Reliability Analysis and Advancements in Fault Tolerant Control of Three-Phase Dual Active Bridge Converters

Abstract

The single-phase dual-active-bridge (1p-DAB) and three-phase dual-active-bridge (3p-DAB) converters are both widely employed in DC-DC power conversion applications. Compared to the 1p-DAB converter, the 3p-DAB converter’s inherently higher number of switches increases the risk of switch failures and associated gate driver failures. Nevertheless, this increased number of switches also enhances the fault-tolerant capability of the 3p-DAB, allowing it to maintain power transmission even under certain fault conditions, thereby improving overall system reliability. Additionally, the distributed current sharing among more switches reduces current stress, further contributing to reliability. Given these trade-offs, it is challenging to definitively determine which topology offers superior reliability. Therefore, the thesis performs a quantitative reliability comparison between the two topologies. Due to the enhanced fault-tolerant capability of the 3p-DAB converter, various fault-tolerant methods have been proposed to address open-circuit failures (OCFs) for this topology. Among these, the frozen leg method stands out as an effective fault-tolerant approach requiring no additional hardware. This method isolates the faulty leg by disabling its two switches so that the converter can continue operating at reduced power levels. However, two key research gaps remain in the current literature: (1) the lack of analysis for the extended phase shift angle range of [π/3, π/2] under unity voltage gain condition, and (2) the limited research on non-unity voltage gain scenarios, i.e., buck (Vin>nVout) and boost (Vin<nVout) modes. The absence of a thorough investigation into [π/3, π/2] leaves the nature of power transfer in this region unclear, raising questions about the actual power transmission capability under unity voltage gain condition. Additionally, in practical applications, buck and boost operations are often unavoidable. Yet their behaviors under frozen leg operation are complex and require extensive analyses using different analytical frameworks and mathematical models, which have not been comprehensively addressed. To address the first research gap, this thesis provides a completely new analysis to describe the transferred power in the phase shift angle range of (π/3, π/2] under unity voltage gain, which also newly yields the theoretical maximum power transfer of the frozen leg operation compared to normal operation. Based on this full-range analysis, a simple self-embedded fault-tolerant control method is proposed, which enhances the power transfer capability under OCFs. To address the second research gap, the thesis offers the first in-depth investigation into frozen leg operation under both buck and boost conditions. The analysis categorizes operations into multiple distinct cases, each with corresponding derivations for power, current, and voltage. Furthermore, the maximum transferable power for both buck and boost modes is derived, along with a detailed zero-voltage switching (ZVS) analysis. Finally, based on the ZVS analysis in both operating modes, a ZVS-guaranteed control strategy is proposed. The strategy can regulate the output voltage reference in a predetermined manner to ensure that the converter consistently operates within the ZVS region at the target power level while operating in frozen leg mode. Consequently, it helps to better manage switch thermal stress and improve converter efficiency. Experimental results confirm that the proposed strategy performs as expected and effectively guarantees ZVS operation.