Towards fault-tolerant, reliable, efficient, and economical DC-DC conversion for DC grid (FREE-DC)

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The UK is a global leader in offshore wind energy. Its installation will be nearly quadrupled to 40 GW by 2030 according to the government plan. The Climate Change Committee (CCC) also predicts that total offshore wind may rise to 100 GW by 2050 to back the nation's net-zero goal. Such large-scale expansion and the concerted effort of other European countries to battle the climate crisis will drive the development and interconnection of offshore HVDC infrastructures within and beyond the UK into a DC transmission grid (rather than separate point-to-point transmission links) to ensure energy security and maximise economic gains. Research on planning such a DC grid has significant participation from the UK, supported by several large EU and EPSRC projects. While the multi-terminal DC grid assuming a standard voltage level for all joining terminals (likely within one planning phase) has achieved considerable progress such as on the control and DC circuit breakers, the DC-DC converters interlinking DC systems at different voltage levels face serious challenges that need to be addressed. To date, no such equipment or pilot-scale demonstration has been installed, and the reluctance is mainly due to the high cost and power losses with the existing concepts for HVDC-DC conversion. These methods based on topology-level variations often lead to a typical "Whac-A-Mole" situation among key performance indicators (i.e., cost, losses, and capability).The above difficulty stems from the characteristics of the power transistors that are used to construct HVDC converters. The proposed research will enable fundamental-level innovation by complementary use of hybrid power switches for HVDC-DC converters, which will simultaneously achieve reduced cost, lower losses, and enhanced operational flexibility. To realise the full potential of the proposed solution, this project, at first, will systematically perform modelling, design, control, and simulation studies for a new breed of DC-DC conversion architectures. Then, based on the knowledge acquired, a high-capacity test bench for the hybrid switch commutation will be built to experimentally investigate the optimisation of device hybridisation. A downscaled 900 V to 200 V, 9.6 kW DC-DC converter prototype will also be designed and integrated with a 9-converter DC grid demonstrator in the Aberdeen University lab to test its system performance. During the project, the university research and industry expertise will be combined effectively by working closely with experts from Hitachi Energy. The success of this project will set a solid foundation for new research lines in the future such as unique DC grid topologies and operational strategies enabled by the new DC-DC converters.