A transformerless DC-DC MMC based on symmetrically interlinked subconverters

This paper proposes a DC-DC modular multilevel converter (MMC) topology for MVDC and HVDC applications that does not require ac transformers or coupled inductors. The DC-DC MMC comprises two dc-dc subconverters that are interlinked in a symmetric manner, whereby the modulated ac voltages remain isolated from the dc terminals. This enables a substantial reduction in converter size, weight, capital cost and operating losses. The proposed dc-dc MMC has buck/boost capability and can block dc fault currents with half-bridge switching cells. A dynamic controller is developed based on a derived converter model. The converter operation and controls are validated by simulation.