Inter-area oscillations which are undamped or growing oscillations between different areas in a large geographical region, have become quite common in utilities throughout the world. The traditional approach to damp these oscillations was to design a supplementary excitation control, called power system stabilizer (PSS), on a single generator affected by the oscillations. Since the number of oscillation modes have greatly increased and a mode of oscillation has been observed to change frequency and location, the adequacy of single generator power system stabilizer controls has been questioned. Is there need for more than one generator control and is there need for coordination between the power system stabilizer controls implemented in different generators to dampen the multiple modes of oscillation that can change in frequency and location?; If control is to be successfully accomplished, it is necessary to know what measurements and controls are necessary to make the portion of the system states associated with such oscillations observable and controllable. A linearized power system model is shown to be controllable and observable using a single generator field voltage or mechanical power input and observable using a single generator output for small disturbances and variations. This result is clearly not true in practice due to bounded state and control, real time control and state estimation, nonlinearity, disturbances, measurement noise, and operating condition variation. Definitions of strong system network disconnectivity, strong input and output connectivity, and strong local control area are given. The states of all generators and the networks states belonging to a strong local control area are then proven to be strong locally observable and controllable based on these definitions. These definitions of strong local observability and controllability are related to the concept of coherency measure in power system dynamics and voltage control areas in voltage collapse research.; Having a method for determining strong local observability and controllability, we are able to identify directly the actual transmission network branches that cause the weakness of the network boundaries due to faults or any contingencies. Thus, system operating security can be improved. Moreover, we are also able to detect the occurrence of inter-area oscillation which can help in understanding more about inter-area oscillations, leading to better control design. Furthermore, a discussion of the set of controls and measurements required to dampen different types of inter-area oscillations is given. Since measurements and controls must lie within an area of 200 miles radius to prevent time delay problems at the sampling rate needed, certain types of oscillations may not be controllable and observable with measurements taken within a 200 miles radius. In this case coordination and hierarchical control may be needed.
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