  This paper proposes decentralized stability conditions for multi-converter
systems based on the combination of the small gain theorem and the small phase
theorem. Instead of directly computing the closed-loop dynamics, e.g.,
eigenvalues of the state-space matrix, or using the generalized Nyquist
stability criterion, the proposed stability conditions are more scalable and
computationally lighter, which aim at evaluating the closed-loop system
stability by comparing the individual converter dynamics with the network
dynamics in a decentralized and open-loop manner. Moreover, our approach can
handle heterogeneous converters' dynamics and is suitable to analyze
large-scale multi-converter power systems that contain grid-following (GFL),
grid-forming (GFM) converters, and synchronous generators. Compared with other
decentralized stability conditions, e.g., passivity-based stability conditions,
the proposed conditions are significantly less conservative and can be
generally satisfied in practice across the whole frequency range.
