Modular Multilevel Converter for propulsion system of electric ships

The choice of the converter topology feeding the electric machine of the main propulsion of electric ships plays an important role for the feasibility of an all electric ship concept. The medium-voltage machine has to be speed-adjustable in the whole speed range at high power and maybe it is connected by a long cable to the inverter. In other high power industrial drives with long cable length, e.g. in fan motor drives in underground mines, overvoltages at the machine terminals are observed, resulting in premature motor insulation failure. For that reason a multilevel topology for the used converter is indispensable. 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. In contrast the Modular Multilevel Converter allows a nearly unlimited cascading of its sub-modules and therewith of voltage levels at the same complexity in construction. The challenge of using this topology as drive converter is the AC fluctuation of the sub-module voltages rising with sinking output frequency, controllable only by special control schemes and by a special load characteristic of the machine. This paper shows the dimensioning of a 17-level Modular Multilevel Converter feeding a SIEMENS H-COMPACT PLUS 3.9-MW 4.16-kV machine. The advantages and the challenges of the modular multilevel topology are presented and a characteristic diagram of the drive is shown. Simulation results document the chosen control strategies in the full operation range.