Wide-Band Admittance Shaping Control for Grid Forming Converters

Virtual admittance (VA) control has emerged as an effective approach to enhance the performance and exploit the capabilities of grid-forming converters (GFMs) in terms of system robustness, stability, and flexibility. In addition to emulating desired virtual admittance at low frequencies, which enables outer-loop active and reactive power regulation and stable operation under both strong and weak grid conditions, it is also desirable to shape the GFM's admittance at higher frequencies, e.g., to prevent destabilization in the harmonic range. This article first shows how to achieve precise admittance synthesis for the inner loop controllers. Then, it exemplifies how this is relevant for stability. In this regard, two VA controller design approaches are considered: the state-of-the art one and the novel approach, which accounts for the effects of the current-control loop and the voltage feedforward. It is shown that, unlike the former, the latter approach ensures that the GFM's output admittance accurately matches the target across a wide frequency range. In addition, it is revealed how, with the state-of-the art VA approach an instability in the harmonic range may arise in case a low-pass filter with a high cutoff frequency is used for the voltage feedforward. On the other hand, the proposed VA approach is demonstrated to ensure stable operation regardless of this cutoff frequency. The proposed methodology is validated experimentally, using a laboratory prototype of a three-phase voltage-source converter.