Research On Control Strategy Of LCL Three-phase Photovoltaic Grid-connected Inverter

In recent years,distributed generation has developed rapidly due to its flexible power generation and high energy utilization.Grid-connected inverters serve as power interface devices for distributed generation and main grids.The performance of these grid-connected inverters directly affects the reliability and grid-connected current quality of distributed generation systems.LCL-type grid-connected inverters is a research hotspot at present due to its superior high frequency harmonic suppression capability.Taking into account distributed generation systems in remote areas,long power transmission lines and the presence of a large number of transformer links introduce a non-negligible equivalent grid impedance to the grid,resulting in the grid to have weak grid characteristics.This dissertation takes the LCL filter-based three-phase photovoltaic grid-connected inverter as the research object,aiming at the constraints of the parameters of the LCL filter,the suppression method of the resonance peak of the LCL filter,the influence of the grid voltage background harmonic on the grid-connected current,and stability analyze of grid-connected inverter in weak grid.Firstly,according to the topology of LCL type three-phase grid-connected inverter,the mathematical model of three-phase stationary abc and two-phase stationary ?? coordinate system is established.According to the limiting condition of LCL filter,the parameters of LCL filter are designed and parameters are optimized.Secondly,in order to solve the resonance peak problem produced by LCL filter,capacitive current feedback active damping method is used,and the design method of capacitive current feedback active damping coefficient is given.A dual-loop control method for the inner loop of the capacitor current and the outer loop of the grid-connected current is introduced.The quasi-PR controller is used as the outer loop controller to track the reference current and achieve better grid connection performance than PI control.Parameters of quasi-PR current controller are designed,do not need to repeatedly try to meet the design requirements.In order to suppress the influence of grid voltage background harmonics on the grid-connected current,a full-feedforward control strategy for grid voltage is deduced,and the suppression effect of the full-feedforward of grid voltage functions on the grid background harmonics are analyzed.Finally,the three-phase LCL grid-connected inverter under weak grid condition is modeled and analyzed.A weak grid model was constructed to analyze the specific influence of the grid impedance on the LCL-type grid-connected inverter system,including the influence of grid impedance on the resonant characteristics and stability of the LCL filter.By establishing a small-signal model of the grid-connected inverter,the stability criterion of an impedance analysis method is introduced,which can be applied to determine the stability ofthe grid-connected system under consideration of grid impedance.In order to improve the quality of grid-connected current waveforms and the stability of the system,this dissertation proposes a parallel virtual impedance method.The simulation results show that the virtual impedance method proposed in this dissertation can still guarantee the stability of the grid-connected inverter when the grid impedance varies widely,and can ensure that the grid-connected current meets the harmonic standards.