Wide-area monitoring and control system complements the classical protection systems and overcomes their drawbacks. In such systems a dynamic view of power system, using synchronized measurements in short time intervals, smaller than 20 ms, is available. This new kind of system's information provides a novel preventive control action to prevent system's large disturbances. This paper proposes a new method to prevent frequency instabilities immediately after a disturbance occurs in a system. Based on online measurements sent by phasor measurement units, a single machine equivalent model including the governors' effect is computed. An accurate frequency prediction is done based on this novel model. Then a load shedding approach is presented maintaining the line power flows in a permissible limits and shedding the most effective loads in one step.