Research On The Low Voltage Ride Through For Ac And Dc Grid-connected Wind Power System Of Permanent Magnet Synchronous Generator

Wind power is the fastest-growing new energy,and its capacity continued to grow rapidly in recent years,the size of the wind farm is also growing.With the increasing share of wind power in the power system,safe and stable operation,grid-connected consumption and long-distance transmission of large scale wind farms have attracted much attention of the wind power industry.Consequently,LVRT(Low Voltage Ride Though)and VSC-HVDC(Voltage Source Converter-High Voltage Direct Current)transmission have become the key technologies of wind power research.Based on analysis and contrast of two prevalent wind generation systems,this paper chooses PMSG(Permanent Magnet Synchronous Generator)as research object,and focuses on studying the LVRT technology of PMSG AC and DC grid-connected wind,generation systems.The main contents of research are summarized as follows.(1)The precise mathematical model of wind turbine is established and DC Motor is used to simulate the wind turbine characteristics by direct torque control.The mathematical model of PMSG and grid-connected PWM converter is presented.The control circuits of GSC(Grid-Side Converter)and MSC(Motor-Side Converter)are presented.to realize the power decoupling control,the maximum wind energy tracking and other functions.Then the DC motor and PMSG are connected as a wind power generation system and the experimental system of DC motor control and PMSG control is set up.The validation of the simulating results is confirmed by physical experiments.(2)The transient characteristics of PMSG wind generation system as the grid voltage sags are analyzed to raise the main problems to be solved during the period of LVRT.After analysis and contrast of the typical LVRT technologies of PMSG wind generation system,LVRT technology based on hierarchical control is put forward.Then the control principle and algorithm process are analyzed in detail.After detailed simulation analysis,the feasibility of LVRT hierarchical control algorithm is verified through the experimental system.(3)A novel topology of PMSG wind power generation system with additional grid side converter is proposed,and the theory of the unified control of wind power grid connected and APF is proposed.The mathematical model of the additional converter is established,and the control algorithm of the APF operation in the steady state is designed.After the transient characteristics of the new PMSG wind generation system during the grid voltage sags are analyzed,the reactive power support principle of the main and additional inverter is put forward.Combined with PMSG power control and DC crowbar circuit,hierarchical LVRT control strategy is constructed and studied in detail by both simulation and experiment.(4)The power transmission methods used on PMSG AC and DC grid-connected wind generation systems are comprehensively compared,which points out that VSC-HVDC transmission is the most feasible for the centralized grid,long-distance transmission of large scale wind farms.The mathematical equations of VSC-HVDC under various reference frames and mathematical model of DC transmission line are established to form the whole model of VSC-HVDC system.Then the control mode and its selection principles of VSC-HVDC converter stations are analyzed to design the corresponding controller.The structure of PMSG wind farms that grid-connected by VSC-HVDC is proposed and the control strategies under steady state of each converter are researched completely.(5)The transient characteristics of PMSG wind generation system grid-connected by VSC-HVDC are analyzed.LVRT control strategy of VSC-HVDC based on voltage control is proposed and the control algorithm of SEC(Sending End Converter)station is designed.Corresponding hierarchical LVRT control strategy of PMSG wind generation system is proposed and formed the LVRT coordination control strategy.(6)Based on the similar idea,LVRT control strategy of VSC-HVDC based on frequency control and the control algorithm of SEC station is proposed.In order to eliminate the communication delay effectively,the DC voltage of transmission line is proposed to be employed as the criterion of grid fault.Then,corresponding hierarchical LVRT control strategy of PMSG wind generation system is proposed and formed the LVRT coordination control strategy that based on frequency control.