A detailed analysis of six-pulse converter harmonic currents

The assumption of 1/h per unit harmonics, even when modified to allow for the attenuating effects of commutations, will not adequately describe the magnitude of six-pulse converter harmonic currents in many cases. To accurately determine the magnitude of characteristic converter harmonics, a calculation procedure which takes into account the ripple of the DC current reflected back into the AC line current must be performed. Evaluation of these ripple effects tends to increase the magnitude of the fifth harmonic while decreasing the magnitude of the higher-order characteristic harmonics. The FFT method described and implemented in computer software accurately predicts the harmonic currents of the converter across a range of firing angles, commutating reactance and DC link inductance values. The classical Dobinson or Graham-Schonholzer methods can be implemented by hand calculation, but the limitations of each must be considered or large errors may occur