Soft-Switching Solid-State Transformer (S4T) With Reduced Conduction Loss

© 2020 IEEESolid-state transformers (SSTs) are a promising solution for photovoltaic (PV), wind, traction, data center, battery energy storage system (BESS), and fast charging electric vehicle (EV) applications. Traditional SSTs are typically three-stage, i.e., hard-switching cascaded multilevel rectifiers and inverters with dual active bridge (DAB) converters, which leads to bulky passives, low efficiency, and high EMI. This paper proposes a new soft-switching solid-state transformer (S4T). The S4T has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc-link, and controlled dv/dt which reduces EMI. The S4T comprises two reverse-blocking current-source inverter (CSI) bridges, auxiliary branches for ZVS, and transformer magnetizing inductor as reduced dc-link with 60% ripple. Compared to the prior S4T, an effective change on the leakage inductance diode is made to reduce the number of the devices on the main power path by 20% for significant conduction loss saving and retain the same functionality of damping the resonance between the leakage and resonant capacitors and recycling trapped leakage energy. The conduction loss saving is crucial, being the dominating loss mechanism in SSTs. Importantly, the proposed single-stage SST not only holds the potential for high power density and high efficiency, but also has full functionality, e.g., multiport DC loads integration, voltage regulation, reactive power compensation, unlike traditional single-stage matrix SST. The S4T can achieve single-stage isolated bidirectional DC-DC, AC-DC, DC-AC, or AC-AC conversion. It can also be configured input-series output-parallel (ISOP) in a modular way for medium-voltage (MV) grids. Hence, the S4T is a promising candidate of the SST. The full functionality, e.g., voltage buck-boost, multiport, etc. and the universality of the S4T for DC-DC, DC-AC, and AC-AC conversion are verified through simulations and experiments of two-port and three-port MV prototypes based on 3.3 kV SiC MOSFETs in DC-DC, DC-AC, and AC-AC modes at 2 kV.This work was supported by Power America Institute, ARPA-E under DE-AR0000899, and Center for Distributed Energy, Georgia Institute of Technology

Fault current limitation with energy recovery based on power electronics in hybrid AC-DC systems

The active distribution networks are becoming increasingly complicated hybrid AC-DC systems constructed by massive power electronics, the magnitude and direction of power flow may change randomly at any time, making the usual protection potentially insensitive, increasing the negative impacts of single-phase-to-ground (SPG) fault which accounts for the majority of all faults that occurred in medium-voltage (MV) distribution networks in the past. The zero-sequence current in the impedance branch induced between the lines and ground will pass through the SPG fault branch as fault current. This study transfers the zero-sequence current from the SPG fault branch to the power electronic branch connected between the faulty phase and ground involved in the construction of hybrid AC-DC system, thereby limiting SPG fault branch current and reducing fault node potential. This helps to extinguish fault arc and provides engineers with safe conditions to clear faulty elements from the SPG fault branch. The power electronic bears the same fault current and fault phase voltage as SPG fault and will therefore absorb energy in the same way as SPG fault, the energy is recovered and routed back to the hybrid AC-DC system via interconnected power electronics for reuse. The proposed is verified by simulation and experiment