Dynamic Response Improvement for DAB Converter with Constant Power Load under Extended-Phase-Shift Control Based on Trajectory Control

Microgrid has been regarded by many researchers as the fundamental building block for the next generation power grid, where DC-DC converter plays a crucial role as the interface between main DC bus, energy generation facilities, and energy storage systems. Among many different topologies of DC-DC converters, bidirectional isolated dual-active-bridge (DAB) converter receives increasingly more attentions because of its many merits, including allowance on bidirectional power flow, isolation between power supply and load, and voltage level conversion. However, as the control scheme used to regulate DAB converter evolves, its output voltage and current could be roughly regarded as constant, which implies the converter actually feeds a constant power load (CPL). Nevertheless, because of its equivalent negative resistance effect, CPL could introduce instability to the system, causing output power to oscillate or drops to zero. This paper comprehensively explains the impact of this negative resistance effect, and, to cope with this problem, a nonlinear control method called trajectory control based on the natural switching surfaces (NSS) of DAB converter with extended-phase-shift (EPS) control will be derived. To better demonstrate the advantages of using trajectory control, a traditional linear controller, PI controller, is implemented as a comparison. Finally, by comparing the simulation results between traditional PI control and trajectory control, it is found that the start-up stage for trajectory control is much shorter than that of PI control, and the system stability of DAB converter feeding CPL is improved.