Operation modes and design consideration for MMC with unipolar current full-bridge submodules

The unipolar current full-bridge submodule (SM) for modular multilevel converters (MMC) operates with unipolar current and bipolar voltage. Two basic operating modes for unipolar arm currents are presented, described, and analyzed. The differences between the two idealized operating modes are further discussed regarding the system design of an MMC. The design factors for current and voltage in the DClink differ, which leads, for example, to different numbers of SMs and power losses. These aspects are presented as examples and trade-offs are summarized.