Multiple-Modular High-Frequency DC Transformer With Parallel Clamping Switched Capacitor for Flexible MVDC and HVDC System Applications

The direct-current transformer (DCT) plays a key role in bus connection, power transmission, and electrical isolation in medium-voltage (MV) and high-voltage (HV) dc power grids. This article proposes a multiple-modular high-frequency DCT with a parallel clamping switched capacitor (CSC-DCT) for connecting dc buses. The proposed CSC-DCT is mainly composed of several modified dual-active bridges (DABs) where the CSC interfaces are employed to connect DABs. Each CSC is connected to the corresponding H-bridge of DAB in parallel; thus, the concentrated capacitors of the traditional DAB can be eliminated, and the capacitor voltages in CSC can be regulated. The CSC-DCT not only has the universal advantages of modular converters, such as flexible control, high switching frequency and power destiny, and easy installation and debugging, but also obtains direct power conversion, dc fault-handling capability, online redundant design, and capacitor voltage self-balance capability. All of the merits improve the reliability and power quality of dc power grids and simplify the design of the control system. In addition, CSC-DCT has the ideal ON-state current and soft-switching performance, which lead to lower switching current and high-frequency-link (HFL) current stress, as well as higher efficiency. The topology, modulation, control structure, voltage, and current characteristics and switching behaviors of CSC-DCT are investigated in detail, and the experiment results verify the correctness and effectiveness of the analysis and the proposed schemes.