Cascaded Control System of the Modular Multilevel Converter for Feeding Variable-Speed Drives

The modular multilevel converter (MMC) is an upcoming topology for high-power drive applications especially in the medium voltage range. This paper presents the design process of a holistic control system for a MMC to feed variable-speed drives. First, the design of the current control for the independent adjustment of several current components is derived from the analysis of the equivalent circuits. Second, the current and voltage components for balancing the energies in the arms of the MMC are identified systematically by the investigation of the transformed arm power components. These fundamentals lead to the design of the cascaded control structure, which allows the balancing task in the whole operating range of a three-phase machine. The control system ensures a dynamic balancing of the energies in the cells of the MMC at minimum necessary internal currents over the complete frequency range. Simultaneously, all other circulating current components are avoided to minimize current stress and additional voltage pulsations. The performance of the control system is finally validated by measurements on a low-voltage MMC prototype, which feeds a field-oriented controlled induction machine.