Control and Protection of VSC-based Multi-terminal DC Networks

The increasing global energy needs and the high integration of renewable energy generation have changed the requirements for the electricity grid. Countries are becoming unable to cover their energy demands with their own means and the need for power exchange between neighboring countries has increased. Consequently, power needs to be transmitted over longer distances and multi-terminal complex grids need to be created to facilitate the energy evolution. Contrary to the existing AC grids, HVDC is an appealing alternative for future grids. VSC technology has been the focus of recent HVDC research due to its inherent advantages. However, the use of fully-controllable switches becomes a disadvantage in case of DC contingencies. Thus far, opening the AC breakers has been the only way to clear DC faults, by completely de-energising the system and interrupting the power transfer with significant economic and societal consequences. Other protection concepts include multi-level converters with full-bridge submodules, which are able to limit the fault current; and control methods which identify the faulty lines. However, DC switch breakers are necessary to isolate the faulty line from the network, allowing normal operation to be resumed. The main contributions of this thesis are the comparison of different grid operating topologies under fault cases; and the impact analysis of different current limiting measures and control strategies on the developing DC fault currents. A four terminal grid in radial configuration was simulated using Matlab/Simulink®, and the natural fault response of the stations in most common HVDC grid topologies was studied. Additionally, two selective fault detection methods are proposed, which take into account the current direction on DC lines and the rate of rise of the fault currents. Four DC breaker technologies were simulated for all analysed grid topologies, and compared on the basis of the total DC fault interruption time and their influence on the system post-fault coordination and operation restoration. With the concepts analysed in this thesis, MTDC network system designers will be able to understand and tackle DC contingencies to facilitate an uninterruptible power flow between the different interconnected AC grids.