Multiple open-switch fault diagnosis for three-phase four-leg inverter under unbalanced loads via interval sliding mode observer

The three-phase four-leg inverter has garnered significant attention for its advantages in addressing unbalanced loads and other factors. However, compared to traditional three-phase three-leg inverters, the introduction of additional branches and the occurrence of unbalanced three-phase currents increase the complexity of open-switch (OS) fault modes and types, posing challenges for fault diagnosis. In this article, a multiple OS fault diagnosis strategy is designed specifically for three-phase four-leg inverters operating under unbalanced loads. The method begins with the design of a novel interval sliding mode observer (SMO) that accurately and rapidly estimates phase currents through the use of an adaptive reaching law. By using the interval characteristics of the innovative interval SMO, the upper and lower bounds estimations of the observer are then utilized to design a fault detection parameter and its adaptive thresholds to ensure the robustness of the detection algorithm. In addition, single OS fault identification and multiple OS fault identification methods are developed. By combining the detection variables with the identification methods, the proposed approach enables the diagnosis of 36 different types of OS faults. Hardware-in-the-loop test results validate the efficient diagnosis of OS faults in three-phase four-leg inverters utilizing the proposed approach.