| The distributed power generation system converts renewable energy such as wind energy and solar energy into electrical energy and transmits it to the grid.The gridconnected inverter is the core device,and its control performance can determine the stability of the system and the quality of the grid-connected current.Since most renewable energy is located in remote areas,long-distance transmission lines and a large number of transformers are needed to transfer the converted electrical energy to the grid.The impedance of the grid cannot be ignored,and the grid shows weak grid characteristics.Under weak grid conditions,a wide range of grid impedances will change the open-loop transfer function of the system and reduce the stability of the system.This paper takes single-phase LCL grid-connected inverters under weak grid conditions as the research object,discusses the influence of grid impedance on current control control strategy and grid voltage background harmonic suppression strategy,and proposes related improved control strategies.The equivalent impedance property of capacitive current proportional feedback in digital control system changes with frequency.When the system resonance frequency is close to 1/6 sampling frequency,the system is unstable.The resonance frequency of the system will changes due to changes in the grid impedance.When the grid impedance changes over a wide range,the resonance frequency of the system gradually approaches the 1/6 sampling frequency,and the grid-connected inverter system is less robust to grid impedance under weak grid conditions.In order to improve the system robustness,this paper proposes an improved capacitor current feedback control strategy.The delay compensation link is added based on the sampling of the capacitor current,so that the equivalent resistance of the improved capacitor current feedback is constant at a positive value throughout the frequency band.The system will not cause instability due to changes in the resonance frequency of the grid impedance,which improves the system’s robustness to the grid impedance.The stability of the improved capacitor current feedback is analyzed and the design steps of related control parameters are given.When considering the grid impedance,the grid voltage feedforward control scheme that suppresses the background harmonics of the grid voltage will introduce a positive feedback,which will change the loop gain of the system,and will lead to system instability in severe cases.The current control strategy adopted in this paper is based on the fullorder state feedback of the state observer,and the adaptive control scheme is used to offset the adverse effects of the additional positive feedback.In order to reduce sensors,save costs,and avoid the interference of high-frequency noise,this paper proposes an adaptive control based on a grid impedance online measurement algorithm.The method uses the value measured by the grid-connected current sensor to estimate the value of the capacitor voltage,thereby realizing negative feedback of the capacitor voltage,and adjusting the capacitor voltage gain online to correct the capacitor voltage feedforward.The proposed control method was simulated by Matlab / Simulink simulation software,which verified the correctness of the theoretical analysis.Build a single-phase LCL grid-connected inverter experimental prototype to further verify the effectiveness of the proposed scheme. |
