Abstract：The numerous submodules and floating capacitors in the modular multilevel converter (MMC) result in tremendous weight and high cost. In addition, the existing MMC projects based on half-bridge submodules (HBSMs) cannot deal with DC faults and require submodules with more devices, which further increases weight and volume. Recently, an arm-multiplexing modular multilevel converter (AM-MMC) was proposed, which had similar operation performance to the conventional MMC with 25% less capacitors. However, its complex structure and multiplexing modes pose problems to pole-to-ground fault riding-through and energy balance. To address these issues, this article deeply analyzes its fault characteristics and energy balance mechanism, and proposes the submodule configuration and non-blocking pole-to-ground fault ride-through strategy. By configuring full-bridge submodules (FBSMs) in the upper and lower arms and implementing fundamental and second harmonic direct circulating current control, the hybrid AM-MMC (HAM-MMC) can use the negative level output capability to reduce the DC voltage bias of the faulty pole and ride through faults. Firstly, after detecting the pole-to-ground fault, FBSMs in the faulty pole arm should output negative levels to reduce the arm voltage by 50% DC voltage and arm switches alternate according to the switching threshold of the healthy pole. Calculated fundamental and second harmonic currents need injecting to achieve the converter energy balance. Secondly, the charging and discharging status of FBSMs change when output negative levels, and the submodule capacitor voltage balance sorting scheme should be adjusted. Thirdly, the reference DC voltage should be reduced to half for converters controlled by fixed DC voltage, and the reference power be half for the fixed active power converters. Finally, after the fault is cleared, normal modulation and power transmission are restored, and the fault is successfully ride through. This way, the strategy solves the energy imbalance problem between arms and submodules in AM-MMC caused by asymmetric faults. Simulation results on the pole-to-ground fault riding-through of HAM-MMC show that, when only alternating arm switches without circulating current control, the submodule capacitor voltage, arm current, and DC current fluctuate due to the arm energy imbalance. The system is unstable, and arm switches exceed the withstand voltage and current capacity. After adding the fundamental and second harmonic circulating current control, the arm switches operate within the normal range. The middle arm submodules participate in capacitor voltage sorting throughout the entire process. Both the DC current and submodule capacitor voltage remain stable, still able to transmit 50% active power. After the fault cleared, the system can quickly restore, which verifies the effectiveness of the fundamental and second harmonic direct circulating current control. =The following conclusions can be drawn from the simulation analysis: (1) Compared with traditional HF-MMC, HAM-MMC requires the same number of IGBTs but reduces the number of capacitors by 25%, which still has significant advantages in terms of lightweight and cost. (2) By configuring FBSMs in the upper and lower arms and implementing fundamental and second harmonic direct circulating current control, HAM-MMC can output negative levels to reduce the DC voltage bias of the faulty pole to 0, riding through pole-to-ground faults without blocking and maintaining 50% power transmission. (3) During the HAM-MMC alternating multiplexing riding-through, submodules in the middle arm participate in the capacitor voltage sorting and transfer energy throughout the entire process. The voltages of the submodule capacitors in the three arms are basically equal, resulting in a better energy balance effect. Compared with conventional MMC, it can ride through faults and recover faster.
李宇薇, 王毅, 高玉华, 于义轩, 王琛. 桥臂复用模块化多电平变流器单极接地故障无闭锁穿越及能量均衡[J]. 电工技术学报, 2025, 40(1): 190-202.
Li Yuwe, Wang Yi, Gao Yuhua, Yu Yixuan, Wang Chen. Pole-to-Ground Fault Riding-Through and Energy Balance of Arm-Multiplexing Modular Multilevel Converter. Transactions of China Electrotechnical Society, 2025, 40(1): 190-202.
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