Fault Diagnosis and Monitoring of Modular Multilevel Converter with Fast Response of Voltage Sensors

Modular multilevel converter (MMC) has been one of the most popular candidates in high-voltage applications. However, reliability is a critical issue due to a large number of power switching devices and capacitors applied in the MMC. To improve the reliability of the MMC, a fast and reliable diagnosis strategy for the open-circuit fault is proposed in this paper, where submodule voltage sensors are relocated to the upper switching device. Furthermore, a voltage observer based on the submodule voltage sensor is established to monitor the capacitor and realize the power control of the MMC under normal operation. A Boolean logic operation based fault indicator is put forward based on the relationship of the operation state and binaried output of voltage sensors, which could detect and locate the open-circuit faults of the MMC very fast. The ratio of the increment of the observed and measured capacitor voltage during the period of positive arm current is applied to monitor the capacitor. The effectiveness of the proposed fault diagnosis strategy and the capacitor monitoring strategy is verified by the experiment results.