Research On Control Technology Of Current Source Photovoltaic Grid-connected Inverter In Weak Grid

As the interface between photovoltaic power generation and the grid,the gridconnected inverter determines the efficiency and performance of the entire photovoltaic power generation system.Compared with voltage source inverters,current source inverters have the advantages of boosting and short-circuit protection,and are more suitable for photovoltaic power generation grid-connected systems.With the increase of grid-connected power levels of distributed power sources and the wide distribution of access locations,power grids increasingly exhibit the characteristics of weak power grids.This paper takes three-phase five-level current source grid-connected inverter as the research object,and conducts research on its control technology under weak grid.First,establish the mathematical model of the three-phase five-level current source grid-connected inverter in the three-phase static coordinate system,the two-phase static coordinate system and the two-phase rotating coordinate system,and design the main circuit parameters and active damping.The carrier phase-shifted SVPWM modulation strategy of the five-level current source grid-connected inverter is studied.According to the vector action time calculation method of the three-level current source inverter SVPWM,the logic switching state and vector of the carrier phase-shifted SVPWM are analyzed.Action time.Aiming at the imbalance of the DC side inductor current of the multi-level current source grid-connected inverter,combined with the carrier phase-shifting SVPWM modulation strategy,the influence of each vector state in the α-β coordinate system on the DC side inductor current is analyzed,and the dynamic redundancy is studied.Vector distribution current sharing strategy,according to the relationship between the AC side capacitor voltage and the DC side inductor current,the redundant vector is reasonably selected to realize the DC side inductor current sharing.Aiming at the traditional control methods of current source grid-connected inverters in weak power grids for poor harmonic suppression and complex parameter design,a control method based on optimization theory in weak power grids is proposed.The error discrete-time state equation of the system in the d-q coordinate system is established,the optimal solution in the form of the value function matrix is solved,and the one-beat advance link in the control command is designed.The stability of the closed-loop system including the grid impedance is analyzed in the z-domain,and the control parameters are designed.The parameters of the control method are simple to modify,and the system can operate stably in both ideal and weak grids,effectively suppressing the CL filter resonance,and suppressing the influence of harmonics and imbalances in the grid voltage on the grid-connected current under the weak grid.Simulation and experimental results verify the effectiveness of the proposed control method.Aiming at the poor adaptability of CSI to grid impedance under weak grids,a current source inverter output impedance model is established,and the relationship between inverter output impedance and harmonic suppression capability is analyzed,and virtual parallel impedance and series-parallel impedance are introduced to the grid.The output impedance characteristics of the inverter are corrected,so that the current source grid-connected inverter can take into account the grid-connected current harmonic suppression capability and the robustness of the control system,thereby greatly improving the adaptability of the inverter to the weak grid.Simulation and experimental results verify the effectiveness of the proposed control method.