Closed-Loop Control of Common DC-Link Voltage of Solid-State Transformer From Medium-Voltage Stage Using Quasi-Square-Wave Modulation

A closed-loop control architecture and a quasi-square-wave modulation method are proposed for two-stage (ACDC and DC-DC) solid-state transformer (SST) that regulates isolated common DC-link voltage in a closed-loop manner without needing frequency control from series resonance converter (SRC) at DC-DC stage. Closed-loop control uses common DC-link voltage as feedback variable, allowing it to be realized from LV side, reducing isolation requirements. Input-series output-parallel (ISOP) modular architecture and resonance operation balance all DC-link voltages of AC-DC cells naturally. AC-DC stage is mainly realized by Si-IGBT switches, and only a voltage source converter (VSC) is realized by SiC-MOSFET switches, reducing semiconductor costs. A modulation strategy is proposed for MV stage utilizing natural voltage balancing condition, switching all IGBT cells at power grid frequency to minimize switching losses. VSC unit is switched at 40 kHz to achieve equivalent high switching frequency, enabling low-frequency harmonic filtration and transient power support. Proposed control and modulation together achieve control simplicity, modularity, higher efficiency, and cost savings, ensuring desired dynamic response. Proposed technique is validated experimentally on a 600 V(AC) / 200 V (DC), 5 kW SST system.