Research on the stability of a three-phase dual-frequency grid-connected inverter based on state-space model

To improve the quality of grid current, inverters typically connect to the power grid through various filters. Among these, the LCL filter is widely used due to its superior high-frequency harmonic attenuation capability. However, the inherent resonance problem of the LCL filter can lead to instability in weak grids and make parameter design more complex. This paper investigates a novel three-phase dual-frequency grid-connected inverter, which reduces switching losses by lowering the switching frequency of the power devices and energy transfer paths. Harmonic suppression is achieved using an L filter and auxiliary units, ensuring high-quality grid current while eliminating the inherent resonance issue of the LCL filter. Weak grids are often characterized by high impedance and low short-circuit ratios, which tend to cause system instability and limit power transmission capability. This study establishes a state-space model of the three-phase dual-frequency grid-connected system under weak grid conditions and analyzes system stability using eigenvalue analysis. The results show that system stability exhibits a nonlinear decline as grid strength weakens. The study further explores the impact of Phase-Locked Loop parameters on system stability. Expanding the control bandwidth and avoiding frequency band overlap are shown to enhance system stability. Finally, simulation and experimental results are provided to validate the theoretical analysis, confirming that the proposed three-phase dual-frequency grid-connected inverter system exhibits good stability.