A Mixed-Frequency and Hybrid-Module Based Single-Stage Solid-State Transformer With Quasi-Square Wave Modulation

Solid-state transformer (SST) has been extensively researched for decades, but their high cost and complex structure, due to numerous switches and passive components, limit practical applications. Although emerging 10 kV switch devices offer potential for simplifying the structure, their immature manufacturing processes and switching dynamics pose significant implementation barriers. Recently, a mixed-frequency (MF) based single-stage SST has shown promise due to its lower switch and capacitor count. However, challenges remain, such as the difficulty in capacitor balancing caused by multifrequency coupled arm voltage and high dv/dt issues from the arm high-frequency (HF) square wave. Additionally, the optimal design of arm submodules has not been thoroughly analyzed, affecting system efficiency and cost. To address these issues, this article proposes a MF and hybrid-module based SST. The arm topology is modified with an additional HF half-bridge to alleviate the multifrequency burden. Then, a simplified balancing control method is introduced without HF current sensing and edge detection. Moreover, a quasi-square wave (QSW) with lower dv/dt replaces the traditional square wave. Based on the model and soft switching characteristic under QSW modulation, the given optimal number design of submodules leads to improved efficiency and reduced cost. Under a 10 kV/1 MVA condition, the proposed single-stage SST can achieve 96.1% efficiency, with a 52% reduction in switch count compared to two-stage SSTs and a 35% reduction compared to existing MF SSTs, and the estimated device cost can be decreased by 23% and 20%, respectively. Simulations and experiments confirm the feasibility.