Analysis and Experimental Validation of Current-Fed Switched Capacitor-Based Modular DC Transformer

Medium-voltage dc (MVdc) power distribution grid is a key link to build dc power networks. In this article, a novel input-series-output-parallel modular dc transformer (DCT) with the high-frequency-link (HFL) voltage buck-boost matching strategy is proposed for MVdc grids application. The proposed DCT is composed of a current-fed switched capacitor structure and corresponding dual-active-bridge converters, and they need to operate collaboratively to achieve the great property. Comparing with the traditional switched capacitor-based DCT (SCDCT), the most salient merits of the proposed DCT include the HFL voltage buck-boost adaptive regulation, the low conducting and switching current in switches and the capacitor voltages self-balance characteristic. These merits are all beneficial for the improvement of efficiency, operating reliability, switching performance, and cost of DCT systems. Meanwhile, the proposed DCT reserves the advantages of SCDCT including redundant submodule design and dc fault isolating capability. The topology, buck-boost modulation principle, capacitor voltage self-balance analysis, switching behaviors, and comprehensive comparison with the traditional SCDCT are completed in detail. Finally, the experimental results validate the effectiveness and correctness of the proposed solution.