The main purpose of an electric power system is to generate and supply a sinusoidal voltage to a large number of customers (residential, commercial and industrial). Ideally the supplied voltage is considered as a perfectly sinusoidal voltage of constant amplitude and frequency. In practice, the use of non-linear and time varying elements by the customers causes various deviations from the ideal sinusoidal voltage and current. In fact, these elements corrupt the purity of the power frequency sine waves that should be the only frequency component in the power system.