Objective Neutral Point Clamped (NPC) three-level inverters are increasingly used in ship power systems. The reliable operation of three-level inverters is crucial for ship power systems. This type of inverter contains a large number of switching transistors, making it prone to open-circuit faults under high-frequency switching conditions. However, some problems exist in the existing NPC three-level inverter switching-transistor open-circuit fault diagnosis methods. These include difficulties in fault feature extraction, large computational volume, etc.

Method To address this issue, an open-circuit fault diagnosis method is proposed, which utilizes the direct current (DC) component and the second harmonic component of the fault phase current as fault-sensitive indicators. The proposed approach enables high-speed and precise acquisition of phase current signals through the current sensing module integrated within the power electronic system. By employing Fast Fourier Transform (FFT), the method efficiently transforms the time-domain stator current into the frequency domain, facilitating spectral analysis without reliance on advanced or computationally intensive signal processing techniques. Under open-switch fault conditions, the frequency-domain representation of the stator current exhibits distinct features—specifically, the emergence of a noticeable DC component and an increased amplitude at twice the fundamental frequency (second harmonic). These spectral characteristics serve as diagnostic signatures for fault identification. To validate the effectiveness and robustness of the proposed diagnostic strategy, numerical simulations are conducted using MATLAB/Simulink. Furthermore, an experimental testbed is constructed, consisting of a 3 kW three-phase induction motor driven by a Neutral-Point-Clamped (NPC) three-level inverter. Experimental results corroborate the feasibility and diagnostic accuracy of the method in practical inverter-fed motor drive systems.

Results The simulation results verify the validity of the proposed fault diagnosis method; the experiments show that the mechanism analysis, simulation and experimental results match, and the proposed open-circuit fault diagnosis method based on the DC component and the second harmonic component of the faulted phase current method is effective and feasible.

Conclusion The proposed method can accurately identify open-circuit faults with two transistors or less, and has the advantages of simple fault feature extraction, small computation amount and strong real-time performance. In addition, the method utilizes the existing current acquisition signal of the inverter without additional sensors, which reduces the hardware requirement and the size of the equipment, and meets the demand for miniaturization and integration of the equipment in the limited space of the ship.