Increasing number of Flexible AC Transmission System (FACTS) devices have been be installed to reinforce the existing grid and build the envisioned "Smartness" into the grid through controls and optimization. However, it has been noticed that adverse interactions among multiple FACTS controllers may occur when they are not properly coordinated with each other and other slowly acting system equipment. These interactions can amplify oscillations and even destabilize the system by influencing the damping properties of individual FACTS controllers or increasing voltage deviations. This paper presents an extensive survey on the existing cases, system studies and assessment techniques to help system planners understand the underlying mechanism of diverse interactions among multiple FACTS controllers and develop coordinated control schemes for preventing or mitigating any harmful interactions. Control interactions are categorized and discussed in terms of their root causes and resulting frequency ranges. Unfriendly interactions involving shunt FACTS devices are detailed in which Korean Electric Power Corporation (KEPCO) is particularly interested.
-This paper focuses on the engineering trade-offs in designing capacitor voltage balancing schemes for modular multilevel converters (MMC) HVDC: regulation performance and switching loss. MMC is driven by the on/off switch operation of numerous submodules and the key design concern is balancing submodule capacitor voltages minimizing switching transition among submodules because it represents the voltage regulation performance and system loss. This paper first introduces the state-ofthe-art MMC-HVDC submodule capacitor voltage balancing methods reported in the literatures and discusses the trade-offs in designing these methods for HVDC application. This paper further proposes a submodule capacitor balancing scheme exploiting a control signal to flexibly interchange between the on-state and the off-state submodules. The proposed scheme enables desired performance-based voltage regulation and avoids unnecessary switching transitions among submodules, consequently reducing the switching loss. The flexibility and controllability particularly fit in high-level MMC HVDC applications where the aforementioned design trade-offs become more crucial. Simulation studies for MMC HVDC are performed to demonstrate the validity and effectiveness of the proposed capacitor voltage balancing algorithm.
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