Improvement Method of Operation Reliability for Modular Multilevel Converter

In the existing flexible HVDC projects, there are some problems such as malfunction of bypass switch, uplink communication failure of the sub-module, resulting in the blocking of the converter. It is equivalent that the fault of a single sub-module can cause the blocking of the whole converter, which seriously affects the operation reliability of modular multilevel converter (MMC). In order to solve this problem, an improvement scheme is proposed in this paper, and it contains three parts. The modification includes the thyristor replacement, the wiring mode change of sub-module and the power board connection between sub-modules. The analysis result shows that the method in this paper is effective in any fault type. In addition, based on the bidirectional thyristor, the half-bridge sub-module based MMC will have the DC fault clearing capability.