Optimized transient modulation and control strategies for bidirectional dual-active-bridge DC-DC converters

Abstract

Due to the advantages of simple structure, wide-range soft-switching features, ease of modulation and control, etc., both non-resonant dual-active-bridge converter (NR-DABC) and series-resonant DABC (SR-DABC) are preferred options for isolated bidirectional dc-dc power-conversion applications. As DABC is more frequently employed in power-electronic systems that demand fast dynamics, its optimal transient performance is an active research topic. It is found that when the control variables, i.e., phase-shift angles, are updated through conventional transient phase-shift modulation, severe transient oscillations and/or dc offsets will be induced in the high-frequency-link currents of DABC. These transient oscillations and dc offsets will lead to high current stresses on power devices, and they can span many switching periods during transient stage, thus introducing excessive time delays between the PWM generator and controller. Consequently, truly optimal dynamic performance cannot be achieved with a high-performance controller alone, and the modulation-induced problems must also be thoroughly investigated. In this thesis, an optimized transient phase-shift modulation (OTPSM) method, known as symmetric single-sided OTPSM (SS-OTPSM), is proposed for single-phase-shift (SPS) modulated NR-DABC. It can fully eliminate all undesired transient dc offsets, and be easily implemented under closed-loop conditions. Furthermore, an enhanced model-predictive controller (EMPC) is proposed for precisely matching the transient energy-transfer model under SS-OTPSM. By integrating SS-OTPSM with EMPC, ultra-fast dynamics can be realized without any transient dc offsets in NR-DABC. To suppress transient oscillations in SR-DABC, a novel sensorless trajectory-switching modulation (TSM) strategy is proposed for cost-effectively achieving the function of transient trajectory planning of the resonant waveforms. Besides avoiding complicated computation for its cycle-by-cycle implementation, the proposed TSM can be compatible with high-gain controllers for demonstrating ultra-fast and oscillation-free transient performance in SPS-modulated SR-DABC. TSM and the other existing OTPSM strategies are mainly developed for suppressing transient oscillations in SPS-modulated SR-DABC, and they cannot eliminate the transient dc offset in transformer's magnetizing current. Hence, this thesis also proposes a generalized TSM (GTSM) method for improving the dynamic performance of multi-phase-shift modulated SR-DABC. GTSM can achieve fast elimination of transient oscillations and dc offsets simultaneously, and ensure safe transient operation of both NR-DABC and SR-DABC. Furthermore, it can be easily adapted to all single/dual/triple/multi-phase-shift gating schemes regardless of power-flow directions and operation modes. This thesis focuses on developing sensorless OTPSM methods for DABC and presents detailed theoretical analyses, mathematical derivations, and real-time closed-loop experimental verifications. The reported findings provide insights on the optimization of the dynamics of DABC using advanced and effective transient modulation schemes and controller design.