Enhanced Ride-Through Capability Under Rectifier-Side AC Fault for Series LCC-MMC Hybrid HVDC System

The series line-commutated converter (LCC) and modular multilevel converter (MMC) hybrid HVDC system provides a more economical and flexible alternative for UHVDC transmission. With the LCC DC voltage reduction, no current cut-off will occur under slight rectifier-side AC faults. However, with the limitation of the MMC voltage modulation ratio, when the rectifier-side AC fault is extremely acute, the current cut-off will unavoidably exist, resulting in enormous impacts on AC and DC systems. To reduce the risk of the current cut-off and improve fault recovery, this paper proposes an enhanced coordinated control strategy. First, under the varying severity of rectifier-side AC faults, the
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operation characteristics are meticulously analyzed. Then, the third harmonic voltage injection (THVI) and reactive power dynamic adjustment (RPDA) are introduced to expand the operation range of MMC DC voltage. Based on the enlarged operation range, a backup DC current control of MMC is proposed to adaptively regulate the MMC DC voltage relying on the fault severity. Finally, the feasibility and effectiveness of the proposed coordinated control strategy is verified through several simulation scenarios of varied fault severity on PSCAD/EMTDC. The simulation results show that, the proposed control enlarges the system operation range, improves the fault recovery, and significantly reduces the risk of current cut-off.