An in-depth study on the rule of change in transient energy distribution in the network can provide a basis for the forecast of power system stability

the recognition of power system vulnerable region

and the rational locating of monitoring and control devices in the network. By analyzing the change rule of branch phase-angle difference

voltage

transient energy distribution after the equivalent two-machine system is disturbed

this paper reveals the internal relationship between them. And the relationship between the change in branch phase-angle difference and the network structure is proved from the perspective of geometry. It is shown that the distribution of branch potential energy coincides with the variation of phase-angle difference. In the process of the power angle gradually broadening after the system perturbation

the energy is not evenly distributed in all the branches. Rather

with the gradual deterioration of stability degree

the nearer the position is to the oscillation center

the more the transient potential energy distribution of per unit reactance will be

and the smaller the stability margin will be. When the system loses stability

for the branches including the oscillation center

their transient potential energy distribution will be unbounded whereas for all the branches not including the oscillation center

their potential energy distribution is bounded. For the multi-machine system

however

the spread of energy still depends on the structure and parameter of network. Simulation results have proved the correctness of the proposed method. This work is supported by National Natural Science Foundation of China(No. 41374189)and Fundamental Research Funds for the Central Universities(No. JD1202B).