Research On Seamless Switching Strategy Of DC Voltage Control For VSC-MTDC System

With the increasing serious problem of energy shortage and environmental pollution,the development and utilization of renewable energy has been carried out in many countries in the world.Multi-level flexible DC transmission system based on modular multi-level converter has attracted much attention because of the feature of high voltage and high-capacity of MMC and the feature of flexible,reliable and economical operation of MTDC system.MMC-MTDC system needs to realize the basic control function in each converter station,it is necessary to have the coordination control strategy between stations to cope with the power disturbance,the change of grid structure and the fault condition,and keep stable operation of the system.In this paper,the coordination control strategy among converter stations of MMC-MTDC system is studied.Firstly,the working principle and characteristics of MMC are analyzed,According to the equivalent circuit of MMC,the mathematical model of multi-terminal flexible DC transmission system is established with the three-terminal system as an example.Then,the hierarchical control structure of MMC-MTDC system is described,and the converter level control and the converter valve level control are introduced in detail.Secondly,the characteristics of coordinated control between converter stations in MMC-MTDC is discussed,The control mechanism and the advantages and disadvantages of the voltage margin control strategy and the voltage tilt control strategy are analyzed emphatically.Finally,on the basis of the above research,this paper proposes a seamless switching strategy of DC voltage control for multi-terminal flexible DC transmission system.The strategy improves the active channels of the outer loop,which enhances flexibility and selectivity of control by setting droop control mode cover fixed DC voltage control mode and constant active power control mode.Only by changing the control mode selection coefficients of the corresponding controllers of serval specific converter stations in a certain principle can realize switching of three kinds of control mode to maintain stability of DC voltage of the system and ensure power balance of the converter stations when facing a variety of circumstances.In addition,A simulation model of five-terminal flexible DC transmission system based on MMC was set up in RT-LAB environment,and the correctness and validity of the proposed strategy were verified.