Modular Multilevel DC-DC Power Converter Topology With Intermediate Medium Frequency AC Stage for HVDC Tapping

This article proposes an isolated dc-dc modular multilevel converter (MMC) topology intended for tapping power from the high-voltage dc (HVdc) link. The proposed converter operates with two stages of power conversion, i.e., HVdc to ac and ac to medium-voltage dc, using an intermediate medium-frequency transformer. The proposed topology reduces the number of arms in MMC to two, along with a series LC passive filter tuned at a medium frequency at the primary side of the transformer. A high-voltage transformation ratio is possible in the proposed converter. Operation at themedium-frequency ac ensures the reduction in the size of the transformer and other passive elements. A sinusoidal level-shifted pulsewidth modulation technique is implemented for modulating the arm voltages of the converter, which results in a voltage close to the sine wave with small dv/dt being impressed across the transformer primary. The design of the passive filters is also included in the article. The operation of the converter is validated by the simulation of a 400 kV, 10 MW system, and a downscaled experimental prototype of 1 kW, 400 V system.