Research On Start-up And Coordinated Control Strategy Of MMC-MTDC System

Modular Multilevel Converter based Multi-Terminal Direct Current(MMC-MTDC)has the advantages of both MMC and MTDC,which is one of the reliable choices for long-distance and large capacity power transmission.The stable operation of MMC-MTDC needs the corresponding coordinated control strategy.The proper startup method is the basis for the stable operation of the system.Therefore,it is of great practical significance to study both of them.This research mainly focuses on the start-up process of MMC-MTDC system and the coordinated control strategy between converters in normal operation state.(1)After analyzing the topology and working principle of the FBSM,HBSM and the hybrid MMC,the mathematical models of MMC converter in three-phase coordinate system and dq coordinate system are established.Based on that,a double closed-loop controller suitable for MMC is established.The carrier phase-shift modulation strategy is introduced.According to the switching characteristics and the the number of two kinds of sub-modules,a suitable carrier phase-shift modulation strategy for hybrid MMC is described.(2)The charging speed of the FBSM in the uncontrolled charging stage is twice that of the HBSM.Therefore,at the end of the stage,there are problems of unbalanced sub-module voltage,under-voltage of the HBSM and low DC voltage.Then an improved charging strategy is proposed to solve the above problems,and the restricted conditions that should be followed when adopting this strategy are analyzed.The reason of the inrush current in the controllable charging stage is analyzed,and the corresponding suppression strategy is given according to the influencing factors of the inrush current.In MMC-MTDC system,the charging speed of FBSM is more than twice of that of HBSM in the AC side MMC during uncontrolled charging process.According to this change,the restriction conditions in the improved charging strategy are adjusted to make it suitable for the startup process of MTDC system.Then the specific process of the three-terminal system startup are described.Through the simulation,the feasibility of the improved pre-charging strategy of MMC converter is verified and the start-up process of three-terminal system is completed according to the specific steps.(3)The working principles,advantages and disadvantages of master-slave control,DC voltage margin control and DC voltage droop control are analyzed.The traditional voltage droop control is improved by introducing the adaptive factor.The improved control strategy can automatically adjust the droop coefficient according to the current power margin and operation conditions.When the power fluctuates,the system will be closer to the rated value of the DC voltage.In turn,the voltage deviation is reduced.Through the simulation,the effectiveness and superiority of the improved DC voltage droop control in the MMC-MTDC system are verified.