Wavelet-Based Multiscale Coupling Index for Transient Voltage Stability Assessment in Renewable Power Systems

The increasing integration of renewable energy sources (RES) into power systems introduces unprecedented challenges to transient voltage stability due to the fast dynamics of power electronic devices. Traditional stability assessment methods, relying on long-term simulations (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0-10 \mathrm{s}$</tex>) or multivariable analyses, struggle to capture rapid transients (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.1-0.3 \mathrm{s}$</tex>) caused by grid-following inverter controls, leading to computational inefficiency and delayed instability detection. This paper proposes a Wavelet-based Multiscale Transient Voltage Stability Index (WMTVSI) to address these limitations. By leveraging discrete wavelet decomposition (db5 wavelet, 5-level), the method decouples high-frequency disturbances (D1-D3: 12.5-100 Hz) from low-frequency electromechanical oscillations (D4-D5: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3.125-12.5 \text{Hz}$</tex>), enabling rapid feature extraction from voltage signals. Key innovations include as Time-frequency decoupling: Direct mapping of wavelet coefficients to stability criteria, reducing the analysis window to 1 s post-fault; Multiscale dynamic integration: A weighted coupling function (D1-D5) quantifies transient energy distribution and oscillation damping, collapsing multidimensional features into a single index (WMTVSI); Feature dimensionality reduction slashes computation time to 0.6 s for severe faults. Validation on the CEPRI-VC benchmark system (100 buses, 46% RES penetration) demonstrates WMTVSI's efficacy in identifying instability within 0.5 s across scenarios, including critical tie-line faults, topology changes, and load mutations. Results show outperforming conventional methods in accuracy and speed. The WMTVSI establishes a paradigm shift for real-time transient stability assessment in RES-integrated grids.