DC-DC transformer for interconnecting HVDC grids of different technologies and grounding schemes into an HVDC multi-terminal grid

In this paper, a new DC-DC transformer topology is described and analyzed. The Modular Multilevel DC Transformer (MMDCT) is studied for interconnecting high-voltage direct current (HVDC) systems with different grounding schemes and converter technologies. Its operating principle is explained and demonstrated using time-domain simulations, where the MMDCT is connected to a hybrid HVDC system with a Line Commutated Converter (LCC) based system on one side and to a Voltage Source Converter (VSC) based system, specifically a Modular Multilevel Converter (MMC) to the other.