Research On Control Strategy Of LCL Grid-connected Inverter Aimed At Digital Control Delay Effect

In recent years,as the global energy crisis and environmental pollution problems have intensified,people have paid more and more attention to renewable energy power generation.As the energy conversion interface between the renewable energy power generation system and the grid,the grid-connected inverter plays an important role in converting DC power into AC power and connecting it to the grid.Traditional inverters are mostly analog control,but their inherent shortcomings make it difficult to apply better and more complex control strategies.Digital control has gradually replaced analog control and become the main method of inverter control due to its strong processing capabilities,good real-time performance,and easy implementation of complex algorithms.However,the use of digital control will also bring delay problems to the system and adversely affect the stable operation of the system.Therefore,this paper conducts a detailed study on the influence of digital control delay on the system,and proposes a control strategy of duty cycle prediction combined with zero-pole compensation for delay compensation.It has certain theoretical value and engineering application significance for ensuring the stable and high-quality grid-connected current.First,the article establishes a mathematical model of the grid-connected inverter under digital control.According to the main circuit topological structure of the three-phase LCL type grid-connected inverter,its mathematical model in the αβ coordinate system is established,and its control strategy is determined according to the control target of the grid-connected current.It also deeply analyzes the working characteristics of the digital control grid-connected inverter,the generation mechanism and composition of the control delay,establishes a continuous domain and discrete domain control block diagram considering the delay links,and derives the system loop gain.Second,based on the established digital control model and loop gain of the three-phase LCL grid-connected inverter,the influence of the digital control delay on the grid-connected inverter is revealed.According to the equivalent transformation of the block diagram,the existence of the digital control delay makes the capacitive current feedback active damping equivalent to an impedance that changes with frequency,and the damping interval becomes smaller as the delay increases.This impedance will also change the actual resonant frequency of the system while damping the resonance peak of the system,and when the equivalent resistance exhibits a negative resistance characteristic,a pair of unstable poles will appear in the system loop gain,which will reduce the stability of the system and the robustness to the grid impedance.In addition,the digital control delay will also bring phase lag to the system loop gain,reducing the bandwidth and low-frequency gain of the grid-connected inverter control loop,resulting in poorer grid-connected current dynamic performance and waveform quality.Then,in view of the adverse effects of the digital control delay on the system,a control strategy of LCL grid-connected inverter with improved duty cycle prediction combined with zero-pole compensation is proposed.The traditional duty cycle predictive control strategy will have the problem of insufficient compensation effect when the delay value changes,and the impact of the zero-order holder delay on the system at low sampling rates cannot be ignored.Therefore,two improved duty cycle prediction combined with pole-zero compensation control strategies are proposed.Through comparison,it can be seen that the adaptive duty cycle prediction combined with the zero-pole compensation control strategy has the best compensation effect and the dynamic performance of responding to delay changes,which can effectively improve the steady-state and dynamic performance of the system,and at the same time,the robustness of the LCL grid-connected inverter to the change of the grid impedance is improved.Finally,based on theoretical analysis and simulation results,a three-phase LCL type gridconnected inverter experimental platform is used for experimental verification.The experimental results show that the adaptive duty cycle prediction combined with the zero-pole compensation control strategy proposed in this paper can effectively eliminate the impact of digital control delay and ensure the stability of the inverter system.