Medium frequency diode rectifier unit based HVDC transmission for offshore wind farm integration

The offshore wind farms are the main trend of the wind power development in the future. The medium frequency diode rectifier unit based high voltage direct current transmission system is proposed here for offshore wind farm integration. Unlike conventional high voltage direct current solutions, the rated frequency of the offshore alternating current (AC) grid is increased to about 100 similar to 400 Hz in the proposed system. Besides, a grid-forming control scheme for the wind turbine converters is also proposed in this paper, where the machine side converter controls the direct current (DC) voltage and the grid side converter controls its output alternating current voltage in the global unified reference coordinate system. As a result, smaller and lighter passive components, such as the transformers, alternating current filters and reactive power (VAR) compensators, could be adopted in the offshore grid, which is beneficial for achieving higher economic efficiency. The small stability analysis and the time domain simulation of the proposed system are carried out based on a simplified model and power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) respectively, and the feasibility of the proposed schemes is verified.