Hybrid Isolated Modular Multilevel Converter Based Solid-State Transformer Topology With Simplified Power Conversion Process and Uneven Voltage Ratio

This article proposes a novel triple-port solid-state transformer (SST) topology based on a hybrid isolated modular multilevel converter (MMC), which overcomes the limitation of voltage ratio R_(pu) in the conventional MMC-based SST. By integrating a high-frequency link into the MMC structure and applying various types of isolated submodule (ISM), the proposed SST can function as a step-down rectifier, where U_MVDC < U_MVAC(p-p). In this case, the power electronics converters used to connect downstream medium-voltage direct current (MVdc) link and distributed energy resources or battery storage require fewer active devices and may minimize the cost of protection equipment. In addition, the proposed SST retains the significant advantages of single-stage SSTs, such as single-stage power conversion, saving capacitor, and a simple control system. First, the topology and modulation strategy of two types of ISMs are discussed. In addition, this article provides a detailed analysis of operation principles, power flow, grid-tied control, and system reliability. Moreover, the proposed converter is compared with other SST topologies in terms of key performance indicators, such as peak efficiency, system volume, and number of active semiconductor devices. Finally, a 300-V/20-kW scaled-down prototype is developed, and the experimental results demonstrate the performance and verify the correctness of the proposed converter.