Study On The Vector Control Of NPC Three-level Inverter Based On Capacitor Voltage Balance

With the rapid development of our industry, Multi-level inverter used in the AC variable frequency speed regulation system of high voltage and large capacity is a hot research topic in the field of power electronic. Compared with the traditional two-level inverter, Multi-level inverter is widely applied in power systems and electric drive systems with high voltage and large capacity because of the advantages of low harmonic content of waveform and low switch frequency.The vector control system of induction motor based on the NPC (Neutral-Point-Clamped) three-level inverter is selected as the research object. The flux observation used in the vector control system is studied in theory. The algorithm of Space Vector Pulse Width Modulation (SVPWM) is analyzed and discussed thoroughly. In addition, the neutral-point potential unbalance problem existed in the NPC three-level inverter is discussed profoundly. Propose the corresponding solutions to the above problems. The main aspects of this paper are completed as follows:Firstly, AC variable frequency speed regulation system is overviewed briefly. The research status of multi-level inverter is discussed. The multi-level inverter applied in the AC variable frequency speed regulation system is investigated. The topology structure of multi-level inverter is analyzed, and the mathematical model is built. The basic principle of induction motor vector control is elaborated.Secondly, since the traditional three-level SVPWM algorithm adopted in the NPC three-level inverter is complicated and not conducive to real-time control, a simplified three-level SVPWM algorithm is proposed, in which the space voltage vector is decomposed and transformed. The determination of the fundamental vector and the selection of appropriate switch states are obtained by this simplified three-level SVPWM.Thirdly, since the neutral-point potential unbalance problem existed in the NPC three-level inverter, the mechanism of the neutral-point potential fluctuation is analyzed. A control strategy to precisely compensate the neutral-point potential basedon virtual space vector pulse width modulation (VSVPWM) calculated under the gh coordinate is proposed. The virtual medium vector and voltage adjustment factor are introduced. Different positive and negative short vectors were distributed different time which improves the influence of short vectors on the neutral-point potential control effect and its flexibility. At the same time, the calculation of the basic voltage vectors working time under the gh coordinate is reduced.Fourthly, according to the above theory research, the simulink platform is built. The simplified SVPWM algorithm and the control strategy to balance the neutral-point potential based on VSVPWM are analyzed by simulation. The results of simulation prove that the simplified SVPWM algorithm and the control strategy to balance the neutral-point potential are true.Lastly, the experimental platform of vector control system for induction motor based on NPC three-level inverter is designed and built. Writing the software program in the DSP controller which use the TMS320F2812as control core. The experiment results show that the three-level simplified SVPWM algorithm and the control strategy to balance the neutral-point potential are effective and feasible.