A DC/AC Modular Converter With Mid-Point Transformer for Integrating MVDC With MVAC for Railway Traction Power Supply

This work presents a half bridge (HB) submodules (SMs) based modular multilevel converter (MMC) topology intended to integrate MVdc grid with a single phase MVac grid for railway traction power supply. The topology uses reduced arms/chain-links (two arms) of conventional HB-MMC and a mid-point transformer at the ac side to employ galvanic isolation needed for this application. Further, by utilizing the 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 precharging methodology for SM capacitors 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.