A diagnostic strategy for IGBT open-circuit faults in modular multilevel converters based on improved diagnostic indices

Capacitor voltage deviations between two submodules (SMs) are always used as the diagnostic index for the insulated gate bipolar transistor (IGBT) open-circuit fault diagnosis in modular multilevel converters (MMCs). However, it is difficult to determine when MMCs operate in different conditions. To solve this issue, this article proposes a diagnostic strategy for IGBT open-circuit faults in MMCs based on improved diagnostic indices. First, two diagnostic indices, i.e., the concept of the count of periods during which the switching function is fixed (CSF) and capacitor voltage sorting sequence number (CSN) are proposed. On this basis, fault detection is implemented by judging the changing trend of the CSN in the SM whose CSF is the highest. The remaining malfunctioning SMs are located in turns by a similar process to fault detection. The proposed diagnostic indices show consistent faulty characteristics under different operating conditions, thus avoiding readjusting the threshold according to different operating conditions. In this sense, the proposed strategy can be easily used and promoted in different operating scenarios. Experimental results in a hardware-in-the-loop (HIL) platform verify the effectiveness of the proposed strategy.