Compared with the typical Ultra HVDC (UHVDC) system

the inverters of the UHVDC system with a hierarchical connection mode (UHVDC-HCM)

connect two receiving-end systems with different operational conditions. This requires the corresponding conversion units (CUs) at the different terminals to be differentiated in reliability modeling

and the spares should also be separately set. A reliability model of the UHVDCHCM system is proposed in this paper. The operating modes are classified by the capacities of the total system and the transmission powers to the two receiving-ends. Considering the independent spares of the components of the CUs at different terminals

the state space is derived. Two sets of indices are newly proposed to more accurately evaluate the system reliability. Based on the matrix description of the frequency & duration (F&D) method

the sensitivities of the reliability indices to the reliability parameters of the components are quantified. Numerical results validate the feasibility of the proposed model. The vulnerabilities are recognized by the sensitivity analysis

and the impact of different spare schemes on the reliability indices and sensitivities are compared. The proposed model and indices provide a reference for the practical UHVDC-HCM projects.