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A characterization of energy transfer in swing instability of hierarchical power network based on mode decomposition

Swing instability is an undesirable emergent phenomenon of a power system, in which generators coherently lose synchronism after an unexpected injection of a disturbance. In this paper, we analyze the mechanism of this phenomenon for a power network where subsystems with multiple identical generators are hierarchically interconnected. We first decompose the dynamics of each subsystem into oscillatory and zero eigenvalue modes by mode decomposition. As we see that the zero eigenvalue modes govern an unstable behavior, we define the transient stability margin in terms of the modes. As a main result, we approximately characterize the energy transfer to the zero eigenvalue modes by using the margin, the collective dynamics of the entire system and the strength of the connection between subsystems. Based on this characterization, we demonstrate that this transfer increases as a predictive phenomenon of the swing instability.