This thesis develops a new class of modular multilevel dc/dc converters, termed the DC-MMC, that can be deployed to interconnect HVDC networks of different or similar voltage levels. Advanced operating features include buck/boost functionality and bidirectional dc fault blocking analogous to a dc circuit breaker. A key advantage of the DC-MMC is that it eliminates the full-rated intermediate ac conversion stage traditionally employed by modular multilevel converter (MMC) based two-stage dc/dc converters, i.e. dc/ac-ac/dc conversion. This is enabled by a new single-stage bidirectional dc/dc conversion process that requires only a fraction of the dc power throughput to be internally circulated as ac power. In comparison to state-of-the-art MMC based two-stage dc/dc conversion, the DC-MMC enables up to a 50\% reduction in total semiconductor expenditure and total capacitive energy storage requirements, translating to commensurate savings in converter capital cost and operating losses.

The DC-MMC principle of operation is validated via simulation of a comprehensive switched model in PLECS. To gain deeper insight into DC-MMC dynamics and internal power transfer mechanisms, this thesis exploits the use of dynamic phasors to accommodate the multiple frequency components that exist at steady-state. Large-signal and small-signal time-invariant dynamic phasor models are derived. The large-signal phasor model enables:​ 1) solving for the full steady-state solution of the DC-MMC under arbitrary loading, and 2) evaluation of DC-MMC open-loop stability via eigenvalue analysis. Based on the small-signal phasor model, a dynamic controller suitable for dc network applications is proposed that regulates output power while ensuring individual capacitor voltages remain balanced. The developed analytical models and dynamic controls are validated by simulation. Bidirectional dc fault blocking capability is also confirmed by simulation. Experimental results for a 4 kW prototype validate the single-stage dc/dc conversion process for both step-down and step-up operating modes.