Technical challenges and innovative solutions for the optimal operation of the future meshed HVDC-VSC systems

The European Union’s regulatory framework governing energy supply and the decarbonization of the power generation sector outlines a unified commitment to accelerating the green transition, promoting the development of innovative and sustainable technologies for the energy transmission networks. In this context, high voltage direct current (HVDC) transmission systems will play an increasingly crucial role, enabling the connection of remote renewable generation centers, such as off-shore wind farms, to major load centers, both within and beyond national borders. Among the possible architectures, the multi-terminal configuration is emerging as the predominant one, promising greater grid interoperability and the integration of different market zones. This thesis investigates and proposes innovative solutions for next-generation multi-terminal HVDC systems, adopting a “multi-layer” approach, incorporating considerations ranging from the semiconductor device level up to innovative protection strategies. In this regard, the complete design of a three-terminal HVDC-Voltage Source Converters (VSC) system lab-scale prototype, equipped with a silicon carbide (SiC)-based hybrid circuit breaker, has been realized. In addition, a detailed analysis of switching overvoltages and, in particular, the voltage polarity reversal on dc links upon circuit breakers intervention has been conducted. The contributions obtained include the design of a novel protection device, called Polarity Reversal Inhibitor, capable of efficiently suppressing voltage polarity reversal phenomena. Eventually, the possible use of high-voltage SiC devices to enhance the reliability and efficiency in transmission grids nodes in presence of HVDC converter stations has also been envisaged. Overall, this research confirms the feasibility of advanced multi-terminal solutions to facilitate renewable energy integration and improve grid reliability.