Voltage Stability Assessment in Modern Power Systems: A Structured Review of Methods and Indices

The increasing penetration of inverter-based resources and the growing complexity of modern power systems have made voltage stability assessment a critical aspect of transmission network operation. This paper presents a review of the main methods and indices adopted for voltage stability assessment, with emphasis on their applicability to real-time monitoring and inverter-dominated grids. The review covers both long-term and short-term voltage stability approaches, including L-index methods, Jacobian- and singular-value-based techniques, Lyapunov-exponent-based methods, trajectory-based indices, and recent artificial intelligence applications. For each method, the main advantages, limitations, computational requirements, and suitability for PMU-based monitoring are discussed. The analysis shows that no single indicator can fully describe all voltage instability phenomena. Instead, combining complementary static and dynamic indices provides a more complete basis for wide-area monitoring and real-time operational support in future power systems. Moreover, the main open challenges related to threshold definition, robustness under evolving grid conditions, and the integration of data-driven techniques into operational stability assessment schemes are discussed.