Research Associate E.ON Energy Research Center, Institute for Power Generation and Storage Systems, RWTH Aachen E.ON Energy Research Center, Institute for Power Generation and Storage Systems, RWTH Aachen
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 interconnect 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.