SiC-Based 5-kV Universal Modular Soft-Switching Solid-State Transformer (M-S4T) for Medium-Voltage DC Microgrids and Distribution Grids

• Published in: IEEE transactions on power electronics 2021-10, Vol.36 (10), p.11326-11343
• Main Authors: Zheng, Liran, Han, Xiangyu, An, Zheng, Kandula, Rajendra Prasad, Kandasamy, Karthik, Saeedifard, Maryam, Divan, Deepak
• Format: Article
• Language: English
• Subjects: Alternating current; Capacitors; Coaxial cables; Conduction losses; Current sources; Current-source converter (CSC); dc transformer (DCT); Distributed generation; Electric bridges; Electric converters; Electric potential; Electric power systems; Electric vehicle charging; Electromagnetic interference; ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATION; Energy conversion; ENERGY STORAGE; ENGINEERING; Fly by wire control; high-frequency link (HFL); Inductance; input-series output-parallel (ISOP); Insulation; isolated bidirectional dc–dc converter (IBdc); medium-voltage direct-current network; Modular equipment; Modules; MOSFETs; power electronic transformer (PET); POWER TRANSMISSION AND DISTRIBUTION; Silicon; Silicon carbide; SOLAR ENERGY; Solid state; Switching; Topology; Transformers; Voltage; WIND ENERGY; Zero current switching; Zero voltage switching; zero-current switching (ZCS); zero-voltage switching (ZVS)
• ISSN: 0885-8993; 1941-0107
• DOI: 10.1109/TPEL.2021.3066908

Medium-voltage dc (MVdc) grids are attractive for electric aircraft and ship power systems, battery energy storage system (BESS), fast charging electric vehicle (EV), etc. Such EV or BESS applications need isolated bidirectional MVdc to low-voltage dc (LVdc) or LVac converters. However, the existing Si-based solutions cannot fulfill the requirements of a high-efficiency and robust converter for MVdc grids. This article presents a 5-kV SiC-based universal modular solid-state transformer (SST). This universal current-source SST can interface either an LVac or LVdc grid with an MVdc grid in single-stage power conversion, while the conventional dual-active bridge (DAB) converter needs an additional inverter. The proposed SST module using 3.3-kV SiC MOSFET s and diodes is bidirectional and can serve as a building block in series or parallel for higher voltage higher power systems. The topology of each module is based on the soft-switching solid-state transformer (S4T) with reduced conduction loss, which features reduced electromagnetic interference electromagnetic interference (EMI) through controlled dv/dt, and high efficiency with full-range zero-voltage switching for main devices and zero-current switching for auxiliary devices. Operation principle of the modular S4T (M-S4T), capacitor voltage balancing control between the cascaded modules, design of components including a medium-voltage (MV) medium-frequency transformer (MFT) to realize a 50-kVA, 5-kV dc to 600 V dc or 480 V ac M-S4T are presented. Importantly, the MV MFT prototype achieves very low leakage inductance (0.13%) and 15-kV insulation with coaxial cables and nanocrystalline cores. The proposed universal modular SST is compared against the DAB solution and verified with dc-dc and dc-ac simulation and 4-kV experimental results. Significantly, the MV experimental results of a modular dc transformer with each module at MVdc are rarely covered in the literature and reported for the first time.