Research On Control Strategy For NPC/H Bridge Five-Level Inverter

Multilevel converters have many advantages,such as large output capacity,low harmonic contents and switching frequency,so it is widely used in high-voltage and high-power applications such as power traction and fan driver.This topic takes NPC/H bridge five-level inverter as the research object,whose topology characteristics and applicable modulation strategies are analyzed.In addition,the key problems of the topology are also discussed,including the balance of DC-side capacitor voltages and the suppression of system common-mode-voltage(CMV).This paper introduces the topology structure of NPC/H bridge five-level inverter and analyzes the working modes of single-phase circuit under five switching states in detail.In view of the characteristics of bridge structure,the PWM modulation strategies based on vertical carrier distribution and hybrid distribution are adopted.In order to improve the utilization of DC-side voltage,this paper describes the implementation steps of the traditional SVPWM algorithm applied to the NPC/H bridge five-level inverter in detail.Because of the complex process and poor real-time performance of traditional SVPWM algorithm,a fast algorithm of SVPWM based on g-h coordinate system is introduced,which improves the efficiency of algorithm obviously.The correctness of the traditional modulation strategy is verified by simulation and experiment,which lays the foundation for subsequent control strategies.Similar to diode-clamped topology,the imbalance of three-phase DC-side capacitor voltages exists in NPC/H bridge five-level inverter topology.By establishing the simplified mathematical model of system circuit,this paper analyzes the effects of 125 voltage vectors on the DC-side capacitor midpoint potential in detail,which expounds the mechanism of the imbalance.Combined with the characteristics of topology and the mechanism of imbalance,a hardware control scheme for the midpoint potential balance of diode-clamped topology is introduced.The balance of capacitor voltages is realized by adjusting the charge and discharge process of each capacitor.In addition,this paper introduces the traditional MPC algorithm with current tracking,which efficiently balances the DC-side capacitor voltages and has strong dynamic response capability.However,the cost function needs to be optimized 125 times during each control cycle,so the computation is too complex.To solve this problem,an improved MPC algorithm based on voltage tracking is proposed in this paper.By locating the reference voltage vector and selecting corresponding position vectors and balancing vectors,the improved algorithm can obviously increase the control efficiency.In order to further reduce the contradiction between the output performance and capacitor voltage balance,combined with the six-segment SVPWM strategy,a dual-pulse mapping control algorithm is proposed.Simulation and experiment verify the correctness of the control strategies mentioned above.The amplitude of CMV in NPC/H Bridge five-level inverter is larger.This paper analyzes its mechanism in detail.Two modulation strategies to suppress CMV are introduced,including an improved five-segment SVPWM algorithm,zero CMV vector modulation.By comparison and analysis,zero CMV vector modulation has better effect on system CMV suppression.In addition,an equivalent modulation strategy to completely eliminate the CMV is studied in this paper.The NPC/H Bridge five-level inverter is equivalent to two three-level inverters and the CMV is suppressed respectively.Simulation and experiment verify the effectiveness of the three CMV suppression strategies.