The Current Shaping Modular Multilevel DC-DC Converter

The recently proposed modular multilevel dc-dc converter (M2DC) and the HVDC-dc auto transformer (HVDC-AT) enable direct power transfer between dc networks at low-to-moderate step ratios using only a single power electronic conversion stage. In both converter structures, an internal circulating sinusoidal ac current is established to balance the energy within the converter. In practice, however, the frequency of the circulating ac current is limited to a few hundred Hz, requiring significant ripple power to be filtered by large voltage submodule (VSM) capacitors. In this article, a novel energy transfer mechanism for interfacing dc networks is introduced wherein the frequency of the ac currents can be increased by one to two orders of magnitude, thus enabling a commensurate reduction in VSM capacitor size and cost. This is achieved by eliminating the ac chokes within the modular converter structure. Instead, a current source submodule (CSM) is employed to shape the circulating current, leading to a current of nearly square-wave shape with a frequency in the kHz range equal to the switching frequency of the CSM. This article describes one particular topology based on this current shaping mechanism, namely a high step down ratio dc-dc converter. The viability of the mechanism is demonstrated via simulation and experimental results from a laboratory scale implementation.