Robust Online Stability Margin Monitoring of Grid‐Connected Inverters via Current‐Ratio‐Based Rational Algebraic Stability Criterion

ABSTRACT

Online stability margin monitoring is critical for ensuring the reliable operation of grid‐connected inverters under varying grid conditions. However, existing impedance‐based methods are constrained by measurement accuracy and struggle to balance flexible margin regulation against mathematical simplicity. To address these issues, this paper proposes a robust online monitoring framework. First, a measurement scheme based on the injection‐to‐grid current ratio is introduced. Unlike approaches relying on impedance ratios, this method uses the dominant perturbation energy to improve the signal‐to‐noise ratio (SNR) and eliminate the dependency on high‐bandwidth voltage sensors. Second, a Rational Algebraic Stability Criterion (RASC) is developed. It constructs a polynomial boundary that decouples gain margin and phase margin constraints, offering high design flexibility without using complex transcendental functions. Finally, based on the RASC, a normalised stability margin index (NSMI) is derived to transform geometric constraints into a continuous scalar indicator for online stability margin monitoring. Experimental results validate the robustness of the proposed scheme under various grid conditions.