Modern electric power systems are gigantic networks that connect numerous power stations to a multitude of load centers dispersed over a wide geographical area that may span several neighboring countries. These networks incorporate sophisticated communications and control systems that are used to optimize the performance of the network. Three-phase systems are almost universally used for the generation, transmission, and distribution of electrical energy because of the many advantages they offer, compared to single-phase systems. Practical three-phase systems are nominally balanced under normal operating conditions; that is, their voltages and currents possess a certain symmetry.