In order to realize the transient angle stability prediction

an on-line transient stability identification method is proposed

in which the maximum Lyapunov exponent(MLE)of the dynamical system is used to analyze the characteristics of rotor speed difference-rotor angle difference in the phase plane. Firstly

MLE stability principle and calculation mechanism are described. Secondly

with the single machine system as an example

the relationship between rotor speed difference-rotor angle difference characteristics and transient angle stability in different scenarios is analyzed strictly from the perspective of physical mechanism. Furthermore

following an improvement of the MLE method by using the recursive least square algorithm(RLS)and the automatic optimization of parameters according to phase trajectory characteristics

the criteria of transient power angle stability is proposed

and the online identification method is formed. Finally

the method is extended to the multi-machine system by double machine equivalent. The simulation results of the New England 10-machine 39-node system verify the effectiveness of the method. The proposed method is valid for both first swing and multi-swing stability judgment problems

and has strong fault tolerance to generator grouping. This work is supported by National Natural Science Foundation of China(No. 51477091)and State Grid Corporation of China(No. XTT1-15-056).