Harmonic analysis of parallel-connected 12-pulse uncontrolled rectifier without an interphase transformer

The author presents a harmonic analysis of a parallel-connected 12-pulse uncontrolled bridge rectifier without an interphase transformer (IPT). Five normal operating modes of the specific rectifier circuit are classified, using an analytical method by deriving instantaneous AC current and DC voltage waveform equations. The discrete Fourier transformation algorithm is then carried out to obtain both current and voltage harmonic contents. Both the AC current and DC voltage harmonics for rectifiers with and without an IPT have been investigated to discuss the influence of the IPT on rectifier harmonics. Simulation results reveal that discrepancies in harmonic contents of both AC current and DC voltage of the two rectifier circuits are marked. It is noted that, for extremely light load up to rated load conditions, the total harmonic distortion (THD) of linesupply current is reduced, while the THD of DC output voltage is increased when the IPT is deleted from rectifier circuit.