An Investigation on Robust Control of Isolated DC-DC Dual Active Bridge Converter

Abstract

In recent years, DC-DC power converters are comprehensively used for numerous applications such as in dc-microgrids, energy storage systems, distributed generating systems, power filters, electric vehicles, etc. With the increase in usage of renewable energy sources and electric vehicles, the DC-DC converter finds larger application area in nearby future. The dual active bridge (DAB) converter is an isolated DC-DC converter which has risen in popularity for its notable advantages such as ohmic isolation, higher efficiency, bidirectional power flow, high voltage gains and smaller size and weight, makes it a preferable option for various applications. The control of DAB converter is an important research area to achieve an efficient and robust system. With various control methods proposed earlier, the converter provides satisfactory performance, however, the robustness, voltage regulation and stability need to be improved further. This thesis provides sliding mode-based control methods for the DAB converter to obtain a robust and stable system. The frequent disturbances including load change and input voltage change is handled to obtain a fix output voltage. The parameters of the converter deteriorate over the period of time and changes the converter overall performance. Therefore, the robust controller presented in this thesis, handle the parametric uncertainties and provide better performance. The sliding mode based direct power control (SM-DPC) is proposed and the experimental results are compared with feedback linearization control (FLC), sliding mode control with harmonic modelling (SM-DPC) and virtual direct power control (VDPC). The SM-DPC shows the improved in voltage regulations and handles disturbances and uncertainties. With several advantage over classical sliding mode control, super-twisting sliding mode control (ST-SMC) provides better performance under large disturbance and uncertainties. Thus, the ST-SMC control for DAB is proposed in this thesis. The experimental results are shown and compared with conventional SMC and proportional-integral (PI) method, which conforms the improvement in the overall performance of DAB converter system and eliminates the problems of chattering in the classical sliding mode control. As the major applications of DAB are in electric vehicles, dc-microgrid, renewable energy systems, solid-state transformers (SST), and energy storage systems. These systems are multi-converter systems with back-to-back connections of power electronics converters. In multi converter systems, the power electronics converter acts as feeding converters as well as loads to other converters. The tightly regulated power converters act as constant power load (CPL) and has destabilizing effect on the connected system. The sliding mode and current observer based direct power control (SMC+O) is proposed in this thesis, which make the DAB converter stable while feeding CPL. The current observer eliminates the requirement of current sensor from the system and help in minimizing the chattering phenomenon. The SMC+O method is compared with PI and SMC methods in experimentation and the results shows the improvement in transient and steady state performance of the system with CPL. Thus, the methods proposed in this thesis can be adapted for various DAB applications as the proposed robust control of the converter makes the system efficient and reliable.