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

Abstract

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 the medium-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.