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Operation and control of multilevel current source converters for wind energy conversion system and STATCOM applications
Alskran, Faleh
Alskran, Faleh
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2020
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Abstract
Large Wind Turbines (WTs), exceeding 10 MW, are expected to become more available in the near future. Most large WTs utilize Permanent Magnet Synchronous Generators (PMSGs) to convert the mechanical power of the WT into electrical power. One of the most important components in a Wind Energy Conversion System (WECS) is the Power Electronics Interface (PEI). The PEI converts the AC power produced by the WT generator, with a magnitude and frequency that varies with the wind speed, into AC power with a fixed frequency that can be injected into the grid. The most common PEI is the Back-to-Back Voltage Source Converter (BTB-VSC). As the rated power of PMSGs increase, the rated current of the switches in the BTB-VSC must increase. To accommodate the high currents, PEI designers resort to either connecting multiple switches in parallel or connecting multiple BTB-VSCs in parallel. However, ensuring equal current division among switches or BTB-VSCs can be particularly challenging. Alternatively, employing a BTB structures that utilize Current Source Converters (CSCs) instead of VSCs could offer a better solution. CSCs are inherently current scalable since each CSC module has its own sharing inductors. Moreover, when connected in parallel, CSCs can operate as a Multilevel CSC (MCSC). MCSC offer advantages such as lower THD and di/dt, which reduces the size of the grid-side and generator side filters. Proper operation of MCSC requires individual CSC modules to have the same average current flowing through them. In practice, the average values of theses currents can deviate from their nominal values due to disturbances and/or differences in electrical parameters. This dissertation presents a unique Current Balancing Algorithm (CBA) to solve this problem. In addition, a modified Level-Shifted SPWM (LS-SPWM) technique that is suitable for MCSC is presented. The modified LS-SPWM is shown to offer less THD and di/dt when compared to the Phase-Shifted Carrier SPWM (PSC-SPWM), which is the most common modulation technique for MCSC. The concepts of CBA and modified LS-SPWM are, also, extended to MCSC operating as STATCOMs. Modeling and control of MCSC and MCSC-based STACOMs are discussed; and closed-loop control strategies are proposed. To validate the CBA, modified LS-SPWM, and control strategies, a proof-of-concept prototype that can operate as either an MSCS or MCSC-based STATCOM was designed, built, and tested. Lastly, a 10 MW WECS model utilizing a BTB 5-level MCSC was created in MATLAB/Simulink using the Simscape Electrical toolbox. The BTB 5-level MCSC utilized the CBA, modified LS-SPWM, and control strategy developed in this dissertation. Simulation results for various wind speeds and different power factors, at the grid side, are presented.
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