A Dual-Loop Control to Ensure Fast and Stable Fault-Tolerant Operation of Series Resonant DAB Converters

When a single-switch open-circuit fault occurs in the series resonant dual-active bridge (SRDAB) converter, the output dc voltage will drop by a half or rise by twice. To maintain a continuous power supply, a fault-tolerant control method based on the voltage single-loop control is proposed in this article, where the rectifier-side output square voltage is regulated. Nevertheless, it may excite the resonance between the resonant inductors and dc capacitors, leading to severe low-frequency oscillations, (appearing as the envelope of the high-frequency current). This may trigger the overcurrent protection and the SRDAB fails to ride through the fault. To address this issue, low-frequency equivalent models are proposed first for the bidirectional power-flow of the SRDAB, enabling frequency-domain analysis of the single-loop voltage control. The analysis reveals that the oscillation depends on the duty-cycle and control parameters, and it is more likely to occur when the converter operates in the boost mode. However, it is not possible to suppress the oscillations by the voltage single-loop control. Thus, a dual-loop fault-tolerant control method is developed. The proposed control strategy includes an outer-loop voltage control, an inner-loop current envelope control, and a nonlinear correction unit. Experimental tests on a 1-kW SRDAB are performed, which validate the effectiveness of the proposal in terms of oscillation suppression.