In order to enable the massive introduction of renewable energies the need for high-voltage direct current (HVDC) grids is anticipated. Large, globally interconnected HVDC networks will likely be the most cost-efficient means to balance electricity demand and available generation. In a meshed system it is important to ensure reliability, robustness, failure management, and fast protection of equipment. In case of a failure somewhere in the grid, the remaining system must be kept operational. State-of-the-art converter implementations are either not adapted to future system requirements or lead to increased losses, cost, and converter footprint. Therefore, this thesis examines several aspects of how to improve the HVDC converter design and functionality with the ultimate aim of developing reliable, highly efficient, cost-effective, more compact and lightweight converters.

Advancements are made on several levels of the converter hardware hierarchy. Main circuits, submodule (SM) topologies, and auxiliary power supply (APS) concepts are investigated and new solutions are proposed. On main-circuit level, different voltage-source converters (VSCs) are evaluated in terms of their energy storage elements. This is useful to compare the physical volume of capacitors required by each topology and, thus, to address the need to develop more compact converter stations. The theoretical analysis indicates that the required energy storage of the alternate arm converter (AAC) is smaller compared to the modular multilevel converter (MMC).

On SM level, new topologies are evaluated with the goal to find topologies, which enable efficient handling of dc-side short circuits, reduction of power loss, and lower SM capacitance. The semi-full-bridge (SFB) SM is identified as one of the most promising topologies from this point of view and is investigated in detail. A control concept for capacitor balancing and several options for improved operation of the SFB are presented. Furthermore, a novel SM cluster topology is proposed which features low conduction losses and increased protection against explosion.

The availability of a reliable APS system is crucial for equipment in future HVDC grids. Therefore, APS solutions are investigated considering design complexity, reliable performance, and power consumption. This thesis presents a novel combined optical power and data transmission concept which is tailored to the specific requirements of HVDC converters employing high-voltage (HV) silicon carbide (SiC) devices. The proposed concept offers a robust solution for isolated APS and signal transmission across any voltage barrier.