The coupling of transient stability limit power among multiple AC transmission sections brings great challenges to the security and stability operation of the power grid. Based on digging the corresponding relationship between the transient response trajectory characteristics and the stability mode in the disturbed system

the concept of the key transmission section and the associated sections from the transient stability perspective is proposed. The associated transmission sections are classified as the mode-coexistence mechanism determined and mode-evolution mechanism determined sections. The index of the synchronization force coefficient is put forward for the disturbance severity of the branch

which is used to reflect the system stability model based on the electrical quantity changes. A practical algorithm for identifying the key section and related sections is then put forward by combining the index

the graph theory and the transient stability mode identification method. The proposed algorithm is helpful for overcoming the lack of inaccuracy and inefficiency of the artificial experience method which is lack of mechanism analysis guidance. It provides the basis for the stability analysis or operation monitoring of the power grid security and stability coordination control based on the accurate identification of transmission section. Based on the example of IEEE 10-machine 39-node system

the principle and effectiveness of the proposed method are illustrated.