Impedance characteristic analysis and control strategy research of single-phase LLCL grid-connected inverter under weak grid conditions

Abstract

Grid-connected inverters are core components for integrating clean and renewable energy technologies into the power grid and play a vital role in enhancing the stability and efficiency of the entire power system. During long-distance power transmission in grid-connected generation systems, the presence of line impedance often results in weak grid characteristics, which can trigger resonance and harmonic amplification, thereby affecting the stable operation of the grid. This paper focuses on a single-phase LLCL grid-connected inverter. First, a mathematical model of the LLCL inverter is established considering the impedance characteristics of a weak grid. By analyzing the amplitude and phase characteristics, the influence mechanism of weak grid impedance on the inverter system is revealed. The study finds that the inherent resonance peak and frequency shift of the LLCL inverter are key factors contributing to increased harmonic content in the system. To address these issues, a hybrid damping method combining active and passive damping is proposed to suppress the resonance peak. Furthermore, a composite current control strategy is designed, which parallelly integrates quasi-proportional resonant control and repetitive control, aiming to enhance the inverter’s adaptability to grid frequency deviation. The results of this study provide important theoretical support for the engineering design of renewable energy grid integration and offer valuable insights into the optimization of grid-connected inverter systems.