Research On Transient Stability Analysis Of Grid-connected Inverter Of Wind Turbine

The proportion of wind power and other new energy sources in the power generation system is increasing,and their applications are becoming more and more widespread.Because wind farms are generally located in remote areas and are far apart from each other,in the process of wind farm interconnection,more power electronic devices and transformer devices are applied to the transmission lines,and the transmission lines are generally longer,accompanied by the larger line impedance,more power electronic switching devices on the system pollution and affect the stability of the system.As an interactive device between wind power plant and power grid,grid-connected inverters of wind power system will be affected by the interaction between power plant and power grid.At the same time,the stability of grid-connected inverters also affects the stable operation of wind farm and power grid.Therefore,it is necessary to analyze and control the transient stability of grid-connected inverters.Specific research contents are as follows:Aiming at the harmonic instability of grid-connected inverters,the stability of grid-connected inverters was analyzed from the perspective of impedance stability.An on-line identification method of grid impedance was proposed,and a current control strategy based on two-phase rotating coordinate system was proposed.The influence of current loop was only considered in the impedance modeling of traditional grid-connected inverters.On this basis,a detailed output impedance small signal model considering delay,filtering,decoupling and other segments was established.According to the impedance small signal model,the stability of the system was analyzed and the stability criterion was proposed.The influence of PLL bandwidth and PI controller bandwidth on output impedance was analyzed,and two improved methods to improve system stability margin were proposed.The validity of the two improved methods was verified by building simulation models.The difference between traditional master-slave control and droop control and virtual synchronous generator control was compared.The small signal model of droop control was established,the control strategy of current-controlled virtual synchronous generator was proposed,the small signal model of current-controlled virtual synchronous generator was established,and the stability of the small signal model of the system was analyzed.The influence of control parameters on the eigenvalue of the system was analyzed.The simulation model of the current-controlled virtual synchronous generator was built.The simulation results verified the accuracy of the control strategy of the current-controlled virtual synchronous generator proposed in this paper.The control principle of three-phase synchronous phase-locked loop was introduced,the small signal model of three-phase synchronous phase-locked loop was established,and the working principles of generalized second-order integrator,double generalized second-order integrator and frequency-locked loop were analyzed.Aiming at the disadvantage of traditional phase-locked loop(PLL)in resisting DC bias interference,this paper quantitatively analysed the causes of poor DC interference ability of generalized second-order integrator and its influence on unbalanced phase-locked loop(PLL).An improved double generalized second-order integrator type PLL structure was proposed.The performance of the improved PLL was verified by simulating the power grid fault through simulation experiments.The model of grid and grid-connected inverters was built based on d APACE.As simulator,the controller model of grid-connected inverters was built based on FPGA.As controller,the proposed method to improve the stability margin of the system was verified by the hardware-in-the-loop experiment of the controller,and the parallel operation of two grid-connected inverters under different control strategies was verified.The improved control strategy of double generalized second-order integrator was validated by the interaction of rows.A back-to-back converter was built,and the controller of the converter was built based on FPGA.The experiment of off-grid and grid-connected converter was carried out,and the performance of the back-to-back converter was tested and improved.