In Chap. 2 , we discussed the oscillations that may occur in interconnected power systems. By looking at different models and with different disturbances, we showed examples of the different types of oscillation that can occur. To do this, we performed a considerable number of 10-second nonlinear simulations. It is apparent that in larger systems the use of transient simulation for the analysis of system oscillations could be very time consuming. To study inter-area oscillations, it is often necessary to run simulations for longer than 10 seconds. In practice, 30 seconds is quite common. Not only is the use of nonlinear simulation time consuming, but also it is often difficult to interpret the results. Larger systems may have a number of inter-area modes at very similar frequencies, and it can be quite difficult to separate them from a response in which more than one is excited.