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Developments in Voltage-Source Converter (VSC) technology has enabled the massive integration of renewable energy resources using High Voltage Direct Current (HVDC) technology. Large-scale HVDC grids provide the required flexibility and reliability to future transmission systems with high penetration of intermittent renewable energy sources. However, in case of faults such as DC-side short-circuits, fast rising currents with high steady-state value appear, resulting in a widespread disturbance to the system. To avoid these adverse effects, the DC-side fault current should be cleared in the millisecond time range, which is at least ten times faster than in an AC protection system.
HVDC circuit breakers (DCCBs) are expected to be widely installed in large-scale meshed HVDC grids. However, the selection of the most appropriate DCCB parameters (i.e. DCCB design), such as DCCB operating time, line inductor value and interruption capabilities, is complex and challenging for several reasons. First, many DCCB technologies exist with a variety of operating times, but no choice has been made yet on which technology is optimal for large-scale meshed HVDC grids. Second, the DCCB design influences the operation of the grid during DC-side faults and in post-fault conditions. Third, no efficient means are available for calculating DC-side fault currents. At present, there are still no complete methods exist for determining all DCCB parameters.
This thesis provides the fundamental concepts, approaches and tools to develop, investigate and evaluate the selective protection scheme using DCCBs for large-scale meshed HVDC grids, considering various DCCB technologies. First, three scenarios are proposed, describing the high-level performance of the HVDC grid during DC-side faults, ranging from the most to the least strict performance. Second, a visual approach is developed characterizing the DCCB parameters. This visual tool provides an insight into the interdependence and trade-offs to be made on the DCCB parameters, thereby allowing for an optimal selection. It is shown that moving from the most to the least strict scenario leads to a relaxation in the DCCB parameter selection. However, relaxing DCCB constraints may lead to more complex operation of converters in post-DC-side fault conditions. Third, the post fault recovery is investigated, by providing an in-depth analysis and insight into the interactions between the converters and the grid during the recovery process, in addition to developing new methods to enhance the post-fault recovery performance. These methods show an effective mitigation of unwanted oscillations during the recovery process. Fourth, necessary approaches are developed to efficiently estimate the DCCB parameters. These approaches help in facilitating the investigation of DCCB requirements for multiple scenarios in future large-scale meshed HVDC grids in an automated way with acceptable computational effort and sufficient accuracy. Fifth, an enhanced active-resonance DCCB topology is proposed to improve the interruption performance of the mechanical interrupter. This allows for an increase in the DCCB breaking capability, a shorter interruption time and consequently reduced DCCB requirements. In summary, this thesis opens a wider view of the selective protection scheme design, which allows for the integration and design of DCCBs with a wide range of operating times.