Thyristor-Assisted Hybrid MMC with Enhanced DC Fault Blocking Capability and Reduced Power Losses

Hybrid modular multilevel converters (HMMCs), consisting of full-bridge submodules (FB-SMs) and half-bridge submodules (HB-SMs), are well known for their dc fault blocking capability and operational flexibility. However, the dc fault blocking performance of HMMCs depends heavily on a high proportion of FB-SMs, which substantially increases both the converter's semiconductor cost and the power losses. This indicates an inherent trade-off between dc fault blocking capability and loss performance. To address this issue, this letter proposes a thyristor-assisted HMMC that enhances dc fault blocking capability. By redirecting the dc fault current based on the instantaneous ac phase voltages, a portion of the HB-SMs can participate in the fault blocking process. As a result, the required FB-SM ratio is reduced, thereby lowering the converter's semiconductor cost and power losses.