Transient stabilization of power systems in response to large disturbances is a challenging problem of high complexity. The complexity arises from the large number of system states and the nonlinearities in system dynamics which evolve at various time scales. The first contribution of this paper is a two-level approach to multi-time scale modeling of power system dynamics which includes dynamics of power-electronically controlled devices, such as Flexible AC Transmission Systems (FACTS) and High Voltage Direct Current (HVDC) lines. This approach proposes an interaction variable-based power system model suitable for the design of the FACTS controllers. The second contribution is an ectropy-based cooperative controller for FACTS capable of transiently stabilizing generator dynamics and extending the critical clearing time of a fault.