Power system interconnections using high-voltage direct-current (HVDC) technologies between different areas can be an effective solution to enhance system efficiency and reliability. Particularly, the multi-terminal dc grids that could balance and ensure resource adequacy increase asset utilization and reduce costs. In this paper, the technical feasibility of building dc grids using the line-commutated converter-based (LCC) and voltage source converter-based (VSC) HVDC technologies is discussed. Apart from presenting the technical challenges of building LCC dc grids and LCC/VSC hybrid dc grids, the reliability modeling and analysis of theseDCgrids are also presented. First, the detailed reliability model of the modular multi-level converters (MMCs) with series-connected high-voltage and low-voltage bridges is developed. The active mode of redundancy design is considered for the reliability model. To this end, a comprehensive whole system reliability model of the studied systems is developed. The reliability model of each subsystem is modeled in detail. Various reliability indices are calculated using this whole system reliability model. The impacts of the redundancy design of the MMCs on these indices are presented. The studies of this paper provide useful guidance for dc grid design and reliability analysis.