Recognition and Analysis of Unexpected Modes of Modular Multilevel Converter

The modular multilevel converter (MMC) with the clamp double submodule (CDSM) performs outstandingly on the handling of DC fault current. As CDSM is composed of multiple switching devices, different operating modes can be produced according to the combination of switching states. Among the effective operating modes, there are inevitably some unexpected modes, which may cause abnormal operation or even failure of the MMC. In order to improve the reliability of MMC, it is necessary to identify all the effective operating modes of CDSM and analyze their trigger conditions. In this article, a novel mode identification method for power converter with multiple switches is used. According to reasonable combination of switching states, the components of CDSM are connected in an orderly manner to obtain all effective operating modes. Based on the operating principle of CDSM, the unexpected modes are further recognized from the effective operating modes, so that the impact of the unexpected modes on the MMC can be obtained. Finally, the recognition and analysis results of the unexpected modes in CDSM is verified by Simulink and Plecs RT Box.