The on-line microprocessor based measurements of phase angles and magnitudes of voltages and currents at selected locations on a power system are proposed. These measurements can be used to assess the resolution of a transient trajectory, and, by considering the velocity and acceleration of machines in synchronous frames of reference, may be employed in determining the nature of a transient in order to take the necessary control action. This work addresses those systems which behave like two coherent systems of generators for the disturbances under consideration. Placement of phasor measurement systems, techniques of on-line transient assessment and the requisite control action will be considered. A realistic system where such a control problem exists in practice will be used to confirm the developed techniques.