Research On Control Strategy Of Flexible HVDC Transmission System Based On MMC

Modular multilevel converter has many advantages,such as high output waveform quality,small switching loss and easy expansion of capacity.It is more and more favored in the field of flexible HVDC transmission as the most promising new generation voltage source converter.Based on the analysis of MMC basic working principle and related characteristics,the control strategy of the MMC-HVDC transmission system is deeply studied in this paper.Firstly,not only the MMC topology,sub-modules working mode and the working principle of the three-phase MMC are analyzed in detail,but also the mathematical model of MMC in d-q coordinates is established.By comparing the principle and characteristics of various modulation modes,the nearest level modulation is selected as the modulation method of MMC-HVDC transmission system.Secondly,the fluctuation mechanism of the sub-module capacitor voltage,the application effect and improvement of the traditional capacitor voltage balance control strategy are analyzed,and four evaluation indicators for comparing the advantages and disadvantages of different capacitor voltage balance control strategies are established.In order to improve the sorting efficiency of the instantaneous capacitance voltage,the Counting Sort algorithm is selected,and a segmented preprocessing method for the capacitor voltage values before sorting is proposed.In order to reduce the MMC switching frequency,the constant threshold method and the priority coefficient method for capacitor voltage balance control strategies are proposed respectively.Combined with the above control strategies and sorting methods,the fast balanced control of the capacitor voltage is realized.Furthermore,an improved cold standby redundant sub-module capacitor voltage control strategy is designed.The Matlab/Simulink simulation results show that the proposed control strategy is effective.Finally,the three-level control structure of the MMC-HVDC transmission system is studied.The converter station-level control adopts direct current control,and a double closed-loop control system capable of independent control of active and reactive power isdesigned.In the control level of the valve group,the mechanism of MMC phase-to-phase circulation is analyzed,and the double-frequency circulation suppressor is designed for the characteristic of high content of double-frequency components in the circulation.In addition,the self-excited start-up control strategy of the MMC-HVDC transmission system was designed by putting a current limiting resistor in the pre-charging stage.The feasibility of the designed control strategy and controller is verified in the simulation model of the two-terminal active MMC-HVDC transmission system built by PSCAD/EMTDC.