Research On Stability Analysis And Robust Control Strategy Of LCL-Type Grid-Connected Inverter In Weak Grid

Driven by the national strategic objectives of "emission peak" and "carbon neutrality",the penetration rate and installed capacity of new energy generation systems are increasing year by year.The power grid shows the "double high" characteristics of high proportion of renewable energy and high proportion of power electronic equipment.Since the distributed generation systems are usually located in remote areas with inverse distribution of load,long distance transmission lines and other reasons make the grid show the characteristics of the weak grid with low short-circuit ratio.The coupling effect between the grid-connected inverter and the phase-locked loop in the weak grid exacerbates the oscillation and instability risk of the system,which seriously threatens the stability of grid-connected system.Accordingly,single-phase LCL-type grid-connected inverters are used as the research object of this thesis.The robust control strategy for grid-connected inverters that can adapt to wide-range variation of the grid impedance in weak grid is researched from the perspectives of both improving the phase-locked loop and the current loop.Firstly,in order to clarify the influence of the phase-locked loop on the impedance characteristics and stability of the grid-connected inverter,a small-signal model of the time-delayed phase-locked loop is derived by adding disturbance variable.Combining the inverter control method,the impedance model of the inverter considering the effect of the phase-locked loop is obtained.The robustness of the inverter in weak grid with conventional time-delayed phase-locked loop is analyzed based on impedance stability theory.Secondly,aiming at the problem that the traditional time-delayed phase locked loop weakens the stability of inverter in weak grid,two robust improved phase locked loop structures are proposed regarding improving the phase locked loop,namely the fixed frequency structure second-order second-order generalized integral phase locked loop and the modified first-order all-pass filter phase locked loop based on the complex coefficient filter.For the two types of phase-locked loops,the parameter optimization design method considering the operating characteristics of the phase-locked loop and the stability margin of the inverter is proposed.And the effect of the phase-locked loop on the output impedance characteristics of the inverter before and after the improvement is compared.It can be shown by the anylysis that the proposed phase-locked-loop structure can guarantee the robust operation of the inverter in weak grid,and the proposed phase-locked-loop structure can also reduce the negative impact of the additional term of the phase-locked-loop on the stability of the inverter,which greatly improves the adaptability of the inverter to the weak grid.The effectiveness of the proposed robust control method for grid-connected inverters based on two types of improved phase-locked loops is verified by simulation and experiment.Finally,in order to further broaden the stability domain of grid-connected inverters in weak grid,an impedance reshaping method based on compensation link is proposed regarding improving the current loop.An optimized design method for the lead compensator parameters is given and the parameter adaptive adjustment strategy based on impedance online measurement techniques is designed.The proposed combined robust control strategy based on the improved phase-locked loop and compensation link further improves the adaptation range of the grid-connected inverter to the grid impedance,while expanding the stability range of the grid-connected system.The effectiveness of the proposed combined robust control strategy for grid-connected inverters is further verified by simulation and experiment.The thesis includes 56 figures,3 tables and 105 references.