Nonlinear Koopman modes of coupled swing dynamics and coherency identification

We report modal analysis of coupled swing dynamics of a multi-machine power system. The analysis is based on the so-called Koopman operator, a linear, infinite-dimensional operator that is defined for any nonlinear dynamical system and captures full information of the system. The modal analysis makes it possible to extract single-frequency modes, which are called Koopman modes, embedded in the coupled swing dynamics and to identify a coherent group of generators in which they swing together in frequency and phase.