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Differential Wye Modular Multilevel AC Converter (DW-M2AC) for Direct AC/AC Power Conversion
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- Author / Creator
- Herathge, Shehan S
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Direct AC/AC MMCs are used in a wide variety of power system applications such as interconnecting asynchronous grids with different frequencies, renewable energy integrations and medium voltage variable frequency motor drives. Due to the advantages such as lower component count and lesser control complexity led by a low number of control states currents, the development of direct AC/AC MMCs with a lower number of arms has been a research trend in the recent literature. This research work proposes a direct AC/AC MMC with a simpler converter design (low number of arms) that has an integrated transformer to provide necessary galvanic isolation. A defining feature of the DW-M2AC is the adoption of a novel power transfer mechanism where the transformer windings carry current with multiple frequency components, while the core is rated only for a single AC frequency flux similar to conventional AC power systems transformers. This multi-tasking of the transformer provides a lower total semiconductor effort and, due to an inherent voltage boosting effect, makes the DW-M2AC more suitable for applications where high AC voltage step ratios are needed. This research work also presents a control strategy based on dq reference frame with decoupled control of differential mode and common mode currents in converter arms. The DW-M2AC’s performance is tested under different scenarios using simulations. The simulation results are used to study the converter power transferring capability in fixed frequency grid connected operation and variable frequency operation. A scaled down 208/104 V, 1 kW three-phase laboratory prototype is developed and the converter operating principle is further validated by conducting a series of experiments.
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- Graduation date
- Fall 2022
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.