Open-Switch Fault Diagnosis in Back-to-Back NPC Converters of PMSG-Based WTS via Zero Range Value of Phase Currents

This study aims to enhance the reliability and availability of a three-level neutral point clamped (NPC) converter of a permanent magnet synchronous generator (PMSG)-based high-power wind turbine system by proposing a multiple open-switch (OS) fault diagnosis method. The OS faults lead to severe torque pulsation and distorted grid current in the system, hence requiring early detection and isolation. To do this, principal diagnostic variables are first formulated using the zero range value of instantaneous current amplitude and dc-link capacitor voltages. Then, the diagnostic variables detect the state of power electronic switches of each leg of the converters without being affected by torque pulsations, speed variations, and system power factor. Second, a fault localization scheme is introduced with identification thresholds to indicate the faulty switches of the machine side converter (MSC) and grid side converter (GSC). In GSC fault identification, an equivalent current transformation is employed to avoid errors in current normalization. Finally, the effectiveness of the proposed method is confirmed by verifying the results of comparative studies, simulations, hardware experimentation, and robustness against false indications in MSC and GSC.