A Unified DC Short-Circuit Fault Current Limiting Strategy for Hybrid-Arm MMC-Based DC Transformers

DC transformers (DCTs) based on modular multilevel converters (MMCs) are becoming critical equipment in medium-voltage DC (MVDC) systems. However, under MVDC fault conditions, the short-circuit current is typically limited by blocking all submodules (SMs), which is unfavorable for rapid fault clearance, accurate fault location, or uninterruptible power supply to critical loads. Therefore, this paper proposes a hybrid-arm MMC-based DCT for interlinking MVDC and low-voltage DC (LVDC) systems.A DC short-circuit current limiting strategy is proposed, which utilizes the negative voltage output capability of full-bridge submodules (FBSMs) together with duty cycle control. Consequently, the HAMMC-DCT can operate over a wide MVDC voltage range (from zero to the rated value) resulting from fault-induced voltage drops. In addition, this strategy eliminates the requirement for a separate fault ride-through design, offering a unified and straightforward control approach. Meanwhile, zero-voltage switching (ZVS) is achieved under both normal and fault conditions. Furthermore, the proposed voltage-balancing algorithm ensures well-balanced capacitor voltages for FBSMs under different output-voltage states, both before and after a fault. Finally, experimental results are given to validate the feasibility and effectiveness of the proposed control strategy.