A fault-tolerant operation scheme for a modular multilevel converter with a distributed control architecture

Hot reserved redundant sub-modules significantly increase the reliability of MMC systems equipped with considerable sub-modules. This paper presents a fault-tolerant operation scheme for an MMC with a distributed control architecture. The MMC output, internal dynamics and the average capacitor voltages are balanced and well-regulated while asymmetrical sub-modules in arms are bypassed due to faults. With the designed operation scheme, the multiple control loops for the MMC are scarcely influenced by bypassing the faulty sub-modules. By simply resetting the Enhanced Pulse Width Modulator (ePWM) module period and phase registers in local controllers according to the number of sub-modules remained in each arm, the re-scaled modulation signals, the symmetrically phase-shifted triangular carriers and the switching harmonics cancellation can be achieved simultaneously. The sub-module capacitor working voltage in the fault-tolerant operation scheme is discussed with the considerations of output voltage requirement and the safe operating voltage of the capacitors. The effectiveness and validity of the fault-tolerant operation scheme are verified experimentally on an MMC prototype. Good agreements are achieved among the analysis, designed objectives and experimental results in the MMC performance under fault conditions.