The fault mode that restricts the power system stability and transmission capacity will change after large scale power systems are interconnected by long distance bulk power UHV AC transmission lines. Taking the UHV AC line between the North China and East China power grids as an example, which is the main channel in planning for sending power from North China to East China, the necessity to analyze its tripping without fault as prescribed in the Guide on Security and Stability for Power System was discussed. It was shown that, when the power flow passing the transmission line is large, the tripping of line without fault caused more severe disturbance in power system than that of a three-phase permanent ground fault on the same UHV AC line. Based on two-machine equivalent system, a criterion for proving the tripping of line without fault more serious than three-phase permanent ground fault were obtained by the minimal oscillation technique. This criterion showed that inertia time constants, transfer impedance angle and initial relative phase angle between two machines are the key factors to determine which one was the most severe fault mode. Effectiveness of this criterion was verified, and the mechanism of difference fault causing disturbance on power system was fully demonstrated through simulation and analysis.