A Composite Inertia Index for Renewable-Dominated Grids: A Case Study on the ERCOT System

Abdelwahab, Ammar Samir and Vokony, István (2026) A Composite Inertia Index for Renewable-Dominated Grids: A Case Study on the ERCOT System. In: IEEE IC_ASET 2026.

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Abstract

The rapid integration of inverter-based resources (IBRs) such as wind and solar in modern power grids has led to a significant decline in traditional synchronous inertia, posing critical challenges to grid stability. Conventional inertia metrics are insufficient to capture the multidimensional frequency support mechanisms present in renewabledominated systems. This paper proposes a novel Composite Inertia Index (CII) that holistically integrates six components: synchronous inertia, fast frequency response (FFR) capability, load resource response, inverter headroom, non-spinning reserves, and real-time system vulnerability to contingencies. The methodology combines Principal Component Analysis (PCA) and entropy weighting to objectively determine component importance, while TOPSIS ranking is employed for comparative stability assessment. Applied to ERCOT operational data (2013–2023), the CII reveals a structural transition from inertia-dependent stability to FFRdominated resilience. The index consistently identifies low-resilience conditions during the 2021 winter event (CII = 0.335, rank 9/11) and peak resilience in 2023 (CII = 0.651, rank 1/11), demonstrating strong alignment with documented system stress periods. The results validate the robustness of the proposed framework as a practical tool for inertia adequacy evaluation and strategic planning in modern low-inertia power systems.

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