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Title: Hybrid multi-level HVDC converter and multi-terminal DC networks
Author: Merlin, Michael Marc Claude
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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This thesis explains the working principles of an AC/DC converter topology intended for HVDC applications, called the Alternate Arm Converter (AAC). It consists of a hybrid between the modular multi-level converter (MMC) through the presence of H-bridge cells and the 2-level converter because of the director switches in each arm. Thanks to its cells, the AAC is able to generate a multi-level staircase AC voltage waveform which results a low distortion AC current while the director switches control which arms are conducting at any given time. By synchronising the conduction period of an arm with the zero-crossing points of the AC voltage waveform, the voltage rating of the stacks can be reduced, hence minimizing the number of cells. In case of a DC-side fault, an AAC with enough cells is able to keep control of the current in the phase reactor and even be operated to support the AC grid by providing reactive power similarly to a STATCOM. Since the AAC relies on pre-charged H-bridge cells, an effective energy management is required to control their level of charge. An ideal working point has been identified, called 'Sweet-Spot'. This operating point describes a set of conditions where the incoming and outgoing energy flows equate, in effect nullifying the average energy drift of the cells. Additional energy techniques based on current modulation have been developed in order to redistribute the energy inside the converter. The study of the AAC has provided an understanding of its working and device requirement and a hierarchical structure for its control system has been developed. Simulation results confirm these findings both on the operation of the AAC under normal and abnormal situations and on the effectiveness of the developed energy management system. Post-processing of the simulation data has also shown that the AAC is on par with the half-bridge MMC on power efficiency.
Supervisor: Chaudhuri, Balarko ; Green, Tim Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available