Power direction control of medium frequency isolation DC/DC converter for modular double cascade converter

Multilevel converters have strongly arrived to the medium voltage drive market. A new voltage source multilevel converter topology, called modular double cascade converter, is discussed. The converter uses cascaded H-bridges as input and output converters and a medium frequency DC/DC converter for galvanic isolation between the H-bridge modules. The key feature for the DC/DC converter is the six-winding transformer, that is able to balance the DC link voltages of the connected converters. It also enables power delivery in both directions, making four quadrant operation feasible. In this paper, different DC/DC converter control schemes are researched. The objective is to find the best method for balancing the DC link voltages between the isolated modules, and to minimize the transformer winding currents.