Research on Adaptive Proportional Coefficient Current Limiting Control Strategy for Hybrid MMC

Applying the negative level output capability of full bridge sub module (FBSM), the flexible DC transmission system based on hybrid modular multilevel converter (MMC) can effectively eliminate DC faults, but using more FBSMs will increase investment cost and steady-state loss. In the paper, an adaptive proportional coefficient current limiting (APCCL) control strategy based on hybrid MMC with 50% of the FBSMs is proposed. The control strategy adaptively adjusts the bridge arm voltage of the converter during DC fault, so that the MMC can output negative DC voltage to suppress the fault current. This paper introduces the basic principle of hybrid MMC dealing with faults, studies the implementation mode and controller structure of the proposed scheme, and finally verifies its fault clearing ability in a typical dual-terminal flexible DC transmission system. The simulation results show that the fault clearing strategy proposed in the paper is similar to the hybrid MMC with high proportion of FBSMs, but it has more economic advantages. It has a certain prospect in overhead line flexible DC transmission system.