Research On Harmonic Suppression Technology In Three-phase Four-wire System

Due to large number of single phase nonlinear loads being put into use, harmonic problem in three-phase four-wire system has become more and more serious. Zero sequence current existed in three-phase four-wire system makes the harmonic suppression quite different from that in three-phase three-wire system and lots of works need to be done. This thesis mainly focuses on harmonic suppression technology in three-phase four-wire system, the key technologies of three-level three-phase three-leg capacitor split inverter based shunt active power filter used in three-phase four-wire system are studied in detail.Two typical three-level inverter topologies (three-level three-phase four-leg topology and three-level three-phase three-leg capacitor split topology) are analyzed deeply. A novel unified three-level topology which unifies these two typical topologies is proposed for three-phase four-wire system. The mathematical model is established and current tracking ability is compared in detail by using this model. Besides, this thesis also analyzes current compensation ability from the perspective of space vector allocation and application potential from the perspective of cost and stability.Current control strategies are also analyzed in this thesis. Considering the periodical character of reference signal, repetitive controller is adopted in the a-b-c coordinates. Design considerations and processes are given. In d-q-0 coordinates, an improved deadbeat controller is analyzed. A state observer is adopted to eliminate the effect caused by one beat delay in digital control system; a repetitive harmonic predictor is adopted to predict the harmonic reference in the next beat. Simulation results are given to validate the effectiveness of these two strategies.A generalized 3-D space vector modulation scheme which is suitable for digital implementation is analyzed. According to the neutral point unbalance problem in three-level converter used in three-phase four-wire system, the mechanism of effect on neutral point unbalance is analyzed and a concept named"Factor of neutral point balance"is proposed. Based on this concept, a novel 3-D space vector modulation scheme which can suppress the neutral point unbalance without adding commutation complexity is proposed. Simulation results verify the validity of proposed method.A prototype based on DSP TMS320F2407 is established, experimental results for improved deadbeat control and novel 3-D space vector modulation are given to demonstrate the correctness of theories above.