Control Strategy for Potential Vector Remolding Fault Ride-through in Grid-forming MMC

[Objective] The operational characteristics of the grid-forming modular multilevel converter （MMC） in offshore wind power transmission systems under short-circuit faults in the receiving-end grid are unclear， and the interaction mechanism between fault ride-through and grid-forming active support is not well understood， which poses a significant threat to the reliable operation of offshore wind power flexible direct current systems. In addition， traditional fault ride-through strategies face the issues of large transient impact currents and slow reactive power support responses during the switching of strategies before and after faults， in conjunction with grid control. [Methods] This study proposes a low-voltage fault internal potential reconstruction ride-through control strategy for grid-forming MMCs. Specifically， it analyzes the short-circuit current characteristics of MMCs under grid voltage dips， constructs a fault ride-through current-limiting boundary based on the voltage equation space vector of the grid-forming MMC， and reconstructs the internal potential using current optimization commands. This study aimed to demonstrate the capability of active voltage support in grid control under different grid voltage dips. [Results] The proposed fault ride-through strategy was validated using a semi-physical experimental platform， and the results indicated that low-voltage ride-through could be achieved under varying degrees of faults. As the grid-voltage dip deepens， the reactive power generated increases accordingly； however， the grid-forming current is limited at deeper dip levels. Compared with traditional fault ride-through strategies， this approach can reduce transient impact currents while quickly providing reactive power support and improving the recovery rate of the grid connection point voltage. The analysis revealed the operational characteristics of active voltage support in a grid-forming MMC and provided insights based on the vector angle correction of currents. [Conclusions] This study lays the foundation for the study of fault ride-through technology under severe voltage drop conditions in deep-sea environments， promoting the safe and reliable operation of large offshore wind power bases and effectively advancing innovation in the energy industry.