In large wind turbines (in MW and multi-MW ranges), which are extensively utilized in wind power plants, full-scale medium voltage (MV) multi-level (ML) voltage source converters (VSCs) are being more preferably employed nowadays for interfacing these wind turbines with electricity grids. For these VSCs, high power density is required due to limited turbine nacelle space. Also, high reliability is required since maintenance cost of these remotely located wind turbines is quite high and these turbines operate under harsh operating conditions. In order to select a high power density and reliability VSC solution for wind turbines, first, the VSC topology and the switch technology to be employed should be specified such that the highest possible power density and reliability are to be attained. Then, this qualitative approach should be complemented with the power density and reliability assessments of these specific VSCs so that their power densities and reliabilities are quantitatively determined, which requires extensive utilization of the electro-thermal models of the VSCs under investigation. In this thesis, the three-level neutral-point-clamped VSCs (3L-NPC-VSCs), which are classified as 3L-NPC-VSC, 3L active NPC VSC (3L-ANPC-VSC), and 3L neutralpoint-piloted VSC (3L-NPP-VSC) and are proven to be high power density and highly reliable solution in the MV converter market, are selected and employed as the gridside VSC of a large wind turbine as well as the 3L H-Bridge VSCs (3L-HB-VSCs). As the switch technology for realizing these 3L-VSCs, press-pack IGBTs are chosen to ensure high power density and reliability. Based on the selected 3L-VSCs and switch technology, the converter electro-thermal models are developed comprehensively, implemented practically, and validated via a full-scale 3L-VSC laboratory prototype. Using these validated models, the power density assessments, which include converter power capability and volume determinations, and the reliability assessments, which are based on statistical failure rates of IGBTs and DC capacitors and based on IGBT lifetime determined by junction temperature excursions due to wind turbine power profile, are performed for these 3L-VSCs employed as grid-side wind turbine converters. Hence, the power density and reliability of these 3L-VSCs are quantified and compared for large wind turbine applications. For the 3L-VSCs under investigation, the results of the power density and reliability assessments can be summarized as follows. Among the 3L-NPC-VSCs, 3L-NPP-VSC is the most suitable solution regarding power density and reliability due to its larger power capability compared to the other 3L-NPC-VSC. Among the 3L-HB-VSCs, the 3L-HB-VSC with common DC bus (3L-HB/C-VSC) is the most suitable solution due to its fewer DC capacitors compared to the other 3L-HB-VSCs. Provided that the transformer and switching ripple filter connections of 3L-HB/C-VSC are realized as practical as the ones of the 3L-NPC-VSCs, 3L-HB/C-VSC becomes a competitive solution with 3L-NPP-VSC in terms of power density and reliability.
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Department of Energy Technology, Aalborg University, 2011