  Nonlinear modal decoupling (NMD) was recently proposed to nonlinearly
transform a multi-oscillator system into a number of decoupled oscillators
which together behave the same as the original system in an extended
neighborhood of the equilibrium. Each oscillator has just one degree of freedom
and hence can easily be analyzed to infer the stability of the original system
associated with one electromechanical mode. As the first attempt of applying
the NMD methodology to realistic power system models, this paper proposes an
NMD-based transient stability analysis approach. For a multi-machine power
system, the approach first derives decoupled nonlinear oscillators by a
coordinates transformation, and then applies Lyapunov stability analysis to
oscillators to assess the stability of the original system. Nonlinear modal
interaction is also considered. The approach can be efficiently applied to a
large-scale power grid by conducting NMD regarding only selected modes. Case
studies on a 3-machine 9-bus system and an NPCC 48-machine 140-bus system show
the potentials of the approach in transient stability analysis for
multi-machine systems.
