Flexible HVDC transmission based on modular multilevel converters (MMCs) is a mainstream solution for integrating large-scale wind power over long distances. However, the sending end of such systems lacks strong grid support, making it susceptible to overvoltage and overcurrent issues during asymmetric AC faults. This paper investigates the transient characteristics and control strategies for such faults in an MMC-HVDC system integrated with gravity energy storage (GES) on the wind farm side. The limitations of conventional negative-sequence current suppression strategies are analysed, highlighting their tendency to cause overvoltage in healthy phases and delayed post-fault voltage recovery. An optimised negative-sequence current control strategy is proposed, wherein the sending-end MMC proactively injects a controlled amount of negative-sequence current to limit fault current magnitude. This is synergised with the rapid power support capability of the GES to facilitate voltage recovery. Simulation results conducted in PSCAD/EMTDC validate the effectiveness and superiority of the proposed strategy in enhancing the system's fault ride-through capability.
Li et al. (Fri,) studied this question.