Research On Control Strategy Of LCL Photovoltaic Grid-connected Converter Under Weak Grid

New energy has gradually become the deployment direction of my country's energy strategy because of its clean and renewable characteristics.The development of photovoltaic grid-connected converters has also received extensive attention.Due to the geographical distribution of photovoltaic power generation,the power grid gradually shows the characteristics of a weak grid.The control strategy under ideal power grid conditions,directly applied to weak power grid conditions,may reduce the stability of the system and even cause system shock,which seriously affects the safe and stable operation of the grid-connected converter.This paper studies the control strategy of grid-connected converters in weak grids,analyzes the factors affecting the stability of grid-connected converters,and proposes a control strategy for LCL photovoltaic grid-connected converters suitable for weak grid conditions.Firstly,the mathematical model of LCL grid-connected converter under weak grid is established.The control models in different coordinate systems are deduced and the Nyquist criterion is applied to analyze the stability of the LCL grid-connected control system.The small signal model of phase-locked loop is derived based on the expression method of complex vector.According to the mathematical model,it is analyzed that under weak grid conditions,the voltage feedforward technology that suppresses the background harmonics of the grid will additionally introduce a positive feedback loop of the gridconnected current due to the existence of grid impedance,which will reduce the stability of the grid-connected system.Aiming at this problem,a feedforward strategy for grid voltage improvement based on capacitive current feedback compensation and stable margin is proposed.The analysis and simulation results show that this strategy can well suppress the interference of the grid background harmonics at the grid-connected coupling point,and it can also significantly improve the stability margin of the gridconnected system under weak grids.The negative influence of the asymmetrical structure of the phase-locked loop on the stability of the LCL grid-connected converter system under the condition of weak power grid is analyzed.At present,the d-q impedance matrix model is commonly used in modeling methods for the study of the influence of phase-locked loop on system stability.However,the d-q impedance model needs to apply the generalized Nyquist criterion in the stability analysis,which is not conducive to the design of the control system.Complex vector modeling can eliminate the coupling between d and q axes from the form,establish a single-input single-output complex vector model,simplify analysis,and have natural advantages for system modeling that takes into account the effects of phase-locked loops.However,the current research direction focuses on Research on complex vector modeling of L-shaped grid-connected converter.In view of this,this paper establishes the output impedance model of the LCL grid-connected converter based on the complex vector expression method,and analyzes the influence mechanism of the phase-locked loop to reduce the system stability margin and gridconnected power quality.Aiming at the negative influence of the phase-locked loop in the weak grid,a control strategy based on disturbance voltage feedback compensation to suppress the influence of the phase-locked loop is proposed.Finally,a simulation analysis and verification is established.The results show that this method suppresses the influence of the phase-locked loop on the stability of the LCL grid-connected converter under the weak grid without changing the phase-locked loop structure,and improves the stability margin of the grid-connected system.Power and power quality.