New breed of solid-state transformer mainly combing hybrid cascaded multilevel converter with resonant DC-DC converters

This paper proposes a new breed of solid-state transformer (SST) distribution tail system mainly based on hybrid cascaded multilevel converter and resonant DC-DC converters between medium-voltage distribution and low-voltage customer side. The proposed SST topology mainly combines per-phase cascaded H-bridge converters based on the DC transformers (DCTs) with the square-wave two-level VSC based on the assembled DCT together. The proposed system can offer the operational flexibility for active and reactive power control for medium-voltage side based on the simplified single-stage control system structure concept thanks to the characteristics of the resonant DC-DC converters working under the quasi-resonant mode. Additionally, compared to the existing SST systems, it offers better features such as smaller space volume, simpler control system, lower unit cost, etc. To illustrate the feasibility of the proposed SST distribution tail system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to medium-voltage ac-side unbalance.