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Title: Power converter design for HVDC applications
Author: Judge, Paul Daniel
ISNI:       0000 0004 6496 0254
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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This thesis investigates the design of modular voltage source converters for High Voltage Direct Current (HVDC) applications. The first half of the thesis focuses on the design of existing multilevel HVDC technology. A design methodology for sizing modular converters for a given grid code specification, and with given design constraints in terms of peak sub-module voltage rating and capacitor size, is developed and used as the basis of comparing converter designs. Results show that the half-bridge MMC requires an energy storage in the region of 35 kJ/MVA in order to achieve a good balance between sub-module capacitor size, and required number of sub-modules. The design of the Hybrid MMC, which combines half- and full-bridge sub-modules in the design in order to achieve DC fault tolerance, is then investigated using the same design methodology, an advantage of which is that the optimum modulation index can be determined, rather than assumed. Results show that the highest efficiencies may be achievable if the converter is operated at a modulation index of 1.2. The power-loss and thermal properties of several converters are then analysed. The Alternate Arm Converter and over-modulating Hybrid MMC show the greatest efficiencies, though the AAC suffers from relatively high junction temperatures within its director switches. The potential of designing overload capability into MMCs, to enable them to provide system support services such as frequency response is then investigated. Results show 30% overload ratings may be achievable with only a 10% require increase in the number of sub-modules within the converter. System studies show that significant response improvements to the AC system can be made even if the converters need to be dynamically rated to prevent excessive junction temperatures being reached. The second half of this thesis focuses on a brand new multilevel thyristor-augmented structure called a power-group, which has the potential to allow voltage source converters that are tolerant to faults on both the AC and DC network to be constructed, while having efficiencies similar to those achievable with Current Source Converter (CSC) technology. Results show that this is possible while also retaining high quality current waveforms and independent control of real and reactive power. Results throughout the thesis are backed up by a combination of simulation and experimental work using a lab-scale multilevel converter that was constructed during the project.
Supervisor: Green, Timothy ; Mitcheson, Paul Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral