An MMC-HVDC system using medium-frequency AC collection grid for offshore wind farms

Modular multilevel converter based high voltage direct current transmission (MMC-HVDC) is one of the most reliable and predominant solutions for far distance offshore wind power, but the huge submodule capacitors limit the increase of the capacity of offshore MMCs. In fact, the offshore ac grid is decoupled from the onshore grid and frequency can be a control dimension of offshore MMC. Moreover, the submodule capacitor voltage ripple is verified to be inversely proportional to the line frequency. This paper proposes an MMC-HVDC system using medium-frequency ac collection grid for offshore wind farms to optimize the converter dimension and cost. The volume and cost of the offshore MMC are evaluated. The simulation has been done to verify the proposed medium frequency solution. The evaluation and simulation results show that the offshore MMC in the proposed system has decreased volume and cost and it can operate in steady and transient states.