In this study, based on the analysis of operation principle and approximate equivalent circuit of the three-phase DAB converter, the expressions of harmonic current of three-phase medium-frequency transformer (MFT) with different winding configurations, including wye-wye, wye-delta, delta-delta, are derived by using fundamental wave analysis, and the calculation method of copper loss is proposed. Considering the voltage waveforms and phase shift strategy under different winding configurations, the expressions of magnetic flux density under six-step and three-step voltage waveform excitations are deduced, the extended expressions of various modification expressions of Steinmetz equation are presented to determinate the magnetic core losses of three-phase MFT under no-load and load condition. On this basis, the equivalent thermal network of three-phase MFT with five-legged core topology is built to calculate the temperature rise. Simulation and experimental results on three 5 kHz/15 kW three-phase MFT models with nanocrystalline alloy core verify the effectiveness of the above methods.

three phase medium-frequency transformer; iron loss; copper loss; hot-spot temperature; harmonic current