Control System Of Modular Multilevel Converter Based On EtherCAT

With well expansibility,great redundant characteristics,low harmonic output and less switching frequency,the Modular Multilevel Converter(MMC)has become a researching focus in recent years.After the research has been developed for more than one decade.In the project,in order to achieve the goal of high voltage with large capacity and high quality output waveform,MMC requires a few or even hundreds of sub-module cascade,which brings a series of difficulties.Firstly,single controller is difficult to deal with MMC,it need a method using multi control control strategy to achieve communication,at the same time,we need a communication mode with reliable,high speed and large capacity for data communications,resulting in many control difficulty increases.Then Not only to achieve the real-time control through the high-speed communication between the module and the goal of the controller,but also to achieve precise synchronization between the carrier signal of each controller,both complexity and cost which will cause the control system greatly increasedFirst,the paper analyses the topological structure and the operation principles of MMC,builds the equivalent circuit diagram and mathematic models.Then presents the relationship between the upper and lower bridge arm voltage,the upper and lower arm current,the circulating current,the AC output voltage and the output current of MMC and give the common mode equivalent model and the equivalent model and analyze the MMC modulation strategy of NLM modulation And introduces the working principle of EtherCAT technology and the distributed clock function Then studies the cause of the SM voltage fluctuation and the second harmonic circulation in arm.Studies the reaso n and the danger of the SM capacitor voltage unbalance,based on theoretical analysis and derivation,designs the External energy controller?Internal energy controller?Second harmonic circulation controller,and put forward a SM capacitor voltage independent control program based on the sorting algorithm.Finally,In order to investigate the correct of the theoretical analysis,we set up a three-phase MMC experimental platform,which includes: the design of MMC hardware system and software system.The hardware system design includes MMC hardware system framework design,control system circuit design,main circuit design.The software system design includes the ETherCAT program timing analysis,communication protocol and software design of the two layer controller.The modulation strategy based on carrier cascade,arm energy summation and difference control algorithm and the grid-connect experiment of SM capacitor voltage sorting algorithm,the program is proved true by the result of the above experiments.