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.