Protection of Multiterminal HVDC Grids based on Modular Multilevel Converters: DC Fault Ride-Through and AC Grid Support

High Voltage DC (HVDC) grids provide an efficient solution for the transmission of bulk power over long distances between the energy production and consumption centers. Up to now, most of the implemented dc connections are point-to-point, creating dc links to strengthen the existing, predominantly ac, electricity grid. However, to optimize the use of assets and energy harvest, multi-terminal HVdc grids are envisioned. Based on recent research and industrial trends, the Multilevel Modular Converter (MMC) technology will be the building block for the realization of these grids. Yet, as the grid structure becomes more complex, the protection of HVDC grids poses one of the most important challenges. This dissertation proposes different ways to isolate and ride through dc faults, while maintaining controllability of the converters that can offer ancillary services to their respective ac grid, having as a main objective to reduce the downtime of the grid and the time in which assets are not utilised. More specifically: The characteristics of dc faults and the main parameters which affect the fault response are investigated. A dc breaker optimized design is proposed which allows equipment sharing, offering bidirectional isolation capability for multiple lines at the same time. In case MMC with fault-blocking capability is used, e.g. Full-bridge MMC, a methodology for dc fault ride-through is proposed. Moreover, a dc current controller is presented, ensuring that the MMC can continue its controlled operation towards the respective ac grid. Once the dc side is isolated, the MMC operation as STATCOM is studied. More specifically, a control structure is proposed to maintain internal balancing of the converter, while ensuring Low-Voltage-Ride-Through (LVRT). The capability of the MMC to operate as Active Power Filter (APF) is also investigated. A selective harmonics detection and control method is presented and experimentally veri_ed for the mitigation of high-order current and voltage harmonics up to 13th order. Each of the aforementioned topics is dealt with in a respective Chapter of this dissertation.