| Abstract |
Modular Multilevel Converters (MMC) are power electronics systems used for energy conversion. Onetopology of MMC is Cascaded MMC characterized by series connection of cells. Each cell canprovide at least two different voltage levels. Resulting voltage in the converter arm is a sum of celloutput 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, powergeneration or grid frequency changers. In case there is no intermediate DC link between input andoutput, subgroup called direct-MMC can be distinguished. Following family includes Matrix Converter(M3C), Hexverter and Hex-Y.One of the challenging operation points for direct AC-AC MMC is a low frequency mode. In case ofrequired non-zero torque at low speed, module oversizing is necessary. An example applications with flattorque-speed characteristics are rolling / mining applications, hydro power generators or geralesswind generators.In following paper, low frequency mode with non-zero load torque is being analyzed for two of aforementionedtopologies: proposed by author Hex-Y topology and M3C used as a reference (due to identicalnumber of branches). Control algorithm providing given output voltages and currents plus equal branchenergy sharing has been described in detail.Control loop design for Hex-Y has been verified using Hardware in-the-loop simulator. |
