| Modular multilevel converter(MMC)is the preferred topology in the VSC-HVDC projects owing to its modularity,expandability,good harmonic characteristic and high system efficiency.There are hundreds of sub-modules in the practical MMC projects,whose control system needs to manage all sub-modules.The distributed control strategies,as a promising solution for managing MMC system with numerous sub-modules,have attracted increasing attention from researchers in recent years.Taking modular multilevel converter as the research objective,the problem analysis,theoretical modeling,performance evaluation and optimization improvement of the MMC distributed control system are carried out in this thesis,which has certain theory value and application value.The main contents are as follows:Firstly,the mathematical model,control objectives and control system of a single-phase MMC topology are established and introduced,and on this basis,the existing distributed control strategies are analyzed and compared.Secondly,for a novel MMC distributed control strategy(in which the control algorithm complexity of controller and the communication data of are independent of the number of submodules),its control conflict is studied in this thesis.The equivalent model of the whole MMC distributed control system is established and simplified.The coupling interactions among all control loops are elaborately studied based on the established model,and the root cause of control conflict in the MMC distributed control system is revealed.Then,a control parameter design principle is proposed to constrain the controller parameter selection of capacitor voltage control loops,in order to suppress the control conflicts and improve the control performance of MMC distributed system.model.The correctness of the coupling interaction analysis and the control conflict analysis,and the effectiveness of the control parameter design principle are verified based on the single-phase MMC simulation.Thirdly,considering the shortcomings of the existing MMC distributed control strategies,a distributed control strategy based on the capacitor voltage round-robin transmission mode is proposed in this thesis to avoid control conflicts and reduce the communication burden.the task assignment and two round-robin transmission modes in the proposed distributed control system is presented.The phase average capacitor voltage expression,the transmission frequency with filtering function,and the capacitor voltage control loop model in the two t round-robin transmission modes are derived and established.On this basis,the capacitor voltage control performance of the distributed control system under different transmission mode and frequencies is evaluated.The single-phase MMC simulation is adopted to confirm the performance of the proposed MMC distributed control system and its feasibility at low switching frequency.Finally,the effectiveness of the proposed control parameter design principle,and the performance of the MMC distributed control system based on round-robin transmission mode are verified in the MMC experimental platform. |
