Research On Three-phase Four-wire Three-level Active Power Filter

In the three-phase four-wire low-voltage power supply system,with the wide use of power electronic devices and nonlinear loads,the power grid is injected into a large number of harmonics.Active power filter(APF)can be used to compensate harmonic and reactive power,so it has been paid much attention in recent years.In this paper,the three-phase four-wire three-level mid-point capacitor active power filter is studied.The mathematical model is established in the abc coordinate system and ??0 coordinate system.The relationship between the capacitor voltage and the phase current is analyzed,and the influence of each vector state on the neutral point potential is obtained.The neutral point balance factor is used to measure the deviation of neutral point potential.The three-dimensional space vector modulation is detailedly analyzed in the abc coordinate system.The method to realize the neutral point potential balance by extending the three dimensional subspace is introduced in detail.For the complexity of the three-dimensional space vector modulation,based on the voltage-second balance,a simplified modulation strategy is proposed for the three-phase four-wire three-level mid-point capacitor topology.The simplified modulation strategy only needs simple voltage-second balance calculation for single phase instead of complex vector synthesis in three-dimensional space.Analogy to the subspace expansion in three-dimensional space,the vector state expansion for simplified modulation strategy is proposed to implement the neutral point potential balance.The consistency of the reference voltage vector before and after the vector state expansion is proved by the three-phase vector synthesis.Furthermore,by comparing the results of the three-dimensional space vector modulation and the simplified modulation strategy,the normalization characteristic is proved between the simplified modulation strategy and the 3D space vector modulation.The mean neutral point control factor is used as an index to evaluate the neutral point control capability of the modulation algorithm and the expression of the mean neutral point control factor is derived for the proposed simplified modulation strategy.Finally,the simulation and experimental verification of the proposed modulation strategy are presented.The simulation and experimental results show that the proposed algorithm not only has a small amount of computation,but also has strong control ability for the neutral point potential.This paper expounds the basic idea of repetitive control,and analyzes the characteristics of repetitive control from four aspects of stability,disturbance rejection,error convergence and steady-state error.For the shortcoming of the slow dynamic response of repetitive controller,the detailed design process of repetitive control in series with PI control is given.Because conventional repetitive control can not adapt to the grid frequency fluctuations,fractional order repetitive control is applied to the three-phase four-wire three-level mid-point capacitor active power filter.The fractional order repetitive control is verified by simulation and the simulation results show that the fractional order repetitive control has good steady-state accuracy when the grid frequency changes.Finally,the steady state accuracy and dynamic performance of the repetitive control in series with PI are tested by the unbalanced three-phase load of uncontrolled rectifier bridge.A digital control system based on DSP+FPGA is designed.On the platform of the system,two kinds of different type of three-phase unbalanced load with DC component are selected to verify the modulation strategy and control strategy in this paper by experiments.