Saleh, Beeond M.
(2022)
Modular multilevel DC-DC converters for HVDC grid interconnection.
PhD thesis, University of Nottingham.
Abstract
Since the introduction of voltage source converters, VSCs, which provide bidirectional power flow without reversing the DC voltage, high voltage direct current grids, HVDC girds, are becoming interest solution for expanding the existed ac grids. HVDC systems are preferred over the high voltage alternative current, HVAC, for long distance high power transmission due to lower total investment, lower power losses and HVDCs are technically and environmental superior. Additionally, they have been chosen as solutions for interconnecting off-grid communities, asynchronous AC grids and transmitting power from offshore winds. Because of the HVDC technology advantages and the absence of operational voltage standards, many point to point HVDC links have been established at different voltage levels. Developing these point to point HVDC systems to HVDC grids will require high voltage and high power DC-DC converters to interconnect different voltage levels and provide isolation.
In the earlier of the last decade, modular multilevel converter, MMC, was introduced and this led to a bread of non isolated and isolated modular multilevel DC-DC converters, MMC-DC-DC, for HVDC grids. In modular multilevel topologies, half bridge submodule blocks, SM, are assembled to a chain-link which produces desired voltage rating. However, the number of the required submodule increases as the voltage rating increase. Recently, modular multilevel DC-DC converters with trapezoidal operation (quasi two-level operation) were introduced to reduce the size of the submodlue capacitor and accordingly the footprint, weight and cost of SM are reduced. In comparison to dual active bridge converters (DAB), trapezoidal operation has also reduced dv/dt stress across the transformer in isolated modular multilevel DC-DC converters whereas the reduction in dv/dt stress is limited.
In the first part of this work, a trapezoidal modulation of modular multilevel DC-DC converter is investigated and a modified trapezoidal operation is proposed. Additionally, a control schemes that support the proposed operation of converter under transient conditions are demonstrated. This work also investigates the limitation of transformer dv/dt and semiconductor device utilisation.
The second part of this work presents a new modular multilevel series chain-link DC-DC converter, SCC-DC-DC, which requires only 33% SMs and lower device current rating as compared to modular multilevel DC-DC converter topologies. In the proposed topology many single phase transformers are used instead of one three phase transformer and this permits the generation of voltage with controllable dv/dt across the transformer. Furthermore, the structure of the topology helps to operate the converter at higher fundamental frequency which reduces the overall converter footprint.
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