In solidly grounded bipolar high-voltage direct current (HVDC) grids, DC faults can cause rapid current surges due to the inherent coupling between poles, posing significant risks to system stability. Traditional traveling-wave protection methods, while unaffected by MMC control characteristics, face challenges in bipolar systems where pole-to-pole coupling complicates fault identification and necessitates extensive deployment of costly DC circuit breakers (DCCBs). To address these limitations, this paper proposes a hybrid MMC topology based on an improved dual half-bridge submodule (IDHSM) with self-clearing capability and coordinated control-protection (CCP) strategy. The IDHSM enables dynamic adjustment of activated submodule ratios through adaptive modulation coefficient, actively reducing DC voltage by 14.93% under 300-Ω fault resistance while maintaining arm current constraints. Compared with full bridge submodule (FBSM), this hybrid topology reduces IGBT counts per voltage level by 50%, lowering hardware costs. Due to its dual capacitor structure with a passive current path, IDHSM can block energy injection from the AC side during faults. According to the simulations on ±500 kV four-terminal HVDC grid, fault identification within 1 ms using modulus instantaneous average values, resolving protection sensitivity degradation caused by MMC-based current limiting. In addition, 30% reduction in DCCB breaking current (from 7.0 kA to 4.9 kA) through coordinated current limiting. The proposed strategy exhibits strong performance within the transition resistance range of 0.1-300 Ω.
