the compensation effects have been done.