A DC/AC Modular Converter with Midpoint Transformer for Integrating MVDC with MVAC for Railway Traction Power Supply

This work presents a half bridge (HB) sub-modules (SMs) based modular multilevel converter (MMC) topology intended to integrate MVDC grid with single phase MVAC grid for railway traction power supply. The topology uses reduced arms/ chain-links (two arms) of conventional HB-MMC and a midpoint transformer at the AC side to employ galvanic isolation needed for this application. Further, by utilizing appropriate modulation technique and mid-point transformer, the cancellation of DC flux within the transformer is achieved. The inductance of the arm inductor can be combined with the leakage inductance of the mid-point transformer, thus eliminating the requirement for extra inductors. A unique pre-charging methodology for SM capacitor in the proposed topology is discussed, which differs from conventional approaches. The converter's performance during various fault scenarios is analyzed, showcasing its inherent fault blocking capability. The operational principles are explained mathematically and validated through simulations and a scaled-down experimental prototype under both normal and fault conditions.