Abstract
Modular multilevel converter (MMC) can provide dynamic reactive power and voltage support for the grid during a quick AC voltage drop. However, restrains due to current capability of MMC should not be ignored. A constant reactive power and voltage control mode of MMC is of slow dynamic response and may produce transient impact. This paper focuses on reactive power control strategy of MMC, and a novel control mode combined modified constant AC voltage control and reactive power control is designed. An auxiliary control loop is added with current limitation, and a post-fault integral instantaneous reset control is introduced to suppress the transient impact after voltage recovery. The control strategy is used in Zhangbei $\pm 500\mathrm{k}\mathrm{V}$ four-terminal MMC HVDC power grid simulation model. Results show that the novel control strategy can provide converter station with much stronger reactive power support capability in contrast to the constant reactive control. It empowers converter station to regulate its reactive power quickly and smoothly. The strategy is effective at limiting MMC over-current and can relieve AC over-voltage transient impact after voltage recovery.
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