Precise Modelling and Dynamic Control of Single-phase Dual Active Bridge DC-DC Converters

Dual active bridge (DAB) converters are an appealing DC-DC converter solution to meet the energy conversion needs of globally burgeoning technologies in renewable energy generation, energy storage, and the transportation sector due to their bi-directional energy transfer capabilities and galvanic isolation. The benefits of using a resonant coupling network to link the two bridges of a DAB are well established – a resonant network appropriately tuned to the converter’s switching frequency can reduce the reactive currents circulating between the converter bridges, lowering conduction losses, and facilitating more compact converter designs. However resonant DAB topology variants are known to have limited zero-voltage-switching (ZVS) capability, particularly when operated at low power levels. This work presents detailed modelling of resonant LCL and CLC topology coupling networks that provide for more accurate resonance matching and precise mapping of the ZVS operation boundary conditions of these resonant DAB converter variants that accounts for all the switched harmonic contributions and the practicalities of switching dead-time and device commutation in the analysis. Thereafter, derived expressions for the active and reactive power flows between DAB converter bridges are combined with these detailed network models for a deeper understanding of these power flows in a resonant DAB and to investigate variance in their fundamental and harmonic components across converter operating conditions. A subsequent selective triple-phase-shift (TPS) modulation angle strategy is presented to extend the ZVS operating range of both the LCL and CLC resonant variants, to maximise their inherent reactive current reducing capability within the bounds of soft-switching operation. DAB control with a fast dynamic power response is essential in applications where power demands fluctuate rapidly (e.g. electric vehicles and renewable energy systems). A novel approach for dynamic power control that uses precisely timed synchronous current and voltage sample points for fast, per-cycle power estimation is proposed, using a simple inductive coupling for reduced complexity. This approach provides rapid and accurate, software-based direct power control for the conventional DAB, implemented with a proportional integral (PI) feedback controller, and hastened further with a TPS feedforward term. A novel carrier-shift approach to TPS modulation forms the basis of the direct power control strategy, enabling control over the converter’s entire operating range. The practical implementation of this control strategy demonstrates how to account for phenomena related to controller actuation (i.e. gate drive and switching device propagation delay) and measurement feedback (i.e. current sensor propagation analogue signal conditioning). All modelling and design analyses presented in this thesis have been extensively verified by precisely matched switched simulations and in experiment on a reconfigurable laboratory prototype DAB converter.<p></p>