  Analyzing the stability of the power system by using a few machines is
promising for transient stability assessment. A hybrid direct-time-domain
method that is fully based on the thinking of partial energy function is
proposed in this paper. During post-fault period, a pair of machines with high
rotor speed difference is defined as couple machines, and the stability
analysis of the system is transformed into that of several pairs of couple
machines. Based on the prediction of power-angle curve of couple machines
within a sampling window after fault clearing, the proposed method avoids the
definition of Center of Inertia (COI) and it can also evaluate the stability
margin of the system by using the predicted power-angle curve. Simulation
results demonstrate its effectiveness in transient stability assessment.
