Neutral-point-clamped three-level (3L-NPC) inverters are widely deployed in ship power systems and require high operational reliability. Subject to numerous switching transistors and high-frequency switching, they are susceptible to open-circuit faults. Existing open-circuit fault-diagnosis methods for 3L-NPC transistors suffer difficulties in fault-feature extraction and high computational complexity.

This paper proposes an open-circuit fault-diagnosis method using the DC and second-harmonic components of fault-phase current as sensitive indicators. Phase-current signals are rapidly collected by the built-in sensing module. Fast Fourier transform (FFT) converts time-domain stator-current signals into frequency-domain spectra for analysis, avoiding heavy-duty signal-processing algorithms. Under open-switch faults, stator-current spectra show prominent DC offset and elevated second-harmonic amplitude, which are taken as fault signatures. Validation is performed via MATLAB/Simulink numerical simulation and a self-built 3 kW 3L-NPC-driven three-phase induction-motor test-bench to verify the strategy's effectiveness and robustness. Experiments validate its feasibility and diagnostic accuracy for practical inverter-motor drive systems.

Simulations prove the effectiveness of the presented method. Experimental data show good consistency among mechanism analysis, simulation and measurement results, verifying the feasibility of the diagnosis method based on DC and second-harmonic components of fault-phase current.

The method accurately diagnoses open-circuit faults of up to two transistors. It delivers simple feature extraction, low computational complexity and favorable real-time performance. Using the inverter’s native current-measurement signals without extra sensors, it lowers hardware overhead and equipment volume, satisfying miniaturization and integration requirements for space-limited marine scenarios.