Grid-connected medium-voltage converters with parallel voltage-source active filters

Grid-connected medium-voltage converters are typically operated at switching frequencies of several hundred hertz per switch position, requiring bulky and expensive LCL filters in order to meet the harmonic limits given by the grid code. Commonly, semiconductor current derating and increased switching frequencies are used to reduce the LCL-filter costs, leading to a reduced utilisation 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. The harmonic performance is improved for the operation with small passive filter components, revealing the potential for increasing the utilisation 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 output power are achieved compared to a reference LCL-filter-based converter system.