MMC의 가변 직류링크전압 제어에 의한 풍력발전단지의 직류 연계 운전

Abstract

An offshore wind farm has many advantages such as a high and steady wind speed. However, economically it is hugely problematic. In order to reduce the installation costs of an offshore wind farm, many topologies for high-voltage direct current transmission system (HVDC) have been researched and analyzed by researchers from around the world. DC series topology is one of the solutions for offshore wind farm about grid connection. In this method, the DC link voltage is created from the output voltage of the combined wind turbines connected to each other by the DC series. Because this method can get rid of the need for an offshore platform, which plays a role as a booster or AC/DC converter, it eliminates the need to spend on the installation costs, operation, and management of an offshore platform. However, the DC link voltage of a DC series connected to a wind farm may not be constant due to the variable wind speed. One wind turbine alone must be able to generate a high DC link voltage in order to transfer wind power to grid stably. From this perspective, this thesis proposes the operation of a DC series connected wind farm by using a tap changer transformer technique with modular multilevel converter (MMC)-HVDC. In this study, the wind farm would have a variable DC link voltage based on the number of operating wind turbines. In order to verify the proposed method, a simulation will be conducted for two situations : First, the random step operation of a wind farm. Second, the operation of the wind farm with variable wind speeds. The proposed method will be applied to a 120 MW wind farm with forty wind turbines. The simulation will be carried out by using the PSCAD/EMTDC program.