Design and Verification of a Medium-Frequency Transformer in a Three-Phase Dual-Active Bridge DC-DC Converter for Medium-Voltage Grid Connection of Offshore Wind Farms

This paper covers the development of a medium-frequency transformer (MFT) for a high-power three-phase dc-dc converter submodule of a modular converter system to inter-connect offshore wind farms with medium-voltage dc collector grids. Here, a low-loss MFT is needed. Therefore, a new MFT design concept for a high-efficient dc-dc converter is proposed. To meet the required series inductance of the converter, usually additional bulky inductors are used in series to the MFT. This paper presents the design of a MFT with an increased leakage inductance avoiding the extra series inductor to realize a high-efficient and compact MFT at the same time. Additionally, the MFT needs to withstand a voltage of ±25 kV of the dc collector grid. These requirements are satisfied by a sectionalized stacked winding structure, an encapsulated secondary winding with resin and by using available nanocrystalline cores. The electrical and thermal performance of the realized MFT prototypes are demonstrated.