Impedance-Based Whole-System Modeling for a Composite Grid via Frame-Dynamics Embedding.

The paper establishes a methodology to overcome the difficulty of dynamic-preserving frame alignment in ac impedance models, and thereby enables impedance-based whole-system modeling of converter-generator composite power systems. The methodology is based on an intermediary steady frame between local and global frames, which separates the global (constant) and local (dynamic) parts of the frame alignment. The local frame dynamics can be fully embedded into local impedance models via a unified transformation law. Compared to start-of-the-art impedance-based models, the proposed method retains both frame dynamics and scalability, and is generally applicable to almost all generators and converters. The methodology is used to analyze the dynamic interaction between rotors and phase-locked loops (PLLs) in a composite grid, which yields very interesting findings on the rotor-PLL stability in a low-inertia grid.