Fault-tolerant control of CPS-PWM-based cascaded multilevel inverter with faulty units

Abstract

Bypassing the faulty power unit(s) of the regenerative cascaded multilevel inverter (CMI) directly results in a series of adverse influences. The existing solutions are not easy to implement and only effective for the V-connected devices composed of regenerative CMIs. To bypass the faulty power units seamlessly, the corresponding control strategies must guarantee: 1) constant equivalent switching frequency; 2) constant sampling frequency; and 3) constant fundamental voltage amplitude. To this end, a generic fault-tolerant control strategy, which adjusts the carrier frequency and simultaneously changes the CMI dc voltage and/or the modulation ratio, is presented in this paper. It is applicable for the device composed of carrier phase-shifting pulsewidth modulation (CPS-PWM)-based CMIs to ride through faults seamlessly, whether it is a single-phase system, a three-phase system (being it A-connected or V-connected), or a multiphase system. The detailed implementation is also analyzed by modeling the output voltage of the CPS-PWM-based CMI. The experimental results verify the validity and superiority of the proposed fault-tolerant strategy for the CPS-PWM-based CMI system with faulty power units.