Research On Advanced Control Of Modular Multilevel Matrix Converter

In order to cope with energy crisis,clean energy,such as photovoltaic power generation and wind power,has been vigorously promoted around the world.Offshore wind power has become a clean energy actively explored and developed by many countries bordering the sea because of its advantages of small footprint area and large resource potential.To solve the problem of offshore wind power grid connection and large-capacity long-distance transmission,fractional frequency(low frequency)transmission system(FFTS)based on high-voltage and high-power electronic converter has become an important research direction of experts and scholars in related fields.Modular multilevel matrix converter(MMMC)adopts modular cascade structure,combining the characteristics of traditional multilevel converter and matrix converter,realizes medium/high voltage applications of low voltage power devices.At the same time,MMMC has low harmonic content,high reliability,good extension redundancy and direct AC/AC conversion,so it catches widespread attention in the fields of motor driver,fractional frequency transmission and so on.In this thesis,MMMC is taken as the research object.First,the internal structure and working principle of MMMC are analyzed in detail,and the generation mechanism of capacitor voltage ripple is deduced.Then,the double-Clarke transformation of the MMMC mathematical model in rectangular coordinate system is carried out,and the decoupling model of AC side in αβ0 coordinate system is obtained,which realizes the decoupling of input side component,output side component,circulation component and common-mode voltage.According to the expressions of the bridge arm voltage and the bridge arm power in the αβ0 coordinate system,their physical meanings are analyzed and deduced.Based on the decoupling model of MMMC,this thesis introduces the traditional double-closed loop decoupling control strategy based on PI,including capacitor voltage equalization control,inner loop current controller and outer loop controller,and improves them respectively.Aiming at the large time complexity of traditional sorting algorithm of capacitor voltage equalization and the useless sorting of parts of modules,a capacitor voltage equalization strategy based on BFPRT algorithm is proposed to reduce the sorting time complexity of the system and the requirements on hardware.On this basis,the switching frequency is optimized and the switching loss is reduced.The traditional inner loop current controller based on PI adopts the cascading form,which is difficult to set parameters and easy to overshoot.In this thesis,a model predictive control method with simple structure and flexible control is introduced as the inner loop controller.The continuous time model of MMMC is discretized,the prediction model of bridge arm current is deduced,and the feedback correction term is added to improve the tracking performance and robustness of the control system.The performance of the outer loop control has a direct impact on the inner loop current control.Aiming at the problem that the traditional PI outer loop controller has high parameter tuning requirements and can not satisfy the fast response of the inner loop model predictive controller,a strategy of outer loop fuzzy PI control combined with inner loop model predictive control is proposed.According to the parameter characteristics of controller,an outer loop fuzzy controller is designed to achieve real-time parameter tuning,which further improves the robustness of the system.Finally,the MMMC-FFTS model is built on the simulation platform,and the simulation comparison between the proposed methods and the traditional control strategy is conducted to verify the effectiveness and superiority of the control strategy designed in this thesis.