الفهرس 
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Abstract
This thesis explores the integration of renewable energy sources into Medium/High-voltage DC grids, focusing on developing DC-DC conversion systems. It investigates Modular Multilevel Converters (MMC), covering principles, submodules topologies, control strategies, and capacitor voltage balancing techniques. Different DC-DC converter topologies are explored, including isolated and non-isolated configurations, along with DC-link balancing circuits and soft switching techniques and HVDC circuit breakers. A novel hybrid modular multilevel DC-DC converter topology is proposed to address capacitor voltage imbalance issues during conversion. This converter utilizes a single MMC leg with half-bridge and full-bridge submodules, achieving balanced capacitor voltages through arm energy equalization. The bidirectional converter facilitates power flow from high-voltage to lowvoltage sides, functioning as a DC transformer. Simulation and experimental results validate the proposed approach, demonstrating its viability and investigating DC fault blocking capabilities. Despite lacking DC-fault blocking capability initially, it can be enhanced for this feature by replacing upper HB-SMs with FB-SMs. Efficiency analysis reveals an overall efficiency of 96.32% for a 2MW 10kV/2kV converter. Simulation results and prototype validation confirm the feasibility of the proposed architecture for DC-grid applications.