I. Introduction
Efforts of a large number of scientific research and technical personnel has been inj ected in the CZ railway in China. The conventional load of the power grid along the CZ railway is relatively small, so that the railway traction load will become the main load in this area. In addition, the huge terrain fluctuations along the CZ railway make the load of the line not only has shock and random fluctuation, but also may have frequent and large regenerative braking power different from the conventional line load, which aggravates the uncertainty of the load side. On the other hand, the external power grid conditions are extremely weak, and some areas lack the support of strong power grid. Therefore, in the case of high-proportion traction load connection to the grid in the future, the system will suffer from frequent forward and reverse uncertainty currents, which will have an unpredictable impact on the stability of the system. In order to avoid the occurrence of cascading failures, the research on system vulnerability is imminent, and the identification of vulnerable lines in the system is a key problem that needs to be solved urgently with the current traction load connected.