Modeling and Defect Analysis of Transformer-Less Multilevel Back-to-Back Cascaded H-Bridge Converter

Back-to-back (B2B) soft open point (SOP) is a widely adopted solution for connecting distribution networks with different phases. In medium voltage networks, two commonly used topologies are modular multilevel converters (MMC) and cascaded H-bridge converters (CHB). However, these schemes require a large number of semiconductor devices, which hinders the widespread adoption of B2B-type SOP in medium voltage networks. In this paper, the scheme of the addition of inductance to the transformer-less B2B-CHB topology to mitigate the short-circuit current is proposed. This modification shows promise in reducing the cost and size of the medium voltage B2B-type SOP. A single-phase model is established and the control system is designed to ensure stable operation. Using the mathematical model of the system, certain drawbacks of the transformer-less B2B-CHB topology are identified. Specifically, the output current range of the converters is significantly influenced by the grid voltage of both sides, and there is a sharp variation in the current distribution among the semiconductors. The theoretical reasons are analyzed for these phenomena and verified through simulations.