Research On Three Phase Three-level NPC Inverter System

High power electronic equipments are widely used in many fields such as electric power, transportation. Multilevel topology structure combining with large capacity switching device is the direction of development of power electronic equipment. Among them, the diode neutral point clamped(NPC) three-level inverter has become one of most widely used multilevel converter at present, because of its simple structure, and high cost performance.Firstly the article introduces the research status and research focus of multilevel converter, then a detail analysis of the three-level space vector modulation algorithm is presented. Comparing with the two-level inverter, three-level topology has more vectors,that results in varied methods to achieve SVPWM, which method achieves high accuracy,fast processing speed, small memory occupation of chip and other issues is worthy of in-depth analysis. The research object of this paper is the subway auxiliary converter, the requirement of output waveform quality is very high, the system can still be stable under frequent mutation operation. According to the requirements, this paper presents a discrete PI+ control strategy based on state feedback. As the load current feed-forward decoupling channel in discrete domain is complex, this paper analyzes the mechanism of the controller in detail, then puts forward a new method to solve the problem, simulation and experimental results prove that the method can not only make the system high accuracy, but also can meet the requirements of dynamic performance. Finally, according to the low frequency oscillation phenomenon of NPC three-phase three-level inverter with asynchronous motor system under open loop control, this paper analyzes the mechanism of low frequency oscillation in detail, and points out that the dead zone of NPC three-phase three level inverter is the main reason that causes the low frequency oscillation of the induction motor, premising the inverter is under good control strategy. A novel dead time compensation strategy is proposed to improve this phenomenon.