Research On Circular MMC Topology And Control Strategy For Flexible Interconnection Of Distribution Network

With the large-scale application of distributed energy in the distribution network,the safe and reliable operation of the distribution network is facing new challenges.Therefore,the realization of the intelligent and flexible distribution network has become the main trend of the future distribution network.Flexible Multi-state Switch(FMS)has the advantages of high flexibility and good reliability,and has been widely studied and applied to the fields of distributed energy consumption and power flow regulation of distribution network.However,with the increase of the capacity and voltage level of modern distribution network,the power devices of the voltage source converter FMS topology are limited in withstand voltage,which affects the application in high-voltage and large-capacity fields.Aiming at this problem,this paper bases on the development requirements of FMS topology compactness and modernization,the twoport hexagonal modular multilevel converter(MMC)and the three-port nonagonal modular multilevel converter are unified with the circular MMC topology family.The circular MMC have the advantages of modular expansion,high compactness and good low frequency performance,which can meet the characteristics and requirements of the distribution network.However,the internal mechanism and the control strategy research are still immature.As a result,its performance and cost advantages have not been fully utilized.Therefore,this paper studies from the following aspects:(1)In this paper,the characteristics of the hexagonal MMC topology and the nonagonal MMC topology are combined,and they are unified with the circular MMC topology family.By analyzing the working state of sub-module,its mathematical model is established,and its basic working principle is analyzed.This provides model basis for power balance analysis and control strategy design.(2)Based on the mathematical model of the circular MMC,the power imbalance problem of the circular MMC is studied in this paper.Firstly,the expressions of port power and bridge arm power are deduced,and the frequency components are analyzed.Secondly,the power balance conditions for the port power and the bridge arm power are studied.Finally,in order to explore the reason of the power unbalance phenomenon,this paper conducts research from both qualitative and quantitative analysis,and obtains the mechanism of the circular MMC generating power unbalance phenomenon and the influence of system parameters on the unbalanced power.(3)Aiming at the power unbalance problem of the circular MMC,the power unbalance compensation condition is deduced.Based on this,the power balance control strategy for circular MMC is designed.In addition,considering that the multi-terminal interconnected FMS can realize uninterrupted power supplies in non-faulty areas through switching control methods.Therefore,this paper takes the three-port nonagonal MMC as the research object,and analyzes its working state under the single-port fault.By optimizing the power balance control strategy of nonagonal MMC,nonagonal MMC minus port operation control strategy is proposed,which realizes the uninterrupted operation of the nonagonal MMC under the condition of two ports.(4)Based on the RT-LAB simulation experiment platform,this paper builds simulation model to verify the feasibility of the circular MMC topology and the effectiveness of the proposed control strategy.The simulation results show that the proposed control strategy can realize the stable operation of the circular MMC as the FMS topology in the distribution network system,and further promote the promotion and popularization of the FMS.