Analysis of a Multi-Cell Converter under Unbalanced AC Source

Multi-cell converters offer very useful characteristics for instance, it fulfills the input current harmonic content recommendations and presents a high power factor even at low levels of load at the ac system, and a reduced torque pulsations at the motor side. They also feature a high level of modularity and present a high reliability. Moreover, this converter does not produce electromagnetic interference or common-mode voltages and therefore, it is appropriate for large volt-ampere and high voltage motor drives. Surprising, the analysis under amplitude unbalances at the AC supply has not been reported in the literature. This work shows that the input multi-pulse transformer does not amplify the supply input unbalance. Thus, the secondary voltages feature identical unbalance, which in turn produces unbalanced secondary line currents. Similarly, the overall input currents feature an unbalance which is at most equal to the unbalance of the secondary line currents. It is also found that small voltages unbalances at the input of any cell produce large input line current unbalances. On the other hand, the current injected by the rectifier into the DC link produces a distorted DC voltage waveform, which in turn deteriorates the motor voltage. In this work, using symmetrical components, it is possible to analyze the effect in each of these stages by quantifying the amount of unbalance and distortion produced in the current and voltage waveforms. Besides, this analysis provides a design guideline to compute the DC link capacitor size necessary to reject off the effects of unbalances in the AC supply