Research On Robust Control Of Grid-Connected Current Of LCL-Filtered Photovoltaic Grid-Connected Inverter Based On Active Damping

Under the "dual carbon" goal,China has steadily promoted energy conservation and carbon reduction,coordinated energy security and stable supply and green and low-carbon development,and scientifically and orderly promoted carbon peaking and carbon neutrality.Green development is inseparable from the adjustment and optimization of energy structure and the efficient use of clean energy,which requires the improvement of the standard system to support the construction of new power systems to ensure the development,construction,grid-connected operation and consumption and utilization of large-scale wind power and photovoltaic bases and distributed energy resources.Low-cost,high-efficiency and high-stability photovoltaic inverter is one of the main research directions of photovoltaic power generation systems.Due to the inherent resonance of LCL-type filters widely used in grid-connected inverters,the power quality and stability of the grid-connected inverter system are affected.In this thesis,taking the single-phase LCL type photovoltaic grid-connected inverter system as the research object,aiming at the inherent resonance of LCL-type filters,the limitations of the weighted average current(WAC)active damping control method of the LCL filter,the time delay introduced by digital control and the perturbation of filter parameters,the change of grid impedance and the interference of grid voltage,with the help of improved WAC active damping technology,time delay compensation control strategy and the proposed state and disturbance observer to improve the stability and robustness of the grid-connected current,and improves the grid-connected quality of the grid-connected inverters.The specific research content of the thesis is as follows:(1)The common structure of the filters required for grid-connected inverters are compared.After comprehensive investigation,the LCL filter is selected as the component to filter,as the switching harmonic component in the grid-connected current of the inverter.In view of the system instability caused by the inherent resonance of LCL filter,the damping control methods are introduced into two categories,passive damping and active damping,and their advantages and disadvantages are analyzed respectively,laying the foundation for the subsequent research of WAC active damping methods.Finally,the detailed design process of LCL filter parameters is given.(2)Due to the superior order reduction characteristics,the WAC method in the active damping method is mainly studied.Its limitations are explained from two different angles: the derivation of its transfer function and the analysis of the equivalent circuit.The control loop of WAC active damping is improved in view of this limitation.The improved WAC active damping method can not only maintain the original order reduction characteristics,but also ensure the stability of the controlled object.(3)Since the grid-connected inverter will be affected by the control delay when using digital control,the capacitor current feedback loop is equivalent to a frequency-dependent virtual impedance connected in parallel on the LCL filter capacitor.The negative resistance component of the impedance will become a factor that destroys the stability of the system within a certain frequency range.In view of this,the time delay compensation control strategy is studied,and the simulation experiment of the compensation strategy is verified.The results show the effectiveness of the time delay compensation control strategy.(4)Aiming at the problems of parameter perturbation,grid impedance uncertainty and external interference of grid voltage of LCL filter in the grid-connected inverter system,the designed state and disturbance observer makes lumped estimation of these uncertain disturbance problems,and feeds them forward to restore the system to the nominal system,eliminating the influence of uncertain interference problems on the quality of grid-connected current;Secondly,the state and disturbance observer reduces the number of sensors in the system,can estimate the state of the nominal system.On this basis,the system can be better controlled by WAC active damping and time delay compensation control,reducing the system cost.By changing the parameters of the LCL filter,the size of the grid impedance,and the reference value of the grid-connected current,the designed system is simulated and it is verified that the designed system can not only improve the quality of the grid-connected current,but also make the grid-connected current have better robustness.