As the demand for hydrogen increases, large-scale electrolyzer plants powered by renewable energy sources are gaining attention. High Voltage Direct Current (HVDC) grids can be used to supply these plants, as they are widely employed to transmit large amounts of renewable energy over long distances. In this paper, a low-frequency transformer-based solution for providing the required output voltage for electrolyzers is explored. Specifically, this paper focuses on designing a transformer cascade that enables high-ratio, high-power voltage conversion. The applicability of transformer scaling laws to this scenario is examined and the increase in transformer window size at different voltage ratings is analyzed. A detailed comparison of single-stage and two-stage transformer solutions is performed, considering factors such as material usage, losses, and leakage impedance. The proposed solutions are modeled and simulated using Ansys Maxwell software. The results indicate that the increase in material costs for a two-stage solution is considerably lower than predicted by conventional scaling laws.

The transformerless modular multilevel DC/DC converter (MMC-DC) has been extensively investigated as an alternative to the isolated front-to-front MMC DC/DC converter. The absence of a transformer, which is typically the bulkiest component, makes it an attractive solution, especially in applications where available space is scarce. An MMC converter consists of multiple arms. Each arm is made up of submodules (SMs), which can be either half-bridge (HB) or full-bridge (FB). The types and total ratings of the submodules included in the arms are design choices. This paper presents a comprehensive analysis and comparison of various arm configurations for the transformerless MMC-DC converter operating at high step-down ratios. To support this, a theory of hybrid arms-i.e., arms that include both types of submodules-is developed and verified using a Simulink model. The results show that energy constraints in hybrid arms limit their advantages over fully FB or HB arms.

High-voltage direct current (HVDC) grids are viable for expanding the existing grids. The development of DC transmission corridors by multiple manufacturers requires practical DC-DC converters to create multi-terminal HVDC grids. Unlike their low- and medium-voltage counterparts, these DC-DC converters face special challenges, including physical constraints and high costs, despite having no fixed operating frequency restrictions. This paper investigates power loss behavior of a 1 GW, 500 kV bipolar isolated DC-DC converter through theoretical analysis and simulation results for HVDC interconnections. The research aims to identify suitable operating frequencies which optimize converter efficiency and compactness through individual loss assessments of converter and transformer components. For accurate core loss calculations, finite element analysis (FEM) is performed using ANSYS and combined with MATLAB for detailed converter loss analysis. The results will provide fundamental principles for the design of high-power DCDC converters in HVDC grid interconnections.