Half-bridge (HB) modular multi-level converter (MMC) based bipolar HVDC transmission systems can be employed to transfer bulk power over long distances with high reliability and flexibility. However, this configuration exhibits special fault behaviors that should be taken into account during its design stage. For instance, a single-phase-to-ground fault at the valve-side of the HB-MMCs will lead to overvoltages in the upper arm and high dc components in the fault currents. More importantly, such type of fault currents will bring difficulties for grid side ac circuit breakers to interrupt arcings within the contact parting time. In this paper, such issues are theoretically analyzed in detail. A hybrid MMC using both HB and full-bridge submodules is proposed to solve the aforementioned problems. The proposed solution is verified through simulations performed in PSCAD/EMTDC. The studies in this paper will contribute to the design and protection of MMCs in bipolar HVDC transmission systems for station internal ac grounding faults.