A Comprehensive AC-Side Single-Line-to-Ground Fault Ride Through Strategy of an MMC-Based HVDC System

When a single-line-to-ground (SLG) fault occurs on the ac side of the modular multilevel converter (MMC) in an high-voltage direct-current transmission (HVDC) system, it results in the ac-side voltage sag and leads to an instantaneous reduction of the MMC power capacity. Thus, it calls for the fault ride through (FRT) strategy to coordinate two MMC stations in the HVDC system to protect the MMCs against the submodule (SM) capacitor overvoltage in case of the SLG fault. In the meantime, the HVDC system is expected to track the prefault active power as much as possible during the FRT to secure the power system stability. In this paper, a comprehensive FRT strategy is proposed, which is free of interstation communication. The proposed FRT strategy presents fast dynamic response, and it can prevent effectively the MMC SM-capacitor overvoltage and HVDC transmission line overvoltage. Moreover, the second-order voltage and current fluctuations in the HVDC transmission line caused by the grid unbalance are inherently avoided. Validity of the proposed strategy and its superiority over existing methods are demonstrated by simulation of a 200-kV, 400-MW cable-based HVDC system.