Research On Control Strategy Of Modular Multilevel Matrix Converter For Wide Frequency Operation

As a highly potential member of modular multilevel converter(MMC),modular multilevel matrix converter(M3C)can realize direct AC/AC conversion,and has the excellent performance of high withstand voltage,modularity,and easy extensibility.It has broad application prospects in wind power conversion systems,distributed micro-grids,large-capacity electric drives,etc.Aiming at the problems of high coupling converter system,complex control freedoms,and different operating characteristics at different output frequencies,this paper studies the control and optimization strategies of M3 C when the output voltage changes in a wide frequency range.The main contents are shown as follows:First,the basic mathematical model of M3 C is introduced.The mathematical expression of capacitor voltage oscillations under each output frequency range is deduced.This paper realizes the system decoupling of input system,output system,circulating currents and common-mode voltage through double αβ transformation.The physical meaning of each power component after decoupling is analyzed.Combined with the power-capacitor voltage model,the relationship between the frequency components of capacitor voltage ripples after double αβ transformation and the input/output systems,circulating currents and common-mode voltage is clarified,which provides the theoretical basis for the decoupling control and capacitor voltage oscillation suppression of M3 C.Then,the basic control strategy for wide frequency operation is designed.Based on the double αβ decoupled model,basic control strategies for stable operating conditions are designed,including input and output side control,capacitor voltage balance control,circulating currents control and modulation strategies.This paper especially analyzes the low-frequency pulsations under equal-frequency condition in detail,and utilizes the diagonal transformation matrix to establish two decoupled circulation systems with clear physical meanings.The closed-loop control strategy of capacitor voltage pulsations under equal-frequency condition is designed,which can achieve high control accuracy and excellent transient-steady performance.Finally,the designed M3 C wide frequency operation control strategy is further optimized.Under equal-frequency unstable condition,aiming at the control components introduced to suppress the low-frequency pulsations,this paper reduces additional capacitor voltage oscillations and optimizes the current stress by changing the amplitude of injected common-mode voltage and the waveform of circulating currents.Under low/high-frequency stable conditions,this paper takes the suppression of capacitor voltage oscillations as the optimization goal.The closed-loop control strategies based on circulating currents and common-mode voltage injection is designed respectively,based on which the control effects of two methods are compared.Besides,in order to reduce the voltage overshoot during the startup process,a four-step precharge strategy is adopted on the basis of the traditional two-step precharge strategy.Based on the above research,The M3 C simulation model and experimental prototype are built in this paper.The simulation and experimental results verify the effectiveness of the proposed control and optimization strategy.