Multi-terminal high-voltage dc (HVDC) grids, or multi-terminal dc (MTDC) grids, are considered enabling technology for the large-scale integration of renewable energy sources and the modernization of electric power grids. MTDC grids offer efficient long-distance power transmission as well as resource sharing across multiple regions with different time-zones and grid frequencies. However, MTDC grids face major technical barriers on all fronts based on their envisioned functionalities and capabilities. Specifically, at the component-/physical-level, power-electronic-based dc circuit breakers (DCCBs) provide dc-side fault interruption capability and are a key building block of MTDC grids, but they are costly, bulky, and slow due to the high fault energy stress and long fault interruption time they experience. Additionally, at the system-/cyber-level, the reliance of MTDC grids on information communication technology presents a significant cyber-attack surface. Malicious players can manipulate measurements to cause unintentional tripping of DCCBs, thereby disrupting grid operations. Considering all the challenges mentioned above, this thesis proposes a suite of methodologies that not only improve the reliability, cost-effectiveness, and power-density of the DCCB hardware, but also ensure malicious DCCB trip-outs can be detected and prevented. Overall, these contributions result in reliable, cost-friendly, and cyber-secure dc technologies, advancing the realization and implementation of dc power systems.