Using virtual impedance network to improve the control performances of LCL-type grid-connected inverter under the weak grid condition

Under the high penetration of renewable energy, the power grid may features as the weak grid, which contains a large set of grid impedance values and varies of harmonics. The grid-connected inverter can be greatly challenged by this weak grid condition when it is operated as the interface to inject the generated power into the grid. Particularly, the stability problem may be aroused by the grid impedance and the injected grid current can be greatly distorted by the grid voltage harmonics. Luckily, the output impedance of the inverter can give an insight into the stable robustness and the harmonic rejection ability of control system, so relationships between them are investigated in this paper. Based on which, the virtual impedance network is proposed to provide additional degrees of freedom for the output impedance optimization. With this method, the grid-connected inverter exhibits high robustness against the grid-impedance variation and also performs strong rejection ability against grid background harmonics at the same time. Experimental results from a LCL-type single-phase grid-connected inverter confirm the theoretical expectations and the effectiveness of the proposed solutions.