Series Thyristor Branches-Based Protection for Hybrid MMCs Against Valve-Side Single-Phase-to-Ground Faults in Bipolar HVDC Systems

The valve-side single-phase-to-ground (SPG) fault is one of the key challenges for hybrid modular multilevel converter (HMMC)-based bipolar high voltage direct current systems. In this article, the valve-side SPG fault characteristics of HMMCs are analyzed in detail, revealing that not only the submodules (SMs) in upper arms are severely overcharged, but also the full-bridge SMs in lower arms suffer unbearable overvoltage, which may destroy the devices. A series thyristor branches-based protection strategy is proposed in this article, which can handle the faults with the advantages of low SM capacitor overvoltage, low semiconductor cost, low ac-side overcurrent, and no arm overcurrent. Electromagnetic transient simulation studies using PSCAD/EMTDC tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of the proposed strategy.