Research On The Capacitor Voltage Balancing Control Strategy Of The MMC-HVDC

The flexible HVDC system based on Modular Multilevel Converter(MMC-HVDC)topology is a new HVDC topology,which is generated in the background of the international energy adjustment and the widespread application of new energy power generation.Compared with other multi-level inverter,the topology has many incomparable advantages in reducing switching losses,capacity upgrades,electromagnetic compatibility,fault management etc.Focused on the MMC control system to carry out related research is one of the necessary conditions for MMC engineering applications,which has important practical significance.Each sub-module DC capacitor of MMC mutually is independent.Due to the module parameter deviation,charge and discharge time difference and the losses difference etc,the capacitor voltage imbalance will be appeared easily in actual operation,If we have not take effective capacitor voltage balancing control measures,it will result the output voltage waveform distortion,affecting the stability of the working device,even lead to the collapse of the entire system does not work properly.To solve the above problems,a mathematical model of MMC-HVDC system is firstly established in this paper and its basic working principle is analyzed.Then MMC control system design is carried out from two aspects of the system-level control and device-level control.The voltage outer loop and inner current loop controller are completed.A capacitor voltage balancing control strategy of MMC sub module is proposed based on model prediction by further analysis the disadvantage of the traditional control method.The AC side current control effect is put on the most important position.Additionally,the circulating current suppressing measure is introduced,and the priority level of sub-module voltage was assigned in order to further balance the sub-module capacitor voltage and relieve pressure of the processor.At last,combined with the system charge and discharge state,the impulse is redistribution,and the superiority and effectiveness of the proposed control strategy is verified by the simulationFinally,in the background of a set of 20 KVA MMC-HVDC experimental device,therelevant experimental procedure is completed in TMS320F28335 control chip,and the preliminary verification of the test circuit is carried out.The design of the controller is verified by experimental results.