Mode Analysis, Transformer Saturation, and Fault Diagnosis Technique for an Open-Circuit Fault in a Three-Phase DAB Converter

The three-phase dual active bridge (DAB3) is a popular dc-dc converter topology for high-power applications, capable of high-efficiency bidirectional power transfer with galvanic isolation. A single point of failure in power converters is the open-circuit fault (OCF) due to failure in a semiconductor device or its gate drive circuit. This study presents detailed waveform analyses for the normal- and the fault-mode operation. A novel logic-based fault diagnosis scheme is proposed based on the unique pattern in the dc bias of phase currents. Unlike previous schemes, the proposed scheme requires low-bandwidth current sensing only one side of the transformer to detect faults on either side, providing a cost and design benefit. Experimental results verify the analyses and the proposed identification scheme, detecting the fault within a few switching cycles. An in-depth study of the transformer under fault mode is presented for the first time, setting a guideline of time available for fault diagnosis and response. Experimental B-H curves and magnetizing currents of the three-phase transformer illustrate the cycle-by-cycle progression toward core saturation under fault mode. The study also reveals a new potential benefit of the three-phase DAB over the single-phase DAB; i.e., even in the presence of a secondary-side OCF, the DAB3 may continue to operate normally at full load.