Comparative Study on the Operating Range of the Traditional and the Compact Modular Multilevel Converters

Modular multilevel converter (MMC) is widely used in the interconnection of two AC systems with the back-to-back (BTB) structure, such as high voltage direct current transmission system, unified power flow controller and so on. However, the BTB MMC requires a lot of devices and large footprint. The compact dual-output MMCs, including nine-arm MMC, bifurcate MMC and modular multilevel matrix converter, can save the required devices at the expense of reduced operating range, which has not been thoroughly investigated. Therefore, this paper carries out a comparative study on the operating range of the traditional MMC and the compact MMCs to further clarify their merits and demerits. The basic operational principles and constraints on steady operation are analyzed, and the operating ranges are illustrated by intuitive power graphs.