Open-Circuit Fault Diagnosis Method in NPC Rectifiers Using Fault-Assumed Strategy

This article proposes a new open-circuit (OC) fault diagnosis method for power switches in three-level neutral-point-clamped (NPC) rectifiers. This method is based on fault assumption by introducing a new voltage deviation for diagnosis, which is the difference between the switching-state-based voltage and the fault-assumed-based voltage, where the fault-assumed-based voltage is the estimated pole voltage when all inner switches are assumed to be in the OC state. After average and sum operations, diagnostic variables are developed from three-phase voltage deviations without extra hardware and precise modeling. Since the fault characteristics of each switch reflected by diagnostic variables are not changed by other concurrent OC faults, multiple-switch faults can be diagnosed in the same way as single-switch faults. As a result, the proposed method can detect both single-switch and multiple-switch faults without complex diagnostic rules and procedures. Besides, adaptive thresholds are designed by considering voltage imbalance, current amplitude, and power factor. Owing to the self-adjusting characteristics of thresholds, the proposed method provides reliable diagnostic results in various operating states of NPC rectifiers and has good robustness against transient changes. Experiments are performed to confirm its effectiveness and robustness.