With the development of power grid interconnection, disturbance propagation phenomena are more and more apparent in the form of electromechanical waves. Once a disturbance occurs, its propagation might cause cascading events and even large blackouts. With the help of advanced communications networks in smart grids, new event-based protection systems can be devised by the reveal of disturbance propagation mechanisms. In this paper, a widely used measurement-based protection system is analyzed and the idea of event-based protection is proposed. By introducing the inverse process of lumped mass method, the impact scope of generator inertia is proposed. Then, the structural frame model of power networks for disturbance propagation is built, which is sectionalized homogeneously. The spatial heterogeneity of propagation velocity of the electromechanical waves is depicted. According to the constant velocity characteristic of the segments in the structural frame model, a weighted undirected graph is formulated. Then, the Dijkstra algorithm based shortest path searching method for arrival time of electromechanical wave is proposed. The simulations are carried out to demonstrate the effectiveness of the proposed electromechanical wave propagation model and disturbance arrival time estimation algorithm. The research work is one of the essential steps for realizing event-based protection strategies.