Pulse Compensation for Phase-Shift PWM with Sawtooth Carrier Applied to Cascaded Multilevel Converter under Fault Condition

A cascaded multilevel converter (CMC) consists of a number of submodules (SMs) connected in series. For this reason, it has highly scalability and thus it attracts much attentions as a converter that is applicable to high voltage and large capacity applications. One of the features of CMCs is that an equivalent carrier frequency of a CMC using phase-shift PWM (PS-PWM) is proportional to the number of SMs. In the previous study about PS-PWM for CMCs, we reveal that total harmonics distortions of line-to-line voltage is lower for PS-PWM using sawtooth carriers than using triangular carriers. This study proposes a scheme that enhances the reliability of the CMC. The concept of proposed scheme is that healthy SMs compensate the voltage pulses that can no longer output from failed SM. The proposed scheme has two significant features as follows: 1) line-to-line voltage waveforms can be maintained the same as ones before a SM faults, and 2) the number of switching times per a SM does not change in the case that the CMC operates with PS-PWM using sawtooth carriers. The effectiveness of the proposed scheme is verified by an numerical simulation and an experiment.