Research On Control Method Of Multi-terminal DC Network Based On MMC

The multi-terminal DC network has played a key role in solving the problem of power transmission and distribution with multiple power supplies or multiple placements,and the connection of multiple AC systems and the isolation of power grids under fault conditions are implemented.In the field of multi-terminal DC network research,the topological structure,complex control system and high protection technology requirements are considered to be the current research problems.Modular multilevel converter(MMC)is considered as a new generation of inverters,which has many advantages such as multiple levels,low harmonic content,lower switching loss,and simple redundant expansion.At the same time,it is rapidly developing toward high voltage and large capacity.Furthermore,it is widely used in the HVDC transmission,DC distribution network,flexible AC transmission systems,custom power and many other fields.In particular,multi-terminal DC network have payed an important role in reducing system costs,improving reliability,and improving overload capacity.In this paper,modular multilevel converter and its control method are studied in detail.Based on the mathematical model of modular multilevel converter,the simulation model of multiple MMC-MTDC systems is established,and the control optimization of multi-terminal DC network system is realized.For the control optimization of multi-terminal DC networks,the following aspects are focused on analysis and research:First of all,MMC technology and multi-terminal DC network control technology,research status at domestic and foreign,and development trends of MMC technology and multi-terminal DC network control are systematically analyzed.MMC control structure and multi-terminal DC network control method based on MMC are summarized.And the problems existing in multi-terminal DC network system based on MMC are analyzed.Secondly,the topology and working principle of the MMC system are mainly studied.Based on that,the switch function model,equivalent circuit model and differential equation model are analyzed in detail.On this basis,the switch function model,the equivalent circuit model and the differential equation model are analyzed in detail.Several modulation methods of MMC system are introduced.The control system structure of the MMC system is introduced,capacitor voltage equalization and loop suppression control system controller structure of MMC system are introduced.Thirdly,according to the mathematical model of MMC system and the basic principle of control system,a converter station controller based on MMC multi-terminal DC network is built,and the MMC-MTDC system-level three-terminal simulation model is established to realize the basic function of master-slave control of the system.Fourthly,the pre-charging and coordinated start control method of the MMC-MTDC system is analyzed in detail.Five-terminal pre-charging and coordinated start-up model of multi-terminal DC network based on MMC is built to realize the pre-charging and starting of the system,reducing the transient energy impact of the system during the startup process.At the same time,for a fault recovery start of several typical short-circuit situations,a control method for controller adjustment suitable for fault recovery is proposed to achieve fast recovery startup.Fifthly,the shutdown control and optimization methods of each stage of the MMC-MTDC system are analyzed.On the basis of the multi-terminal DC network simulation model,several optimization methods for the energy feedback and energy consumption stages of the MMC converter station are simulated to verify the effectiveness of the optimized shutdown control method.At the same time,the coordinated start-stop control method of multi-terminal DC network based on redundant switching is established by redundant control method to maximize energy utilization efficiency and system stability.