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Medium Voltage Solid State Transformer Based Extreme Fast EV Charger

SystemStrcutrue
Fig. 1. System structure and one sub-module.
  To shorten the charging time, increase the efficiency, and reduce the volume of an electric vehicle charger, the key research is to replace the bulky line-frequency, medium-voltage (MV) transformer cascaded by a low-voltage rectifier with a high-frequency MV ac-dc solid-state transformer (SST).

  Fig. 1 shows the whole fast charger system with medium voltage (13.2 kV), input voltage, and 1.1 kV output voltage and a three-level CLLC resonant converter circuit of cascaded submodule. This resonant converter is a dc to dc conversion component respectively connected in SST. It converts 15kW from ac-dc PFC output 1.6 kVDC to 1.1 kVDC dc bus then to a fast charger module.
  Devices for both primary side and secondary side are selected based on device loss evaluation. Magnetizing inductance Lm and deadtime td are selected based on the requirement of zero voltage switching (ZVS). Transformer core material candidates are evaluated using the specialized core-loss measurement techniques developed at CPES and the core material is selected based on the switching frequency. Litz wire is also selected based on the switching frequency. Considering the insulation requirement, the insulation materials and winding structures are evaluated and selected to minimize the transformer volume. The transformer loss is then calculated using the magnetic core loss model and litz wire winding loss model developed in CPES. Finally, all the transformer design parameters, such as number of turns, core loss density, and switching frequency are optimized based on the trade-off between the volume and converter total loss, as well as winding loss and core loss. A prototype is built according to the optimized design, as shown in Fig. 2.

  The CLLC resonant converter operation principals are analyzed and verified through experimental results. The efficiency of one sub-module is tested and compared to the design result. In addition, the insulation capability of the medium-voltage, high-frequency, solid-state transformer is also studied and discussed in the paper, especially its performance on the partial discharge test.
3-level CLLC
Fig. 2. Three level CLLC resonant converter prototype.


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