Design-Oriented Small-Signal Stability Assessment of Grid-Following Inverter-Based Resources Based on Virtual Dynamic Short-Circuit Ratio

Small-signal instability may occur when parallel grid-following inverter-based resources (IBRs) are connected to a grid with low strength, i.e., a weak grid. In such a weak grid, changes in output power and the number of IBRs can significantly impact the small-signal stability margin. However, the short-circuit ratios (SCRs) currently used for small-signal stability assessment of the IBR plant are static indices, and the difference in the IBR plant impedance with different total output power is not considered, resulting in inaccurate stability assessment results. Moreover, the stability margin expressed by the existing SCRs under different operating conditions is not unified. Therefore, the virtual dynamic short-circuit ratio (VDSCR) is proposed in this article, which converts the stability margin of the IBR plant under different working conditions to the rated single IBR unit, so that the proposed VDSCR is accurate and unified. Based on the VDSCR, the IBRs control is designed under the fluctuation of total output power and grid impedance, which can keep the small-signal stability margin at the set value. While ensuring the small-signal stability of the IBR plant, the control performance of the IBRs is further optimized through the control design based on VDSCR. Finally, experiments are conducted to verify the effectiveness of the proposed method.