Coordinated Control and Protection Strategy for Bipolar HVDC Grids Based on Improved Dual Half‐Bridge Submodule MMC Using Modulus Transformation

ABSTRACT In solidly grounded bipolar high‐voltage direct current (HVDC) grids, DC faults can cause rapid current surges due to inherent pole‐to‐pole coupling, posing significant risks to system stability. Traditional traveling‐wave protection methods, though unaffected by MMC control characteristics, face challenges in bipolar systems where coupling complicates fault identification and necessitates costly DC circuit breakers (DCCBs). To address these limitations, this paper proposes a hybrid modular multilevel converter (MMC) topology based on an improved dual half‐bridge submodule (IDHSM) with self‐clearing capability and a coordinated control‐protection (CCP) strategy. The proposed method enables dynamic adjustment of activated submodule ratios through an adaptive modulation coefficient, achieving a 14.93% reduction in DC voltage under a 300‐Ω fault resistance while maintaining arm current constraints. Compared with full‐bridge submodules (FBSM), this hybrid topology reduces IGBT counts per voltage level by 50%, lowering hardware costs. Simulation results on a four‐terminal HVDC grid demonstrate that the proposed strategy enables DC fault identification within 1 ms using modulus instantaneous average values, effectively addressing the sensitivity degradation caused by MMC‐based current limiting. Moreover, the DCCB breaking current is reduced by 30% (from 7.0 to 4.9 kA) through coordinated current limiting. The proposed strategy exhibits strong performance across a wide transition resistance range of 0.1–300 Ω. Moreover, a low‐voltage experimental platform is built to verify the effectiveness of the proposed protection strategy.