Study On The Control And Stability Of LCL Three-phase Photovoltaic Grid-connected Inverter In Weak Power Grid

The rapid development of the world economy has led to the rapid consumption of fossil energy and increasing environmental pollution.Photovoltaic power generation has been rapidly applied and developed as a means of clean energy development.However,the influence of grid impedance and grid fluctuations on the stability of photovoltaic grid-connected inverters under weak grids cannot be ignored.The analysis of factors affecting the stability of photovoltaic grid-connected inverters under weak grids and the optimization of control strategies have become hotspots in the field of photovoltaic power generation.In this study,the LCL photovoltaic grid-connected inverter system is used as an object to study the stability analysis and improvement of control strategies under weak grids.In this paper,the output characteristics of photovoltaic cells are analyzed,and an improved variable step size incremental conductance method is used as a control method for maximum power point tracking of photovoltaic cell output.In the study,a three-level space vector-based pulse width modulation strategy is used.The three-level algorithm is simplified by rotating the spatial reference vector in different sectors to the first sector.The 7-segment voltage vector sequence and simulation result.As the conventional grid-connected inverter is prone to instability in a weak grid environment,this paper analyzes the increase of the impedance of the grid under the weak grid and the small signal disturbance to the phase-locked loop,LCL filter,and DC-side bus in the grid-connected inverter The effects of voltage,including the effects of different discretization methods on the system.Based on the topology of the LCL photovoltaic grid-connected inverter,a grid-side current-LCL filter capacitor-voltage double-closed-loop control strategy was designed in consideration of the grid impedance variation.A mathematical model was established in the dq synchronous rotation coordinate system to understand the coupling and controller design.A system parameter design method considering phase-locked loop disturbance under different grid impedance conditions is presented.Finally,simulations verify the correctness and feasibility of the theoretical derivation,as well as the stability and robustness of the three-phase photovoltaic grid-connected inverter system.In order to further improve the stability and robustness of grid-connected inverters in distributed photovoltaic power generation under weak grid conditions,this study designed a dual grid-connected current loop feedback active damping strategy using second-order generalized integration.The adaptive second-order resonant integrator has an active damping feedback function,a proportional resonance controller is used as a function of the current loop controller,a virtual impedance correction is performed on the control system,and a voltage feedforward controller is designed.At the same time,a sorted phase-locked loop based on a biquad adaptive filter is designed.After demonstration,by adding reasonable virtual impedance correction,this method can reduce the use of measurement devices in hardware and save hardware costs while achieving good damping effects.Simulation results show that the theoretical derivation and design scheme are reasonable and effective,and the control strategy can ensure system stability and robustness.Finally,the semi-physical real-time experimental platform combined with RT-LAB was used to verify the grid-connected inverter.