Research On Neutral Point Potential And Output Voltage Balance Control Of NPC Three Level Inverter

Neutral point clamped(NPC)three-level inverter has the advantages of simple structure,large capacity,low harmonic content of output voltage and low voltage stress of switch tube.It has gradually become the preferred power electronic converter in photovoltaic power grid,medium and high voltage AC drive and other occasions.However,with the increase of power level,the output efficiency and voltage quality of three-level inverter are required to be higher and higher.In this paper,NPC three-level inverter is taken as the research carrier.The detailed research contents are as follows:1.NPC three-level inverter has the problem of neutral point potential offset because of its own structure,and the neutral point potential is maintained at half of the DC power supply,which can not only reduce the voltage stress of power devices,but also reduce the harmonic content in the output voltage,which is of great significance to ensure the normal operation of the inverter.In this paper,three-level space vector modulation based on reference vector decomposition is used to solve the complex problem of traditional methods,reduce the harmonic content,and combine with neutral point potential control.In view of the problem of low precision in the control of the neutral point potential in hysteresis control,the switching function is constructed between them,which is combined with the precise control based on charge conservation,and the problem of offset and fluctuation of the neutral potential is solved.2.The output voltage quality of NPC three-level inverter is not only affected by its own neutral point potential,but also related to the three-phase load.In this paper,the output voltage imbalance caused by the most common unbalanced load is studied.Firstly,the mathematical model of three-level inverter is established,and the parameters of PI controller are designed according to the model.Then,the reason why the double closed-loop PI control can not control the output voltage under unbalanced load is analyzed,and the traditional voltage control strategy based on double synchronous rotating coordinate system is introduced.However,the strategy has delay and error,and the harmonic suppression ability is not strong.According to the internal model theory,repetitive controller has better harmonic suppression ability than PI controller,and can improve the dynamic performance of the controller combined with PI controller.But because the design of the repetitive controller is affected by the stability of the system,the gain at the resonance frequency is not large enough,which affects the control of the output voltage under unbalanced load.On this basis,the resonant controller is introduced to improve the gain of the system at the resonant frequency.Finally,the proposed control strategy is simulated and analyzed.The experimental prototype of NPC three-level inverter based on d SPACE is built,including the debugging and design of main power circuit,driving circuit and sampling conditioning circuit.The feasibility of the proposed control strategy is verified on the experimental platform.