Analysis, Design and Control of Asymmetrically Modulated Dual Active Bridge Converters for Energy Storage Systems

Abstract

Energy storage units are widely used in power systems to increase grid reliability to compensate the risks imposed due to massive integration of intermittent sources, as well as serve as spinning reserves. High efficiency dc-dc converters are required as an interface between the storage units and the grid tied inverter, and one of the suitable efficient converters for this purpose is the Dual Active Bridge (DAB) converter. This thesis presents an alternative modulation method and detailed analysis and control systems for a Dual Active Bridge converter to minimize the transformer RMS current with Zero Voltage Switching to improve efficiency throughout a wide operation region. The proposed modulation operates asymmetrically to achieve the optimized operating point. Various operating modes are analyzed, and their corresponding power, RMS current equations and soft-switching conditions are derived. The RMS equations are minimized using multi-variable optimization method and the corresponding parametric equations are obtained. Moreover, the valid region of operation for asymmetric modulation is also identified. In order to achieve a closed loop operation, large and small signal models of the non-linear DAB converter with large varying signals and operating under asymmetric modulation are derived and linearized. Using the models, the closed loop controller gains for the converter are designed and a comprehensive stability analysis is carried out. In addition, some unique problems arising in the converter due to asymmetric modulation, such as emergence of parasitic dc component in the low voltage side of transformer tank are addressed and effective solutions are presented. Finally, a soft-start method for the DAB converter operating under proposed modulation with active source and load have been proposed. The performance of asymmetric modulation is compared with all relevant existing modulation methods throughout a wide operating region and it is found that the proposed modulation can achieve a 6-10% higher efficiency than the existing modulations at the low power range. Based on the results, asymmetric modulation is combined with other modulations in different regions to provide superior efficiency throughout a wide operating range. Analytical, simulation and experimental results are presented to verify the effectiveness of all the proposed methods and designed controllers.