Analysis and Design of a Nonisolated DC Transformer With Fault Current Limiting Capability

The dc-dc converter, also termed as dc transformer, plays an important role in the future dc grids. It is used to interconnect network segments or lines with different voltage levels. A nonisolated modular multilevel dc-dc converter is presented in this article, which can realize bidirectional power regulation and dc fault current limiting. The control strategy is simplified and the current stresses are decreased due to the employment of autotransformers. The nonisolated structure can reduce the requirements of autotransformers, because only part of the transmitted power flowing though the autotransformers. The operation principle and control strategy of this converter are illustrated in detail. The analyzes of power transmission, submodule (SM) capacitor voltage ripple, the optimal configuration, and the efficiency of this converter are also provided. The optimal design methods of the dc transformer are proposed to reduce the costs and power losses according to the analysis results. Simulations performed in power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) and experiments based on a down-scaled prototype verify the operation principle, control strategy, and theoretical analysis. The simulation results and experiment results show the performances of the converter under steady state, power regulation, and fault conditions.