Abstract: The isolated DC transformer based on high- performance soft magnetic material (SMM) is the key equipment that supports DC collection boost of large-scale new energy, multi-voltage DC networking, and interconnection of DC transmission and distribution. The microstructures and grain orientations of 0.15 mm grain-oriented (GO) silicon steel, 0.10 mm ultra-thin (UT) silicon steel, and 0.025 mm amorphous alloy samples were studied by optical microscope (OM), electron back scatter diffraction (EBSD), and transmission electron microscope (TEM) techniques. The magnetic properties of the threee types of SMM at medium frequencies were researched using an arbitrary waveform magnetic field excitation measurement system. The modeling and simulation analysis of the medium-frequency isolation transformer (MFIT) core and winding in DC transformer were carried out using COMSOL finite element software. Physical models of the MFIT based on the threee types of SMM were built and experimental verification were completed. Results showed that, completely different microstructures of materials lead to different medium-frequency magnetic performance and magnetostrictive noise characteristics. The loss of 0.10 mm UT steel is higher than that of 0.15 mm GO steel at frequencies of 100 Hz~1 kHz. However, the no-load loss of the 0.10 mm UT steel MFIT model is 20.7 % lower than that of the 0.15 mm GO steel MFIT model under the rated condition of 400 Hz/35 kVA. The latter one has the advantage of low loss at frequencies of 100~200 Hz, minimizing noise as well as a large space for capacity expansion. Besides, the amorphous alloy MFIT model has the lowest no-load loss as well as the highest load loss and noise. Application prospects of the threee types of SMM in different scenarios and capacities of isolated DC transformers were discussed.

Key words: magnetic soft materials, DC transformer, coremodel, experimental analysis