Soft-Switching Solid-State Transformer With Reduced Conduction Loss

Abstract

Solid-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 fullrange zero-voltage switching (ZVS), electrolytic capacitor-less dclink, 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 outputparallel (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.

Unified Control Strategy for Microgrid Solid-State Transformers

“…[13] proposes control algorithms to improve the efficiency under light-load conditions in traction systems. [14] proposes control of resonance topology to minimize conduction loss and achieve full-range ZVS to allow for low switching loss and smaller passive components and reduce EMI in a single stage structure. [15] proposes model predictive control (MPC) in a matrix converter-SST connected to ac grids based on the discrete model of the matrix converter-SST (MC-SST).…”

Review of a Disruptive Vision of Future Power Grids: A New Path Based on Hybrid AC/DC Grids and Solid-State Transformers

“…In [107], the analysis, modeling, and control of multiple active high-frequency converters, which employ SiC technology, are proposed, aiming at a new generation of SST. In [108], active semiconductor control strategies for an SST are addressed, based on a single-stage structure, using soft-switching, and having as its main objective the reduction of conduction losses. In [109], a new high-frequency single-stage SST structure is proposed, with the objective of reducing the number of switching devices, based on a dual active bridge converter and a buck circuit that operates in an interleaved mode.…”

LVDC Bipolar Balance Control of I-M2C in Urban AC/DC Hybrid Distribution System

“…In most of the urban DC application scenarios, the LVDC distribution system is linked to MV distribution systems for increasing the power supply reliability and efficiency. Thus, reliable power interfaces like power converters or solid-state transformers (SSTs) are necessary for the grid integration of an LVDC system (Sun et al, 2021c;Zheng et al, 2021). Unlike classical two-port power interfaces, the MMC-based three-port SSTs can connect the LVDC system to both the MVAC and MVDC systems.…”