Browsing by Author "Lyu, Yadong"

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    Modeling and Control Strategy for Capacitor Minimization of Modular Multilevel Converters
    Lyu, Yadong (Virginia Tech, 2017-02-20)
    The modular multi-level converter (MMC) is the most prominent interface converter used between the HVDC grid and the HVAC grid. One of the important design challenges in MMC is to reduce the capacitor size. In the current practice, a rather large capacitor bank is required to store line-frequency related circulating energy, even though a number of control strategies have been introduced to reduce the capacitor voltage ripples. In the present paper, a novel control strategy is proposed by means of harmonic injections in conjunction with gain control to completely eliminate both the line frequency and the second-order harmonic of the capacitor voltage ripple. Ideally, the proposed method works with the full bridge topology. However, the concept also works with half bridge topology with a significant reduction of line frequency related ripple. To gain a better understanding of the nature of circulating energy and the means of reducing it, the method of state plane analysis is employed to offer visual support. In addition, the design trade-off between full bridge MMC and half bridge MMC is presented and a novel control strategy for a hybrid MMC is proposed. Finally, the work is supported with a scaled down hardware demonstration.
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    Modular multilevel converter capacitor voltage ripple reduction
    (United States Patent and Trademark Office, 2019-09-03)
    Aspects of capacitor voltage ripple reduction in modular multilevel converters are described herein. In one embodiment, a power converter system includes a modular multilevel converter (MMC) electrically coupled and configured to convert power between two different power systems. The MMC includes one or more phase legs having a cascade arrangement of switching submodules, where the switching submodules include an arrangement of switching power transistors and capacitors. The MMC further includes a control loop including a differential mode control loop and a common mode control loop. The differential control loop is configured to generate a differential control signal based on a target modulation index to reduce fundamental components of voltage ripple on the capacitors, and the common mode control loop is configured to inject 2nd order harmonic current into a common mode control signal to reduce 2nd order harmonic components of the voltage ripple on the capacitors.