Design and Control of Power Fluctuation Delivery for Cell Capacitance Optimization in Multiport Modular Solid-State Transformers

Modular multilevel converter (MMC)-based solid-state transformers (SSTs) have gained increasing interest lately. Topologies consisting of cells made of directly coupled MMC submodules (SMs) and dual-active-bridge (DAB) modules have been introduced to form multiport SSTs to interface different grid entities. Designated as modular SSTs (M-SSTs), such devices can interconnect hybrid ac-dc distribution systems and enable flexible power flow control among participating grids with different voltage forms and levels. However, the large capacitors needed to suppress power fluctuations in the power cells are a major contributor to the SST's poor power density. This article first proposes a power fluctuation delivery (PFD) control strategy for M-SST cell capacitance optimization. Through a modified phase-shift control, the low-frequency fluctuating power in the MMC SMs is transferred to the DAB's secondary side and is automatically canceled there. As a result, the cell capacitance requirement is significantly reduced. Considering the modified ripple power path, both power semiconductors and the high-frequency transformers in DABs are affected. Therefore, the design considerations of a single M-SST cell (1 MMC SM + 1 DAB) with/without PFD control are elaborated for a 2-MVA M-SST example. It is shown that the cell capacitance can be reduced to 10% of its original value, and the power density of the whole cell is increased by 50% with only 0.22% drop in the overall efficiency. The feasibility and effectiveness of the proposed method are verified by simulation results in this 2-MVA case and experimental results obtained from a 4.8-kVA scaled-down M-SST prototype.