Research On Compound Multi-resonance Feedforward Control Of Grid-connected Inverter In Weak Grid

The promulgation of the carbon peaking and carbon neutrality policy has promoted the rapid development of new energy power generation technologies.The rapid increase in the installed capacity of new energy power generation systems and the frequent switching of a large number of power electronic equipment have resulted in a large number of background harmonics at the common coupling point of grid-connected inverters.This will directly affect the power quality and system stability of the power grid system.Among them,the existence of background harmonics will distort the grid-connected current,which will cause the power quality of the grid-connected current to not meet the requirements of new energy grid-connected.The use of voltage proportional feedforward strategy can moderately suppress the influence of background harmonics,but this strategy has obvious application technical bottlenecks under weak grid conditions,such as the phase margin and open-loop gain of the system will be reduced.Adopting the resonance feedforward strategy can effectively improve the phase margin of the system,and can make up for the insufficient phase margin in the voltage proportional feedforward strategy,but the resonance peak in the resonance link makes the phase of the system lag significantly near the resonance peak,resulting in Insufficient system phase margin and reduced stability.This subject takes the background harmonic suppression under weak grid conditions as the background,and takes the optimization of the resonance feedforward strategy as the goal.Phase margin provides strategies and methods for improving system stability under weak grid conditions.Aiming at the insufficient phase margin of grid-connected inverter grid voltage proportional feedforward strategy in weak grid,a composite resonance feedforward control strategy with broadband characteristics is proposed,and a resonance feedforward control with phase correction function is studied.The composite control strategy of the inverter and the phase compensation link of broadband characteristics,and the influence of the composite resonance feedforward strategy on the stability of the grid-connected inverter system and the improvement of power quality under weak grid conditions are explored.The specific research work of the thesis is as follows:The mathematical modeling method of three-phase LCL grid-connected inverter is studied,and the design basis of LCL filter topology parameters is explored.According to the Nyquist stability criterion,the impedance analysis method was used to explore the influence of grid impedance on the stability of the LCL grid-connected inverter.Study the influence of background harmonics on the voltage proportional feedforward strategy,summarize the impedance characteristics of grid-connected inverters considering weak grid factors,and explore the influence of voltage proportional feedforward strategy on the reduction of system phase margin under weak grid conditions;Summarize the system phase shortage problem existing in the voltage proportional feedforward strategy,combined with the design idea of ??the resonance feedforward strategy,the optimization of the system equivalent output impedance phase of the resonance feedforward strategy with phase correction function was studied.Explore the phase compensation method of this strategy and its potential application bottleneck.In order to further optimize the problem of insufficient stability margin of the multi-resonance feedforward strategy and realize the applicability of the system in a larger grid impedance environment,a phase compensation link with broadband characteristics is studied.The principle of the compensation link to the system phase margin compensation is analyzed,and the design idea of the phase compensation link is explored.Combining the phase compensation link with broadband characteristics and the optimized multi-resonance feed-forward strategy,a composite resonance feed-forward strategy is proposed.Adopting this strategy can provide phase margin for the system in a wide frequency range,so that the system still has strong robustness in a larger grid impedance environment.Finally,the simulation model of the three-phase LCL grid-connected inverter is simulated by building Matlab/Simulink.The grid-connected current waveform of the system under the influence of strong grid and background harmonics is analyzed,and the influence of voltage proportional feedforward on grid-connected current waveform and the influence of weak grid on grid-connected current under voltage feedforward strategy are explored.The performance of the resonant feedforward strategy with phase correction function under different grid impedances is verified and analyzed by the simulation model.The adaptability of the traditional phase compensation link and the proposed phase compensation link to the weak grid in the composite resonance feedforward strategy is compared and analyzed.The simulation results show that the proposed composite resonance feedforward strategy with broadband characteristics can improve the stability of the system under weak grid conditions.