CLLLC Dual Active Bridge with Novel Insulation Approach for SST Applications

This paper proposes a new insulation approach to meet the medium-voltage (MV) isolation requirements of the DC-DC converter modules in a Solid-State Transformer (SST) or Power Electronic Transformer (PET). The proposed approach simplifies the fabrication and cooling requirements of the MV transformer. The isolation strategy results in relatively higher leakage inductance and lowers magnetizing inductance for the transformer. A phase-shift controlled CL’L’LC resonant dual active bridge is proposed as the ideal topology to effectively utilize the transformer and meet the control needs. The designed converter maintains zero voltage switching at all load conditions, even at no-load. The phase-shift modulation provides a smooth transition from forward power to reverse power and vice versa. An 80 kW, 800 V to 800 V, 50 kHz converter prototype is fabricated. The MV transformer is designed to have more than 90 mm creepage and 50 mm clearance, and 12 kV dielectric withstand capability. Detailed fabrication details for the MV transformer are given. Steady-state waveforms at no-load and full-load validate the ZVS of all devices over the entire load range. To prove the ease of controllability, hardware result for a step-change in power from 80 kW to -80 kW is given. Measured rated power efficiency is 98.63%, and peak efficiency is 98.85% measured at 60% load.