Low-Frequency Oscillation Suppression in Series Resonant Dual-Active-Bridge Converters under Fault Tolerant Operation

When an open-switch fault occurs in the inverter-side of the series resonant dual-active-bridge (SRDAB) converter, the rectified DC voltage will drop by a half. One solution to maintain the continuous power supply of the converter is to regulate the duty-cycle of the rectifier output voltage. Nevertheless, it may excite the resonance between the resonant inductors and the DC capacitors and lead to severe low-frequency oscillations, which appears as the envelope of the high-frequency current. This phenomenon may trigger the over-current protection and make the SRDAB fail to ride through the fault. In this paper, a low-frequency equivalent model is proposed for the SRDAB, enabling frequency-domain analysis of the conventional single-loop voltage control. It is revealed that the oscillation depends on the duty-cycle and control parameters, and it is not possible to suppress such oscillations by the conventional control method. Thus, a dual-loop fault tolerant control method consists of an outer voltage control-loop, an inner current envelope control-loop. Also a non-linear correction unit is proposed to suppress the oscillation. Experimental tests on a 1-kW SRDAB are performed. The test results have validated the effectiveness of the proposal in terms of oscillation suppression.