Research On The Fault Ride-through Method And Coordination Control Strategies Of VSC-MTDC

The high-voltage direct current(HVDC)transmission system based on voltage source converter(VSC)is also known as the flexible DC transmission system.It is an economical,flexible and environmentally friendly transmission mode,which has obvious advantages in connecting renewable clean energy to the grid and power supply to passive loads such as offshore platform and isolated island.VSC-MTDC(Voltage Source Converter Multi-terminal DC System)has multiple delivery and receiving ends.It can realize multi-power supply and multi-load power,which is more flexible and more reliable than the two-terminal flexible DC transmission.All of these advantages have created a broad prospect for its development.Therefore,it is of great significance to study the key technical problems in VSC-MTDC control strategies,such as power coordination distribution and fault ride-through.It is crucial for VSC-MTDC to have a good quality of DC voltage and to distribute unbalanced power in a coordinated manner.The thesis studies the adaptive adjustment of the droop coefficient from the control of a single converter station and the coordination between multiple converter stations.For a single converter station,the main adjustment factor which reflects the operating conditions of the system is designed to solve the relationship between the quality of DC voltage and the power tracking.Considering the power coordination between multiple converter stations,the real-time power margin of the converter station is used as the secondary regulator to avoid overload.Through the primary and secondary adjustment factor,the proposed control strategy implements the adaptive change of droop coefficient.A model of four-terminal VSC-MTDC is established via MATLAB/Simulink.The simulation results show the feasibility and effectiveness of the proposed control strategy.Considering the sensitivity of passive loads to voltage drops and significant DC voltage fluctuation during AC fault,the control strategy of VSC-MTDC for AC fault ride-through is analyzed and a coordinated control method is presented in this thesis.The method improves the controllers at passive loads side and the grid side respectively.At passive loads side,the controller has priority to the supply of reactive current.Only inner current loop is used to improve the fault response speed by fast reactive support.In coordination,the active current takes precedence over the reactive current at the grid side.The controller in the grid side is switched from double-loop control to single-loop control.The single-loop control can help the system to realize the power balance of the system quickly and reduce the fluctuation range of DC voltage during fault.A model of three-terminal VSC-MTDC is established via MATLAB/Simulink.The simulation results show that the proposed strategy realizes AC fault ride-through and the stable operation of the system.