In this talk, we will discuss a general methodology for identifying critical lines in long-distance power transmission. The power grid may lose synchrony when it is pushed to its operational limits. In particular, we analyze the Continental European grid in scenarios of large interarea power imbalances. We observe that instabilities emerge due to topological constraints. The system loses its stable fixed point and the grid is separated into asynchronous zones. We will also discuss the modes of the system, which help identify the most susceptable areas.