High-Frequency Transformer Design for Modular Power Conversion from Medium Voltage AC to 400V DC
Due to the increasing use of cloud computing and big data, the power consumption of data centers alone will reach 10% of the total electrical power consumption in the world by 2020. Considering the booming data center load development and the high cost of copper, conduction loss due to low voltage (480VAC) power distribution outside the server hall needs to be reduced. We propose to utilize a medium voltage 4.16 kV ac line as the distribution bus within the data center facilities, as shown in Fig. 1. A medium voltage of 4.16kV is sent into each server hall. Inside each server hall, a solid-state transformer (SST) in a cascade configuration is employed to convert the medium voltage directly to 380V dc. The structure of the proposed SST is shown in Fig. 2. Five cascaded full bridges handle the high input voltage and achieve power factor correction with low switching frequency, while a high-frequency isolated dc/dc converter follows to convert 800V dc to 380V 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.