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The control delay is a crucial factor that induces negative damping in the impedance of modular multilevel converter based-high voltage direct current (MMC-HVDC) system at high-frequency range, which potentially lead to high-frequency resonance. Delay compensation is an effective approach to mitigate the impact of control delay. In this paper, the control structure and composition of delay time in MMC-HVDC are first introduced, and the influence of control delay on the impedance characteristics of MMC is analyzed. A method for delay compensation in MMC-HVDC control systems is then proposed. An equivalent impedance model of MMC-HVDC system with delay compensation is established by modifying the reference phase of dq/abc axis transformation. The impact of delay compensation on the impedance characteristics is analyzed. The improvement effect of delay compensation on MMC impedance is influenced by the proportional coefficient of the PI controller in the inner current loop and the compensation time. Finally, the correctness of impedance model and resonance characteristics analysis is verified by impedance scanning and electromagnetic transient simulation.
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Zhao et al. (2024) studied this question.
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