A Hybrid Medium Voltage Multilevel Converter with Parallel Voltage-Source Active Filter

In consequence of high semiconductor losses, grid-connected medium voltage power converters are typically operated at switching frequencies of several hundred Hertz per switch position. Therefore, conventional converter systems require bulky and expensive LCL-filters in order to meet the harmonic limits given by the grid code. It is only possible to reduce the LCL-filter costs by semiconductor current derating and operation at increased switching frequencies, leading to a reduced utilization and efficiency of the converter system. To overcome these disadvantages of conventional converter systems, the presented hybrid converter uses a parallel voltage-source active output filter and thus allows a significant reduction of the passive component demand. An excellent harmonic performance is achieved for the operation with small passive filter components, revealing the potential for increasing the power density and efficiency of high power medium voltage converters. As a result, significant reductions of the filter losses and passive components as well as an increased utilization are achieved compared to a reference LCL-filter based converter system.