High-Frequency Transformer Design for Modular Power Conversion from Medium Voltage AC to 400V DC
With the advent of SiC and GaN devices, we propose to operate the dc/dc stage at an unprece-dented high frequency: 500 kHz, which is 25 times higher than industry state-of-the-art SSTs. At 500 kHz, the selection of WBG devices and converter topologies are critical. A novel bi-directional CLLC resonant converter with a power rating of 15kW, instead of the popular back-to-back con-nected dual active bridge, is used to reduce switching loss and guarantee soft switching under all conditions.
For such a 4160V ac to 380V dc system, insulation is critical for the entire system. We propose to use a sectionalized coil with a UU core structure to guarantee enough clearance and creepage distance between the primary and secondary windings. Fig. 3 shows a prototype of the high-frequency transformer with medium voltage isolation. The tests show that it can pass a lightning impulse test of 30kV, applied voltage testing of 12kV, and partial discharge test of 5.4kV, which are the standard testing requirements of the IEEE and IEC for a 4.16kV isolation application.