Stability assessment of inverter-dominated power systems considering coupling between phase angle and voltage dynamics

The integration of renewable energy sources (RESs) with inverter interfaces has fundamentally reshaped power system dynamics, challenging traditional stability analysis frameworks designed for synchronous generator-dominated grids. Conventional classifications, which decouple voltage, frequency, and rotor angle stability, fail to address the emerging strong voltage-angle coupling effects caused by RES dynamics. This coupling introduces complex oscillation modes and undermines system robustness, necessitating novel stability assessment tools. Recent studies focus on eigenvalue distributions and damping redistribution but lack quantitative criteria and interpretative clarity for coupled stability. This work proposes a transient energy-based framework to resolve these gaps. By decomposing transient energy into subsystem-dissipated components and coupling-induced energy exchange, the method establishes stability criteria compatible with a broad variety of inverter-interfaced devices while offering an intuitive energy-based interpretation for engineers. The coupling strength is also quantified by defining the relative coupling strength index, which is directly related to the transient energy interpretation of the coupled stability. Angle-voltage coupling may induce instability by injecting transient energy into the system, even if the individual phase angle and voltage dynamics themselves are stable. The main contributions include a systematic stability evaluation framework and an energy decomposition approach that bridges theoretical analysis with practical applicability, addressing the urgent need for tools for managing modern power system evolving stability challenges.