Research On Control Strategy Of Flexible HVDC System Based On MMC

Modular multilevel converter(MMC)has obvious modular structure characteristics,which can continuously output high-level,meet the operation requirements of multiple voltage levels and power levels.Without adding special filter deviceit can complete the operation and meet the needs of high voltage and high power flexible DC transmission.The flexible DC transmission technology based on MMC has been applied to many scenarios,such as urban power supply system,isolated load power supply,passive network power supply,etc.So it will have far-reaching influence and significance to analyze and study its modulation technology and control technology.Firstly,the paper focuses on the structure and working principle of the multilevel converter,and then establishes a complete mathematical model for analysis.Based on the working principle and mathematical model of MMC converter,the paper introduces the selection requirements of MMC power parameters,and provides theoretical basis for the realization of control strategy and modulation strategy.The modulation strategies of MMC topology are compared and analyzed.Based on the characteristics of MMC,the paper selects the nearest level approximation as modulation strategy,and simulates NLM.Then,MMC control strategy is designed.The inner and outer rings are respectively equipped with DQ axis current decoupling and fixed active power,reactive power and constant AC voltage and DC voltage.In order to make MMC have better output waveform,analyze the principle of circulation generation,improve the design of loop suppression controller,effectively reduce the voltage fluctuation,and design the voltage balance strategy of sub module,and propose an optimal sorting algorithm based on NLM modulation.The simulation results show that the optimization strategy is effective.Finally,in the two-phase static coordinate system,combined with the advantages of the classical double closed-loop vector control and model predictive control strategy,based on the idea of functional equivalence,the double closed-loop vector control is divided into two model prediction links,which forms an improved model predictive control.In view of the problems of large computation and complex weight factor adjustment in traditional predictive control,two different control command acquisition methods are studied.By means of discrete MMC mathematical model,the output prediction model of MMC valve side voltage control command is established,and the equivalent function of the inner loop controller is realized,and the inner loop control function is integrated into a controller.The effectiveness of the control strategy is verified from the aspects of power tracking,negative sequence current suppression and power fluctuation suppression.