The Research Of The Control Strategy For Three-level Diode-clamped Convertors Based On Zero-sequence Voltage

In recent years, diode clamped multilevel converters have been widely used in minings, locomotive tractions, rollings and other fields by the advantage of huge capacity, high voltage and low current harmonic content. This paper focuses on the control strategy of bus midpoint voltage in diode clamped three-level inverter.After introducing multilevel converter's background and development status, combined with practical application in industry, this article states the necessity of multi-level converters'application and importance of its related research. Then, the research direction and development of multi-level transform technology is studied.Based on introducing the topological structure and basic principle, we analyze the PWM method for three-level diode clamped (NPC) inverter. The research focuses on two types of PWM method-Carried Disposition PWM and Space Vector PWM. The relationship and between them gives us a way to realize the PWM for the three-level inverter by simplifying and summarizing the PWM algorithm. Therefore, a common PWM method is studied and discussed. On the work above, with the purpose to balance the bus neutral-point potential in three-level inverter, a metod called searching-optimization algorithm is studied and analyzed. In the further research, The relationship between the neutral current and zero-sequence voltage in neutral-point-clamped converters is discussed and summarizing, on this basic, a theoretically optimal scheme to control the neutral point voltage is presented.With MATLAB/Simulink's help, we establish a three-level diode-clamped inverter simulation model. In different modulation and power factor conditions, control strategies of bus neutral-point potential based on zero sequence voltage is simulated. The search results validate the searching-optimization algorithm and the optimal zero-sequence voltage method are correct, at the same time, the law of bus neutral-point voltage fluctuation is summarized.A NPC three-level inverter test platform based on DSP control is set up, and the optimal zero-sequence voltage method is approached to test validation. The results show that the control strategy based on zero sequence voltage method optimal can effectively inhibit the bus midpoint voltage fluctuation, which verify the theoretical analysis and simulation results.