| With the international energy adjustment and the widespread application of new energy power generation,a flexible HVDC system based on Modular Multilevel Converter(MMC-HVDC)topology is posed.Compared with traditional voltage source DC transmission(VSC-HVDC)system,and the modular structure makes it have advantages of multilevel output,high quality output waveform,low breaking frequency,low switch consumption,and strong versatile structure etc.Therefore focused on the MMC control system is one of the necessary conditions for MMC engineering applications,which has important practical significance.Each DC capacitor of the MMC sub-module is independent of each DC capacitor.Due to the data of the sub-module difference,charge and discharge time difference and the damage degree difference etc,It is easy to produce capacitor voltage imbalance,if we take no effective capacitor voltage balancing control measures,it will not only result the output voltage waveform distortion,affecting the stability of the working device,will lead to the collapse of the whole system stop working.To solve the above questions,first,establish a mathematical model of MMC-HVDC system in this paper and analyze its basic working principle.Then,to achieve the outer loop and inner loop controller,working on the control design of the MMC-HVDC system,witch is carried out from two aspects of the voltage outer loop control and inner current loop control.A capacitor voltage balancing control strategy of MMC sub module is proposed based on bias control and Shell sorting algorithm by further analysis the disadvantage of the traditional control.The number of on-state SMs of the previous and the present control cycle were compared and the deviation between capacitor voltages and their average value in bounds was detected to determine whether to recalculate the trigger pulse,using Shell sorting algorithm in a different state of bridge arm when meet the corresponding decision condition of sub-modules sorting,according to the sequence arrangement after input.Finally,a modular multilevel converter model was set up on the MATLAB,the feasibility and the correctness of the design control method is tested by experimental results.With the improved control strategies,better capacitor voltage balancing performance has been reached. |
