**Basics on Power Harmonics**

The following figure illustrates the typical waveforms of line current and line voltage for a three-phase variable speed drive.

The line current is distorted as a result of the non linearity of the input rectifier. The voltage distortion is the result of the circulation of the distorted current through the line impedance.

In this example, the Total Harmonic Current Distortion THD i is equal to 40%, and the Total Harmonic Voltage Distortion THD u is equal to 4.3%. (See definitions below).