<p>Key parameters of different scenarios.</p>

As grid-forming converters (GFM) and grid-following converters (GFL) continue to be integrated into low-inertia power systems in place of traditional synchronous generators, the characteristics and forms of system inertia have undergone significant transformation. This evolution poses significant challenges to conventional inertia response mechanisms and analytical methodologies. To address these challenges, this study proposes a novel grid index frequency stability margin (FSM) from the perspective of frequency stability, encompassing its definition, quantitative evaluation, and practical applications. This paper first introduces the mathematical foundations and operational definitions of the FSM. It then systematically investigates the factors influencing FSM and presents a comprehensive mathematical model specifically developed for low-inertia power systems. The FSM calculation method based on aggregated system modelling was developed, followed by the derivation of a simplified estimation approach suitable for practical engineering applications. The effectiveness of the FSM in analyzing the frequency stability of low-inertia grids was validated through case studies based on provincial-level power grid data from China and a modified IEEE 39-bus system. The findings establish a theoretical framework for optimizing the planning and development of new energy power plants, as well as for formulating grid operation control strategies. This framework offers essential guidance to ensure the secure and stable operation of low-inertia power systems.随着电网形成型变流器（grid-forming converters, GFM）与电网跟随型变流器（grid-following converters, GFL）逐步替代传统同步发电机接入低惯量电力系统，系统惯量的特性与表现形式发生了显著转变。这一演变给传统惯量响应机制与分析方法带来了严峻挑战。为应对上述挑战，本研究从频率稳定视角提出了一种新型电网指标——频率稳定裕度（frequency stability margin, FSM），涵盖其定义、定量评估方法与实际应用场景。本文首先阐述了FSM的数学基础与运行定义；随后系统分析了影响FSM的各类因素，并针对低惯量电力系统构建了完整的数学模型；接着提出了基于聚合系统建模的FSM计算方法，同时推导了适用于实际工程场景的简化估算方案。基于中国省级电网数据与改进型IEEE 39节点系统的算例分析，验证了FSM在低惯量电网频率稳定分析中的有效性。研究结果为新能源发电厂规划开发优化、电网运行控制策略制定搭建了理论框架，可为保障低惯量电力系统的安全稳定运行提供关键指导。