Three-Level Submodules Based Modular Multilevel HVDC Converters With DC Fault Blocking Capability

The DC faults can impose adverse influences on the reliable operations of modular multilevel converters (MMCs) and destroy the devices in MMCs. The improvement of the submodule (SM) to provide blocking capability for MMC is an effective mea- sure to handle DC faults. In this paper, different SMs capable of blocking DC faults are analyzed in detail. According to that, the second-type three-level blocking SMs are superior to other SMs in the tackling of DC faults. Then, a three-level blocking SM is newly designed in this paper. In this SM, two half-bridge SMs (HBSMs) are recombined by the jointed unit. Besides, a branch in one of the HBSMs employs the bidirectional voltage-withstanding structures. With these features, the configuration, control and device stresses of the MMC built by the three-level SMs are analyzed to ensure the normal and blocking operations of the proposed MMC. The comparisons of operation performances among different MMCs further reveal that the cost, operation losses, fault clearance time and driver power supplies of the proposed MMC have advantages. Eventually, the feasibilities of the proposed MMC in normal mode and blocking mode are testified by simulation studies.