A Novel Diagnosis Strategy for Switches With Common Electrical Faults in Modular Multi-Level Half-Bridge Energy Storage Converter

In this article, a novel diagnosis strategy including both open-circuit faults (OCFs) and short-circuit faults (SCF) is proposed for the switches in modular multilevel converter topology based on half-bridge energy storage submodules. The OCF diagnosis strategy is to analyze the current flow path of the open-circuit submodule in each OCF state, and then deduce the output voltage expression of the submodule under each OCF state. Finally, by analyzing the expression of output voltage, the average value of half cycle of sampled output voltage is proposed to diagnose all OCF states. The SCF diagnosis strategy is based on the carrier phase-shift (CPS) modulation strategy, using the characteristics that the fault of the power switching device mainly occurs at the turn- on and turn- off instants, combined with redundant submodules to replace the diagnosed submodule and interrupt its driving signal. Finally, the SCF diagnosis is carried out according to the output voltage of the submodule measured under the condition of interrupted drive signal. The diagnosis of SCF is transformed into a diagnosis similar to OCF, which greatly improves the security of the system. In addition, the interruption time point of the drive signal of each diagnosed submodule of the converter is deduced according to the improved CPS modulation diagram. besides, the corresponding fault-tolerant control strategy is also proposed, which uses redundant sub modules to replace fault submodules. Finally, the simulation and experimental results verify the feasibility of the fault diagnosis strategy.