| The multi-terminal flexible dc transmission technology(MMC-MTDC)based on modular multi-level converter has the advantages of flexible operation of multiterminal system,high reliability,multi-source power supply and multi-drop point power supply.It is one of the most effective solutions to the grid-connection and passive power supply of renewable energy and the realization of energy Internet.However,the structure and control strategy of MMC-MTDC are very complex,therefore,the coordination control of MMC-MTDC needs to be studied in depth.As stable dc voltage and reasonable power distribution are the core issues to be considered in the MMC-MTDC control strategy,this paper takes them as the research focus and takes the hierarchical control(valve level control,converter station level control and system level control)of the MMC-MTDC system as the framework to carry out the following relevant research work.Firstly,modular multilevel converter(MMC)is used as the research object.The principle of MMC submodule is analyzed in detail,and the nearest level approach modulation strategy(NLM)is given.The mathematical model of the converter station in the three-phase static coordinate system and the dq rotating coordinate system is established.On this basis,the direct current control method for the converter station is given,and the corresponding double-loop current controller is designed.The proposed control strategy is verified by PSCAD/EMTDC simulation software,and the simulation results show that the designed dual-loop controller can realize independent decoupling control of active power and reactive power of the converter station.And then analyzes the MMC,MTDC system coordinated control strategy: a single point of dc voltage control strategy,and put forward the modified on the basis of this control strategy,and in the PSCAD/EMTDC simulation software to build three end system simulation verify the improved control strategy regardless of the main converter station work in rectifier and inverter state,without communication,still can achieve good dynamic characteristics,improve the flexibility and reliability of multiterminal system,application is wider,dc voltage control is superior to the traditional single point.Finally,this paper proposes a dc voltage droop control strategy,and to make an improvement on the basis of this,the article introduces the first and second in the droop controllers priority to adjust parameters to hang on the demand of the system operation condition of the adaptive adjustment of control,at the same time,the controller can according to the real-time power margin between the converter station size unbalanced power distribution system.A four-terminal HVDC system is built in PSCAD/EMTDC simulation software.The simulation results show that the improved droop control can automatically change the droop coefficient,effectively reduce the dc voltage deviation,reasonably distribute the active power,improve the utilization rate of the converter station,and make the control more stable.When the system is unbalanced by the disturbance power,the controller can allocate the power adjustment task reasonably according to the power margin of the converter station itself.Figure [66] table [5] reference [55]... |
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