  The paper establishes a methodology to overcome the difficulty of dynamic
frame alignment and system separation in impedance modeling of ac grids, and
thereby enables impedance-based whole-system modeling of generator-converter
composite power systems. The methodology is based on a frame-dynamics-embedding
transformation via an intermediary steady frame between local and global
frames, which yields a locally defined impedance model for each generator or
converter that does not rely on a global frame but retains all frame dynamics.
The individual impedance model can then be readily combined into a whole-system
model even for meshed networks via the proposed closed-loop formulation without
network separation. Compared to start-of-the-art impedance-based models, the
proposed method retains both frame dynamics and scalability, and is generally
applicable to various network topologies (meshed, radial, etc) and combinations
of machines (generators, motors, converters, etc). The methodology is used to
analyze the dynamic interaction between generators and converters in a
composite grid, which yields important findings and potential solutions for
unstable oscillation caused by PLL-swing coupling in low-inertia grids.
