Abstract: When the AC line fault of modular multi-level converter (MMC) inverter occurs, the MMC short-circuit current amplitude is greatly affected by the control strategy, which probably affects the operation performance of the three-section current protection additionally. To solve this problem, a flexible AC/DC distribution network system supplying power to passive network is built on the real time digital simulation system (RTDS) and then an AC fault ride-through strategy is designed in the MMC controller. Considering the integration of controlling and protection, this paper compared the different operation performances of three-section current protection in the condition of fault ride-through strategy input and non-input. The results demonstrate that when the asymmetrical fault occurs on the AC line of MMC inverters, neither section I nor section II of the main current protection operates under the AC fault ride-through strategy, but only the closest backup production section III is able to remove the fault with delay. In conclusion, the three-section current protection, setting for the fault current characteristics of the pure AC system, has a poor performance on current protection accordingly no longer available for the AC line of MMC inverters.

    Key words: flexible AC/DC distribution network; passive network; MMC; fault ride-through strategy; short-circuit current; current protection