Modulation, control, and applications of multilevel converters for power systems with high penetration of wind energy
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The proposed research focuses on developing modulation and control methods for multilevel converters so as to optimize their applications in wind energy generation and transmission. This dissertation first establishes the inherent relationship between the space vector modulation (SVM) and a phase-voltage modulation technique (called the nearest-level modulation): the two modulation methods are functionally equivalent. Consequently, a simplified SVM scheme for multilevel converters is proposed, which is independent of the level number of the converter and for the first time achieves the same easy implementation as phase-voltage modulation techniques. The three-level active neutral-point-clamped (ANPC) converter is well suited to control high-power wind turbine generators, but suffers from unequal power loss distribution among its semiconductor devices. This dissertation proposes a new modulation scheme, called the adaptive doubled frequency PWM (ADF-PWM), to achieve the power loss balancing control for the ANPC converter. In applications of high voltage direct current (HVDC) transmission systems connecting large wind farms over a long distance to a utility network, the modular multilevel converter (MMC) is the best choice, because of its modularity and scalability to meet any voltage level requirements. This dissertation proposes an optimized control method for the MMC based on the proposed simplified SVM scheme, which significantly improves the capacitor voltage balancing and circulating current suppression.