Conceptualization and Experimental Verification of a 50 kW 600 V/4 kV/600 V DC Power Supply System for Ultra-Deep-Sea HyDrones

Remotely operated underwater vehicles (ROVs) are crucially important in deep-sea exploration. Today, ROVs are typically supplied from a surface vessel via a medium-voltage (MV) 50 Hz/60 Hz ac umbilical. The thus bulky and heavy step-down transformer aboard the ROV complicates achieving neutral buoyancy of the ROV. Further, for a given diameter, dc cables offer lower losses and/or higher power transfer capability than ac cables. Therefore, this paper conceptualizes an MVdc power supply system, which is based on using identical modular dc-transformer (DCX) converters in input-parallel output-series (IPOS) configuration aboard the vessel and in input-series output-parallel (ISOP) configuration aboard the ROV. The DCX concept ensures natural voltage sharing among the series-connected converter modules, full soft-switching operation of the 900V and 1200V SiC power transistors of the converter modules, and does not require any closed-loop control. A detailed analysis of the dynamic behavior of this open-loop DCX ROV power supply, including the MVdc umbilical, is conducted and safe operation in the presence of sudden load changes can be assured. Further, the performance limits of the converter system and the umbilical cable are systematically assessed by means of multi-objective Pareto optimization. A higher MVdc transmission voltage benefits the umbilical design and efficiency, but lowers the IPOS/ISOP DCX converter efficiency and/or power density. Hence, an MVdc level of 4 kV is identified as a system-level sweet spot for a 50kW DCX work-class ROV power supply with a 4000m umbilical, achieving an overall system efficiency (DCX-umbilical-DCX) of around 97%. Finally, two industrial prototypes of 50kW, 600 V/4 kV DCX IPOS/ISOP converters are realized and successfully tested in a back-to-back configuration, indicating a rated-power efficiency of around 99% per DCX converter stage.