Comparison and review of DC transformer topologies for HVDC and DC grids

Driven by the growing share of renewable energy sources that are located far from load centers, high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution will be expanded in the future. DC transformer is needed to connect different dc voltage levels or different dc transmission lines in this development. Although numerous dc transformer topologies have been proposed, they do have relatively advantages and disadvantages when considering from a practical point of view, and there is not one topology that is suitable for all scenarios. Based on three different implemented scenarios, this paper provides a comprehensive review of the latest topologies, and highlights the attributes and limitations of two most promising topologies through time-domain simulations and evaluation of three different parameters in terms of transferable power, power rating of ac transformer and the number of switches.