| Flexible DC transmission(VSC-HVDC)is a high-voltage direct current transmission technology based on voltage source converter technology.VSC-HVDC not only has the advantages of traditional HVDC(LCC-HVDC),but also has the advantages of power supply to long distance small isolated load,multi terminal power supply,flexible and diverse control methods,reduce the flicker of transmission line,and improve the power quality.Therefore,it is of great significance to study the transient characteristics of VSC-HVDC grid connected system with wind power.In order to improve the transient stability operation characteristics of flexible DC transmission VSC-HVDC grid connected system,a coordinated control strategy between grid connected VSC-HVDC DC voltage and VSC is proposed.First,the mathematical model of the double fed wind turbine(DFIG)and the control mode of the rotor side and the net side are designed,and the basic principle of the maximum wind energy tracking control and the running characteristics of the wind turbine are discussed in detail,and the modeling of the control system of the pitch angle is discussed in detail.Secondly,the working principle,basic structure,technical features and application fields of VSC-HVDC are discussed in detail,and the control mode of VSC-HVDC is summarized;the mathematical model and control strategy of VSC-HVDC are written out in detail;in order to solve the problem of VSC-HVDC DC voltage instability in grid connected network,a coordinated control strategy between VSC-HVDC DC voltage and VSC is proposed,and the control strategy is studied;the mathematical model and control mode of doubly fed induction generator(DFIG)in grid connected wind farm are studied.According to the basic structure of VSC-HVDC,the model of wind power grid connected system is established on the MATLAB/Simulink platform,and the transient characteristics of the system under wind speed change,wind electric field side and the shunt three-phase symmetrical grounding fault are simulated and analyzed,and the coordination control strategy is used to comparison of experimental simulation.The results show that the system can quickly restore stability when wind speed changes,wind farm side and node failure,and the adopted coordinated control strategy effectively restrains the sudden increase of DC voltage,thus enhancing the transient stability characteristics of the system.VSC-HVDC technology will be a choice for the ideal grid connected mode for large wind farms in the future.This technology not only improves the transmission capacity,but also enhances the stability of the system and improves the quality of power. |
