Stability Of Wind Power Integration With VSC-HVDC System

With the continuous development of renewable energy and the continuous progress of longdistance transmission technology,the stability and power quality brought by grid connection of renewable energy have also received much attention.Regions rich in wind resources are often far from the main power grid,and the electrical energy generated by the wind farm needs to be sent into the power grid from a long distance.AC transmission is a commonly used wind power grid connection method,but when the transmission distance reaches a certain value,it is easy to cause the terminal voltage to rise beyond the boundary.At this time,the use of flexible DC transmission grid connection scheme is a good choice,especially for offshore Wind power can reduce costs.Due to the large output power and high voltage level of large wind farms,flexible DC converter stations need to use Modular Multilevel Converter(MMC).At present,a number of wind farms via flexible DC transmission grid-connected projects have been constructed at home and abroad.However,the theoretical research on the impact of wind-flexible grid-connected systems on power system stability has just begun.Whether the actual grid-connected system generates oscillations and the conditions for the oscillations is a question that needs to be explored,and its uncertainty greatly hinders the application of wind power-flexible grid-connected projects.This article will mainly study the stability of the wind farm connected to the grid via the MMC-HVDC transmission system,the main work is as follows:(1)On the basis of summarizing the current status of domestic and foreign wind power and flexible DC grid connection technologies and their operation control methods,the mathematical model and MATLAB simulation model of the wind farm with doubly-fed unit are established.On the Simulink platform,the control strategy and low voltage ride-through performance of the wind turbine are verified.(2)Established the mathematical model of MMC flexible DC transmission system,discussed its modulation strategy,MMC rectification and inverter principle,and the control strategy in wind farm and flexible DC transmission system;Simulink simulation model was built to verify the windflexible system Operating characteristics.(3)For a single wind farm flexible DC transmission grid-connected system,a small signal stability impedance model for the interconnected system of wind farm and flexible DC transmission system is derived and analyzed.Through time domain simulation,the correctness of the impedance model analysis is verified.(4)The stability analysis method of multi-wind farm interconnection via multi-terminal flexible DC connection system is discussed,a simplified scheme of the impedance model of the interconnection system is proposed,and its stability criterion is derived,analyze the stability of the overall system.(5)The stability of the wind power-flexible system is analyzed,the coordinated fault ridethrough control strategy of the wind turbine and the flexible DC converter station is discussed,and the effectiveness of the control strategy is verified by simulation.