7.2kV/100kVA Solid State Transformer Based on Half Bridge LLC Resonant Converter and 15kV SiC AC Switch

Abstract

This paper proposes a novel single-stage ac-ac converter for realizing a high-efficiency high power-density 7.2kV medium voltage (MV) solid state transformer (SST) based on half bridge LLC resonant converter and 15kV SiC AC switch. The new topology is proposed to address major challenges in MV SSTs such as achieving soft switching for the MV switches across wide voltage and load ranges. The topology is analyzed through both time domain analysis and first harmonic approximation to provide useful equations for circuit design. The LLC based SST (LLC-SST) always operates at unity voltage gain mode, soft switching behavior over wide voltage range is investigated and proper fully zero voltage switching (ZVS) design methodology are discussed. A novel 15kV SiC AC switch based on 1700V SiC MOSFETs in series connection is developed and tested at a DC voltage of 10kV/15A. Critical parameters, such as Eon, E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">of</inf> f, Eos <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> , Rds, on and leakage current, of the fabricated AC switch are characterized for system design. A full-scale 100kVA and compact LLC-SST module that converts 480Vac to 7.2kVac is developed to verify the theoretical analysis. The LLC-SST modular power density is around 25W/inch <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Peak efficiency is expected to be higher than 98.5%. A 300Vac to 4.2kVac preliminary power test with 1kΩ resistive load bank (18kW) has been demonstrated to verify the feasibility of the proposed single-stage ac-ac converter and validity of the fabricated 15kV AC switch operation. The proposed LLC-SST module can be used as a building block in three-phase architectures (6.9kVac three-wire and 12.47kVac/13.8kVac four-wire) for replacing line frequency transformer (LFT) in the medium voltage power system.