LAUSR

This paper proposes an insulated two-stage ac–dc converter to be applied in unidirectional power flow systems. The two-stage structure is the combination of a Hexverter and a series-type 12 pulse rectifier. The key features of the proposed structure are inherited from the modular multilevel converter concept such as modularity, scalability, and expandability. Besides the topology, this article presents a complete control strategy based on the output (dc) voltage regulation and Hexverter's stored energy. In particular, an unusual method to regulate the Hexverter's branch energy unbalances is proposed. All currents in the Hexverter are controlled which increases the robustness for operation in harsh environments such as subsea systems. Mathematical analysis of the control loops is carried out to develop the necessary control-oriented models for each control loop. The converter and the control strategy are verified through experiments in a 24 cells Hexverter and a current-fed series-type 12-pulse rectifier.