Research On Control Strategies Of Modular Multilevel Matrix Converter

Modular multilevel matrix converter(M3C for short),as a new type of AC-AC conversion circuit,has advantages that other topologies do not have in the field of medium and high voltage,high power AC transmission and motor drive: The singletube voltage drop and switching frequency of the switch are low,the output waveform step is small,and the system is fault-tolerant and scalable.Therefore,it has attracted great attention from many scholars in recent years.This article has done the following research work:First,the research background of M3 C system is introduced in detail,the application prospects of M3 C in the general background are elaborated,and the advantages of M3 C as a direct AC-AC topology in related fields are highlighted through corresponding comparison.At the same time,the development history and research status of M3 C are analyzed.Secondly,in view of the relatively complicated circuit structure of M3 C,this article analyzes it in detail,including: building a mathematical model of M3 C during normal work,and establishing a corresponding circuit equivalent model based on this;analyzing the M3 C system's working principle and detailed analysis of two important electrical parameters under normal operation of the M3 C system-capacitor voltage ripple of the sub-module and bridge arm circulation.In the analysis of the capacitor voltage ripple of the submodule,the relationship between the capacitor ripple of the submodule and the instantaneous power of the bridge arm is derived,and the expression of the capacitor ripple is derived based on the power expression of the bridge arm.It also analyzes several important factors affecting the absolute value of the capacitor module voltage fluctuation and its relative value.In the analysis of the bridge arm current,nine types of internal circulation paths that are independent of the input and output sides are described,and the mutual decoupling of the bridge arm current input,output,and internal circulation is achieved through the double ??0 transformation,providing a control method for the following theoretical basis.The control of several important electrical parameters,such as the energy balance of the bridge arm,the capacitor voltage of the sub-module,the loop current of the bridge arm,and the input and output current,are analyzed in detail,and the corresponding control strategies are given.At the same time,in combination with the existing control strategies,a comprehensive comparison is made in this paper to determine the control method of combining hierarchical control and double ??0 transformation.While not affecting the control effect of the system,it not only achieves direct control feedback of electrical parameters,but also reduces the complexity of the previous control system on the horizontal and vertical levels of the system,respectively.For the pre-charging and pre-starting processes of the system's sub-modules,this paper proposes a staggered group pre-charging method and a multi-linear reference pre-starting control method,which greatly reduces the overshoot current during system startup.Aiming at the working characteristics of M3 C in special frequency bands,this article analyzes them in detail.For low frequency output and co-frequency output,this article gives corresponding solutions based on the same points and different points: at low-frequency output,fine-tuning the input side current or power to regulate the bridge arm current,thereby achieving suppression of capacitor module voltage fluctuations;at the same frequency output,fine-tuning the input and output current or power to regulate the bridge arm current,thereby achieving sub-module capacitor voltage fluctuation suppression.According to the control strategy given in this paper,corresponding simulation studies are performed for various situations of M3 C under different working conditions,including high frequency conversion,low frequency conversion,and co-frequency conversion.The corresponding simulation results are also given.At the same time,corresponding simulations are performed for transient processes such as sudden changes in capacitor voltage reference,load shedding,and frequency conversion.In addition,in view of the fact that some existing M3 C control methods only stay at the research stage of simulation and hardware-in-the-loop simulation,this article also builds an M3 C prototype to further verify the excellent characteristics of the control strategy given in this paper.