Symmetric State Reconstruction for Open‐Circuit Fault Tolerance in Four‐Level ANPC Dual‐Active‐Bridge Converters

ABSTRACT The four‐level active neutral‐point‐clamped dual‐active‐bridge (4L‐ANPC‐DAB) converter features galvanic isolation, high power density, strong adaptability to medium‐ and high‐voltage applications, and enhanced modulation flexibility enabled by multilevel voltage synthesis, making it well suited for medium‐ and high‐voltage DC conversion systems. However, the four‐level bridge contains a relatively large number of switching devices, and an open‐circuit fault (OCF) in a single switch may lead to output‐voltage distortion, volt‐second imbalance across the leakage inductance, DC bias in the inductor current, and cumulative magnetic bias in the transformer, thereby degrading system reliability. To address these issues, this paper proposes a symmetric state reconstruction (SSR) fault‐tolerant method for 4L‐ANPC‐DAB converters under single‐switch OCF conditions. First, the OCF modes on the four‐level side under single‐phase‐shift (SPS) modulation are analyzed, revealing the mechanisms of voltage‐level degradation during the faulty interval and the resulting DC‐bias formation. Then, by exploiting the redundant active‐clamping states inherent in the 4L‐ANPC bridge leg, the output voltage levels during the fault intervals of both inner and outer switches are reconstructed, and the proposed method is compared with the conventional primary‐side lower power secondary‐side bypass arm (PLP‐SBA) method. Furthermore, the proposed SSR strategy is extended to multiple‐phase‐shift (MPS) control conditions, and the post‐fault power‐transfer capability under different phase‐shift ratios is investigated. Experimental results demonstrate that the SSR method can mitigate voltage distortion during the fault interval, suppress current bias, and improve the power‐transfer capability after a fault.