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).
The power harmonics current spectrum, representing the amplitude of individual harmonic currents Ih (%) is shown on the figure below.
The current distortion can be reduced by implementation of power harmonics mitigation. As an example, the improvement provided by a 12-pulse arrangement is presented below. THD i is reduced from 40 to 11%.
Total Harmonic Distortion (THD): ratio of the r.m.s. value of the sum of all the harmonic components of an alternating quantity up to a specified order (H), to the r.m.s. value of the fundamental component.
The voltage THD is commonly noted THDu and is equal to: