Research On Unipolar-to-bipolar Interface DC-DC Converter For Future Dc-based Offshore Wind Farms

With the rapid economic development of the whole society, actively developing clean, renewable energy and improving the utilization and efficiency of new energy has been highly valued. Offshore wind farm has excellent wind resources and less impact on the environment. The concept of the DC grid provides the basis for future applications DC-based offshore wind farm. To realize the bipolar line, the paper determines an overall structure of unipolar-to-bipolar DC-DC interface converter in DC-based wind farm. This thesis focused on the interface converter topologies and possible control strategies.Firstly, the bi-directional full-bridge isolated DC-DC(DAB) converter was introduced. A detailed analysis of the operating mode of DAB converter and the power transfer equation of the converter were made. The average model and the small signal model of DAB converter were established. The voltage closed-loop strategy was designed for unipolar-to-bipolar DAB converter.The bidirectional DC-DC converter technologies for high-voltage and high-power were described, including high-voltage series technology, series and parallel combinations and modular multi-level technology. The principle of modular multi-level technology was analyzed and various types of submodules were introduced. This paper studied two main difficulties of MML technology, researching the principles and methods of voltage balance and circulating current suppression. Models were made in Simulink simulation to perform the voltage control of unipolar-to-bipolar DAB and MMC converter.Based on the theoretical analysis, two low power unipolar-to-bipolar DC-DC converters was designed and constructed. The results show that the unipolar-to-bipolar DC-DC converter can effectively control the voltage of the wind farm in different kinds of conditions.