Modular DC/DC converter: Comparison of modulation methods

This paper presents a bidirectional modular DC/DC converter. It consists of two Modular Multilevel Converters (MMC) linked by a transformer to achieve galvanic isolation. This topology serves as “DC transformer” interfacing low and medium voltage DC grids. By the means of the “front-to-front” connection, the converter is able to handle short circuits without additional protection. A higher operating frequency makes it possible to decrease the size of the transformer, the capacitors and the branch inductances. Two fundamental frequency modulation methods are presented with the aim to reduce the switching frequencies without penalizing the performance of the transformer. The multilevel modulation yields a conventional staircase voltage waveform, whereas the two-level modulation, which is inspired by the conventional double active bridge converter, results in a two-level voltage waveform. A case study of a wind power converter will serve as point of comparison between the two methods. The semiconductor losses are evaluated and the efficiencies for different transformer designs are discussed.