Medium Frequency Transformer Based Configuration for Voltage Source Converter Based Offshore Wind Farm

The biggest challenge for existing offshore wind farms is the bulky and costly offshore substation. Series-connected offshore wind farm configuration, on the other hand, featuring no offshore substation, is getting increased attention in the research. In this study, a medium frequency transformer (MFT)-based configuration is proposed for such series-connected offshore wind farms. The proposed configuration consists of two parts, offshore and onshore. The modular multilevel converter (MMC) is employed at onshore, while the offshore part is composed of numbers of series-connected wind turbine-based systems. Each wind turbine-based system consists of a medium-voltage permanent magnet synchronous generator, a passive three-phase rectifier, and a modular MFT-based converter. By contrast to the commercial offshore wind farm, the proposed configuration features low cost thanks to the series-connected structure; by contrast to configurations in the literature, the present configuration features high power density, high reliability and flexibility thanks to the adoption of the modular MFT-based converter. Apart from fulfilling traditional control objectives (maximum power point tracking, dc-link voltage control, and reactive power regulation), this work endeavors to ensure evenly distributed power and voltage sharing among the constituent modules given the cascaded structure of the MFT-based converter. Finally, simulated results are provided to verify its performance.