Comparative Study of a Multi-MW High-Power Density DC Transformer With an Optimized High-Frequency Magnetics in All-DC Offshore Wind Farm

In this paper, the design of a 1/30 kV, 10 MW modular isolated dc-dc converter is presented. The design and optimization of a high-frequency transformer as the key part of such a converter is addressed. Efficiency curves for different semiconductors and frequencies are presented in order to find an optimum frequency-enabling adequate transformer volume reduction with a suitable compromise with the converter efficiency. It was found that for this voltage level and size, it is no point to go above 6 kHz from the transformer perspective since the isolation requirement that leads to that the size is not reduced much more for even higher frequencies. At 5 kHz, the efficiency of the transformer having a nanocrystalline core reached 99.7% while the power density was about 22 kW/l. For the entire dc-dc converter, an efficiency of 98.5% was reached at 5 kHz switching frequency using SiC metal-oxide semiconductor field-effect transistors as switching elements while the efficiency when using insulated-gate bipolar transistors reached 97.2% at the same frequency.