High Power Converters for Current Source Converter-Type Series DC-Based Wind Energy Conversion System
Creators: Xing, Ling
Publisher: name: University of Alberta Library
Year: 2024

<p>The high-voltage direct current (HVDC)-based wind energy conversion system (WECS) currently used in the industry requires a large and expensive substation. An emerging technology called current source converter (CSC)-type series DC-based WECS, can eliminate the need for the bulky and costly substation, resulting in substantial cost and size reductions for WECS. However, the power converters used in the CSC-type series DC-based WECS present several technical challenges. For example, the grid-side power converters require the use of bulky and costly multi-winding transformers, while the generator-side power converters face issues such as highly distorted generator stator currents, low scalability, and/or high complexity. Therefore, in this thesis, new power converters are proposed for generator-side and grid-side converters to address their respective technical challenges. On the generator side of the CSC-type WECS, a passive rectifier-based converter utilizing a phase-shifting transformer is proposed. It effectively addresses the disadvantage of highly distorted generator stator currents commonly associated with passive rectifiers, while retaining all their advantages including low cost, high reliability, simplicity, and scalability. However, the phase-shifting transformer is bulky and heavy, which poses a disadvantage in WECS where space in the nacelle is limited. To address the challenges associated with a phase-shifting transformer-based converter, while retaining all its benefits, a passive rectifier-based converter using modular medium-frequency transformers (MFTs) is proposed for the generator-side converter. Compared to the phase-shifting transformer-based converter, the MFT-based converter offers reduced size and weight, while retaining all the advantages of the former. Additionally, an active rectifier-based converter using modular MFTs is also proposed for the generator-side converter. The use of an active rectifier ensures superior harmonic performance in the generator stator currents, while the use of modular MFTs contributes to size and weight reductions in the generator-side conversion system. This converter offers benefits of both active rectifiers and MFTs simultaneously.On the grid side of the CSC-type WECS, transformerless series-connected CSCs are proposed. The proposed transformerless CSCs are the first to eliminate the need for transformers in series-connected CSCs. This elimination results in substantial reductions in both cost and size. A modified version of the transformerless CSCs is also proposed. Compared to the original converter, this modification removes the need for series-connected switches by using cascaded half-bridge converters. This change eliminates the requirement for expensive and complex voltage balancing schemes while retaining all the advantages of the original converter. Modulations and controls are developed for proposed converters on both generator side and grid side of the CSC-type CSC. The performance of the proposed converters and the effectiveness of the proposed modulations and controls have been validated through simulations and lab-scale experiments.</p>