Analysis of the module-voltage fluctuations of the Modular Multilevel Converter at variable speed drive applications

For medium-voltage drives multilevel converters are advantageous to reduce the insulation stress of the windings of the machine. State of the art are multilevel converters as Diode Clamped Converter, Imbricated Cell Converter or Stacked H-Bridge Converter with a low number of levels limited by the restrictions in practical construction. The Modular Multilevel Converter is not limited in the number of levels. Its modularity allows a nearly unlimited cascading of sub-modules and therewith of voltage levels at the same complexity in construction. The challenges of this topology used as drive converter are the AC fluctuations of the sub-module voltages rising with sinking output frequency. This paper derives analytically equations for the module-voltage fluctuations for two different controls using the converter in the whole speed range. The results are analysed and compared to simulation results.