The modular multilevel DC converters for MVDC and HVDC applications

A dc structure for an electrical power system is seen to have important advantages over an ac structure for the purpose of renewable energy integration and for expansion of transmission and distribution networks. There is also much interest and strong motivation to interconnect the existing point-to-point dc links to form multi-terminal and multi-voltage dc networks, which can make full use of the benefits of a dc scheme across various voltage levels and also increase the flexibility and ease the integration of both centralized and distributed renewable energy. This thesis investigates both high step-ratio dc-dc conversion to interface dc systems with different voltage levels and low step-ratio dc-dc conversion to interconnect dc systems with similar but not identical voltages (still within the same voltage level). The research work explores the possibility of combining the relatively recent modular multilevel converter (MMC) technology with the classic dc-dc circuits and from this proposes several modular multilevel dc converters, and their associated modulation methods and control schemes to operate them, which inherit the major advantages of both MMC technologies and classic dc-dc circuits. They facilitate low-cost, high-compactness, high-efficiency and high-reliability conversion for the medium voltage level and high voltage level dc network interconnection. For medium voltage level cases, this thesis extends the classic LLC dc-dc circuit by introducing MMC-like stack of sub-modules (SMs) in place of the half-bridge or full-bridge inverter in the original configuration. Two families of resonant modular multilevel dc converters (RMMCs) are proposed covering high step-ratio and low step-ratio conversion respectively. A phase-shift modulation scheme is further proposed for these RMMCs that creates an inherent feature of balancing SM capacitor voltages, provides a high effective operating frequency for reducing system footprint and offers a wide operating range for flexible conversion. For high voltage level cases requiring a high step-ratio conversion, a modular multilevel dcac-dc converter based on the single-active-bridge or dual-active-bridge structure is explored. The operating mode developed for this converter employs a near-square-wave ac current in order to decrease both the volt-ampere rating requirement for semiconductor devices and the energy storage requirement for SM capacitors. For low step-ratio cases, a single-stage modular multilevel dc-dc converter based on a buck-boost structure is examined, and an analysis method is created to support the choice of the circulating current frequency for minimum current stresses and reactive power losses. Theoretical analysis of and operating principles for all of these proposed modular multilevel dc converters, together with their associated modulation methods and control schemes, are verified by both time-domain simulation at full-scale and experimental tests on down-scaled prototypes. The results demonstrate that these medium voltage and high voltage dc-dc converters are good candidates for the interconnection of dc links at different voltages and thereby make a contribution to future multi-terminal and multi-voltage dc networks.