Hybrid-Frequency Cascaded Full-Bridge Solid-State Transformer

Modular solid-state transformers (SSTs) are suitable for various voltage- and power-level applications. State-of-the-art modular-SST topologies contain a considerable quantity of high-frequency transformers (HFTs) and power switches, thereby increasing the costs, difficulty, and workloads of engineering. To overcome this challenge, a new SST topology family, namely, hybrid-frequency cascaded full-bridge SST (HCFB-SST), is established on the basis of the concept of hybrid-frequency modulation. In comparison with modular-multilevel converter SST (M²LC-SST) or CFB-SST, the HCFB-SST uses only 50%-66.7% and 1.11%-10% the device count of power switches and HFTs, respectively, in typical applications, thereby obtaining considerable cost savings. It also achieves a cost reduction of sub-module redundancy. In addition, HCFB-SSTs have simple engineering and compact structures. A hybrid-frequency modulation method is proposed for a low well-balanced average switching frequency and a high equivalent switching frequency without monitoring the bridge-arm current. Analysis and comparison of the semiconductor loss with existing topologies are presented. The simulation studies are conducted to validate the effective function of HCFB-SST and the proposed modulation method.