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This article proposes an open-circuit (OC) fault diagnosis method in three-level neutral-point-clamped (NPC) rectifiers. In this method, OC faults are detected based on the deviation between actual and expected reference voltages, which are obtained without complex calculation, precise modeling, and extra hardware. More importantly, this method innovatively incorporates fault-tolerant control into the diagnosis process to locate the faulty switch more accurately and quickly. Fault-tolerant control is triggered before confirming the faulty switch. It can completely or partly eliminate reference voltage distortion caused by any outer-switch faults. Making use of this feature, outer- and inner-switch faults can be differentiated by checking whether reference voltages return to normal. Unlike other invasive methods that amplify fault characteristics, this method does not aggravate fault consequences. Using fault-tolerant control to replace the invasive location step of other methods can make a timely intervention of OC faults and save computing resources used for fault diagnosis and tolerance. Also, this method considers different operating states of NPC rectifiers, including modulation index and power factor variation. Moreover, this method can be easily extended to multiple-switch and clamping diode fault diagnosis. Experiments are carried out to confirm the effectiveness of the proposed fault diagnosis method.

Capacitors and semiconductor power devices are the most fragile components in power electronic systems. Hence, designing a comprehensive method for online switch fault diagnosis (FD) and capacitor condition monitoring (CM) is conducive to guaranteeing the reliability of the three-level T-type (3L-T) inverter in an all-around way. First, based on modeling the 3L-T inverter on the dc link and ac side, the inherent harmonic components of the dc link under normal and open-circuit fault conditions are analyzed. Furthermore, the capacitor current reconstruction method is designed. Subsequently, a comprehensive FD and CM framework is proposed, which consists of signal injection, extra hardware, 3L-T model, FD, and CM submodules. In FD, the detection variable, which is the function of output current and its changing rate, is designed, and the fault location is carried out based on the phase current residual and the sum of the neutral-point voltage residual. The capacitance estimation method is introduced based on the input current reconstruction technology and recursive least squares algorithm using the inherent or injected ac component on the dc link. Finally, experimental results demonstrate that the proposed comprehensive method merits high estimation accuracy, fast dynamic response, and better FD performance under various conditions.

Traditional fault diagnosis (FD) and condition monitoring (CM) methods for three-level T-type (3L-T) inverters are studied separately, which is not conducive to guaranteeing the reliability of the 3L-T inverter in an all-around way. This article presents a unified open-circuit FD and CM method for the 3L-T inverter based on the proposed on-state voltage measurement circuit (OVMC). The designed OVMC comprises a voltage limiter, an absolute value circuit, and a minimum value circuit, separating all the OVs and actual switching state information with a few devices and terminals. The corresponding sampling method is designed under the space vector pulsewidth modulation mode, and the sampling frequency is only twice the switching frequency. Three exceptional cases are considered during the implementation to achieve accurate OV sampling. The proposed method for fault locating requires only a straightforward lookup table without complicated feature extraction and threshold setting steps. Meanwhile, the FD results would help to correct the OV distribution. Experimental results demonstrate that the proposed unified FD and CM method merits high measurement accuracy, fast response speed, and short FD time.