A Hybrid MMC with Low FBSM Ratio for DC Fault

Flexible DC transmission system using modular multilevel converters (MMC) is facing the problem of DC fault clearance. The existing sub-module hybrid MMC uses full bridge sub-modules (FBSM) to achieve non-blocking fault ride-through (FRT), but the ratio of FBSM needs to reach 50% or more, the cost and operating loss are more than half higher than that of the half bridge MMC. This paper proposes a low ratio hybrid MMC suitable for DC fault clearance. When the ratio of FBSM is less than 50%, the DC output voltage is reduced to zero by reducing the value of the arm voltage proportionally to isolate the faulty line. This paper analyzes the fault isolation sequence and controller structure of the scheme. Finally, a simulation model is built in PSCAD/EMTDC to verify the effectiveness of the scheme.