#### Relative and Mean Motions of Multi-Machine Power Systems in Classical Model

##### Bin Wang, Kai Sun, Wei Kang

It is well-known that in an m-machine power system where each machine is represented by a second-order differential equation, the Jacobian of the system equation contains (m-1) pairs of conjugate eigenvalues and two real eigenvalues, including at least one zero. This letter proves that under the uniform damping condition, the dynamics associated with the two real eigenvalues do not have any impact on the dynamics associated with those complex eigenvalues. This conclusion is important to justify the use of the relative motions or center-of-inertia (COI) coordinate to analyze the rotor angle stability in a multi-machine power system.

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