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Solid state transformers normally include a transformer whose operating frequency is well above the usual grid frequencies. These transformers are supplied by power electronic inverters such as an H-bridge. The average value of transformer supply voltage can drift to a non-zero value which may lead to saturation of the magnetic core. This paper presents an active control method to prevent saturation of a multi-winding mediumfrequency transformer. The control method comprises two control loops: the first balances the flux of the transformer and the other balances the currents in different windings. Simulation results which show the feasibility of the proposed control method are shown.

Medium-voltage motor drives extend the power rating of AC motor drives in industrial applications. Multilevel converters are gaining an ever-stronger foothold in this field. This doctoral dissertation introduces a new topology to the family of modular multilevel converters: the modular double-cascade converter.The modularity of the converter is enabled by the application of multiwinding mediumfrequency isolation transformers. Owing to the innovative transformer link, the converter presents many advantageous properties at a concept level: modularity, high input and output power quality, small footprint, and wide variety of applications, among others. Further, the research demonstrates that the transformer link also plays a key role in the disadvantages of the topology.An extensive simulation study on the new converter is performed. The focus of the simulation study is on the development of control algorithms and the feasibility of the topology. In particular, the circuit and control concepts used in the grid interface, the coupling configurations of the load inverter, and the transformer link operation are thoroughly investigated. Experimental results provide proof-of-concept results on the operation principle of the converter. This work concludes a research collaboration project on multilevel converters between LUT and Vacon Plc. The project was active from 2009 until 2014.