Low Frequency Mode Operation for Direct ac-ac Modular Multilevel Converter Systems. A Comparison of M3C and Hex-Y Topology

Modular Multilevel Converters (MMC) are power electronics systems used for energy conversion. One topology of MMC is Cascaded MMC [1] characterized by series connection of cells. Each cell can provide at least two different voltage levels. Resulting voltage in the converter arm is a sum of cell output voltages. Depending on the application, converters can be divided by number of phases in interconnected systems. One of the applications is 3-phase to 3-phase ac-ac power conversion found e.g. in motor drives, power generation or grid frequency changers. In case there is no intermediate DC link between input and output, subgroup called direct-MMC can be distinguished. Following family includes Matrix Converter (M3C) [2], Hexverter [3] and Hex-Y [4]. One of the challenging operation points for direct AC-AC MMC is a low frequency mode. In case of required non-zero torque at low speed, module oversizing is necessary. An example applications with flat torque-speed characteristics are rolling / mining applications, hydro power generators [5]-[7] or geraless wind generators [9]. In following paper, low frequency mode with non-zero load torque is being analyzed for two of aforementioned topologies: proposed by author Hex-Y topology and M3C used as a reference (due to identical number of branches). Control algorithm providing given output voltages and currents plus equal branch energy sharing has been described in detail. Control loop design for Hex-Y has been verified using Hardware in-the-loop simulator.