Fault-tolerant control strategy for open-circuit fault of two-parallel-connected three-phase AC–DC two-level PWM converter

Abstract

This paper proposes a fault-tolerant control (FTC) strategy for an open-circuit fault (OCF) of a single IGBT in a two-parallel-connected three-phase AC–DC pulse-width-modulation (PWM) converter system. In the case of the parallel-connected PWM converter system, the OCF of the IGBT causes not only DC-link voltage fluctuation and grid current distortion, but also a zero-sequence circulating current (ZSCC). Although the effect of these problems can be reduced by existing FTC methods which is used in the single PWM converter, it is difficult to totally mitigate the problems due to the ZSCC. Therefore, this paper proposes a new FTC strategy to suppress the ZSCC using the healthy converter in the parallel-connected converter. First, it is analyzed how the difference in common-mode (CM) voltage between the two converters causes ZSCC flowing inside the parallel-connected converter under the fault condition. And then, the limitation of the existing control method for ZSCC is explained. To suppress the ZSCC using the healthy converter, the proposed control method uses a d-axis voltage injection of the healthy converter which does not affect an active power control. The effectiveness of the proposed FTC strategy is verified through experimental results.