Efficiency Optimization And Midpoint Potential Control Strategy Of Three-Level Neutral Point Clamped Converter

In multi-level converters,three-level Neutral Point Clamped(NPC)converters are widely used due to their high output power,low device stress and low output waveform distortion.Modulation strategy and DC side midpoint potential control strategy are the key and difficult issues in the research of three-level NPCi converters.The Double Modulation Wave Carrier-Based Pulse Width Modulation(DMWPWM)strategy can achieve midpoint potential balance in full modulation and full power factor range,but it has high switching frequency and low system efficiency disadvantages.In view of the shortcomings of DMWPWM strategy,this paper takes three-level NPC converter as the research object,based on DMWPWM strategy,with efficiency optimization and midpoint potential closed-loop control as the research content,including:new type of three-level NPC c.onverter Modulation strategy,efficiency optimization modulation strategy and improvement strategy,and midpoint potential control strategy research,the specific research contents are:Firstly,a new modulation strategy for a three-level NPC converter is proposed.A three-phase sinusoidal voltage is used as a group of modulated waves.Another set of modulated waves is selected by different zero-sequence voltage duly cycles to ensure a three-level NPC converter.The midpoint potential is balanced,and the analytical formula of the modulation signal of the new modulation strategy is derived in different duty cycle ranges.Secondly,an efficiency-optimized modulation strategy for a three-level NPC converter is proposed.By introducing an odd-period DMWPWM strategy+an even-period DMWPWM strategy to form an efficiency-optimized modulation strategy,the modulation wave is reduced to the maximum value of the system.Therefore,the analytical solution of the modulated wave of the efficiency-optimized modulation strategy is derived,and the analytical formula of the modulated wave of the improved efficiency optimization strategy is derived by injecting different zero-sequence voltages in different ranges.Finally,the efficiency optimization modulation strategy can not suppress the DC capacitor voltage fluctuation and offset caused by non-modulation factors.By introducing the Model Predictive Control(MPC)to select different groups of modulation waves in real time,the efficiency optimization is realized.DC voltage and voltage closed-loop control of the modulation strategy.The simulation and experiment prove that the proposed new modulation strategy can achieve the purpose of modulating and balancing the DC capacitor voltage.The efficiency optimization and improved modulation strategy can improve the system efficiency and the midpoint potential balance compared with the DMWPWM strategy.The midpoint potential model Predictive control can suppress DC-capacitor voltage fluctuations and imbalance problems caused by non-modulation factors.