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500kVA Medium-Frequency Core-Type Amorphous Transformers with Alternately Wound Sheet Winding for Offshore DC Grid

This paper describes a low winding loss design methodology to develop a real-scaled medium frequency transformer (MFT) for an isolated DC-DC converter to be used in a DC-interconnected offshore wind farm system. We assembled a core-type 500 kVA MFT consisting of a lap-joint amorphous wound core and windings with a primary Cu sheet and divided secondary Cu sheets, wound alternately in turns. Then, we compared its loss performance with that of a conventionally designed MFT. The alternately wound winding structure suppressed the medium-frequency proximity effect between the Cu sheets and the in-plane eddy current due to the fringing flux crossing the edges of the sheets and fixtures, and the winding loss at 3 kHz was 61% lower than that of the conventional MFT. In addition, we propose and discuss an accurate estimation method for the winding loss of core-type MFTs, considering the in-plane eddy current loss at the edge of the Cu sheets based on the finite element method.