| It is the development goal of China's power grid to build a reliable,strong economic and environmental protection,high efficiency and advanced intelligent transmission and distribution system.Flexible HVDC technology is the only way to realize the construction of modern power grid.Modular Multilevel Converter(MMC)is a hot research topic in recent years.Modular design is adopted to improve the voltage level through cascade connection,the number of levels is high,the quality of transmission power is high,and the filter device can be omitted.etc.The multi.terminal direct current based based on modular multi.level converter(MMC.MTDC)not only has the advantages of high.voltage and large.capacity of MMC,but also has many advantages,such as high flexibility,reliability and economy compared with traditional DC transmission technology.MMC.MTDC has better technical advantages and development prospects in the HVDC field.In this paper,MMC's modulation,capacitance equalization and vector control are analyzed according to the topological characteristics of MMC,and the control strategy of MMC and MMC.MTDC is studied.The specific structure of this thesis can be described as follows:1)The general situation of HVDC technology is expounded.This paper introduces the research status of MMC and the research status and engineering application of MMC.MTDC with MMC structure,and presents the typical DC transmission project.2)Establish the mathematical model of MMC's sub.module.Start from the structure of MMC,the structure and working characteristics of the three sub.modules are analyzed briefly,including the half.bridge sub.module,full.bridge sub.module and clamped sub.module.Then take the half.bridge sub.module as an example,And the relationship between the current direction of the half.bridge sub.module and the capacitor voltage charge and discharge of the sub.module is studied.According to the switching principle of the sub.module,the switch mathematic model of the MMC and the AC and DC model of the MMC AC.DC decoupling are established.3)Study the modulation technique of MMC arm.Carrier Phase.Shift PWM(CPS.PWM)and stepped.wave nearest.voltage modulation(NLC.PWM)are mainly analyzed.In order to solve the capacitor voltage fluctuation problem of MMC sub.module.The improvement of the CPS.PWM modulation strategy and the implementation of the capacitor sequencing algorithm for the NLC.PWM modulation strategy are realized by introducing the closed.loop current,which achieves the capacitor voltage balance and provides a solid foundation for the subsequent realization of the MMC control system and simulation verification.4)MMC closed.loop vector control.The dynamic time domain mathematical model of MMC in the dq coordinate system is established,and the inner loop and the outer loop controller of MMC are established to form the closed loop control.The simulation results of the double.ended MMC system show that the closed.loop vector control is feasible.5)MMC.MTDC control strategy in the study.Based on the study of the characteristics of MMC.MTDC,considering the complexity of MTDC system control,The main and subordinate control,DC voltage deviation control and DC voltage slope control are studied respectively.Combined with the advantages and disadvantages of the three control strategies,a DC voltage hybrid control strategy is proposed,which combines the advantages of DC voltage deviation and DC voltage slope control strategy.The simulation results are validated by simulating the four kinds of the four projects in Zhoushan,and the simulation model is built in MATLAB/Simulink to verify the effectiveness of the proposed control strategy. |
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