Disturbance-Observer-Based Virtual Impedance for Virtual Synchronous Generators

This paper proposes a novel method for implementing a virtual impedance (VI) based on a disturbance observer (DOB) in the virtual synchronous generator (VSG). In conventional methods, the emulation performance of VI can be impeded by rough approximation or control interactions. In addition, the implementation of VI needs to be carefully designed to prevent instability under weak grid conditions and compensate for nonlinear distortions of inverters. The DOB-based VI utilizes a complete impedance model without the approximation, leading to an accurate emulation of VI. Above all, a target impedance can be conveniently adjusted by simply setting the nominal parameters of the plant model for the DOB, even if the actual parameters are inaccurately known. Additionally, the proposed method enhances the accuracy of impedance emulation by improving the VSG's ability to reject unmodeled disturbances, such as voltage distortion caused by the dead-time effect. The characteristics of the proposed method are examined using comparative analyses against conventional methods, based on eigenvalue trajectories and frequency responses. Finally, the effectiveness of the proposed method is validated with simulation and experimental results.