Circuit Topology and Control Scheme of a High-Power High-Voltage DC/DC Converter for Large Scale Offshore Wind Project with DC Collector Grids

In the last decade, a number of offshore wind projects have been implemented or planned to meet the CO2 emission targets and nuclear fission policy, especially in Europe. Some projects adopt high-voltage DC (HVDC) transmission with a long-distance submarine cable laid from offshore to onshore. The HVDC transmission system has the advantages of high efficiency and low cost, when compared with AC systems. On the other hand, a medium-voltage (MV) collector grid uses AC 33kV instead of DC so that the efficiency and cost will be poor in case of high-power turbines because the higher power rating of turbines causes higher current in the collector grid and requires greater separation between the turbines. Presently, the MW rating of the latest wind turbines exceeds 8.0MW, and the MW rating has been increasing as the years go by. Thus, the MVDC collector grid will be installed, and an HVDC/DC converter is required to step up the MVDC to HVDC.