Controls and Applications of the Dual Active Bridge DC to DC Converter for Solid State Transformer Applications and Integration of Multiple Renewable Energy Sources

The Dual Active Bridge (DAB) converter was first developed in the University of Wisconsin Madison in 1989. Since then the converter has gained popularity because of its high power density, high efficiency due to ZVS operation under wide load range, bidirectional operation and high frequency isolation. The traditional control of the converter is based on phase shift modulation. The primary bridge which supplies power phase leads the secondary bridge which absorbs power. Extensive research has been done previously on the small signal analysis and the dynamics of the converter. In order to further improve the speed of response of the converter and to have a control on the DC bias level in the high frequency transformer current, a digital predictive current mode controller for the DAB converter is proposed in this thesis. The first chapter discusses the proposed current mode control. The controller is shown to track the reference current within a switching period provided the correct inductance information has been provided to the controller. Additional control methods are developed to observe and remove DC bias flux in the high frequency transformer and the high frequency inductor. Experimental results are provided to verify the working principles of the current controller. In the second portion of the thesis, a multi-terminal topology variant of the DAB converter was explored with storage and renewable energy integration. A multi -- limb core transformer based DAB converter (MLC-DAB) is proposed and developed for this application. The equivalent circuit of the MLC-DAB topology is developed using the gyrator concept and the advantage and disadvantage of the proposed topology is shown with respect to a single core multiple winding DAB topology or a series connected multi-terminal DAB topology. A pulse width modulation (PWM) based input current control is developed for the MLC-DAB to integrated different renewable energy sources operating at different maximum power points. In the final chapter of the thesis the application of the DAB converter in the DC stage of a Solid State Transformer (SST) is discussed. A single phase cascaded solid state transformer is considered with the DAB converter in the DC to DC stage. A soft start algorithm is proposed for the SST to reduce inrush currents at startup. The MLC-DAB topology is considered for the cascaded single phase SST and is shown to require simpler control compared to a conventional DAB topology. A micro grid is developed with two parallel connected single phase single stage SST with MLC-DAB. The system is demonstrated experimentally in grid tied mode with power being injected into the grid from renewable energy source (emulated by DC source) integrated to the MLC-DAB stage of the SST. In case of grid failure a black start mode algorithm is developed and is implemented with one of the SST acting as a master and maintain the PCC voltage while the slave SST work in constant current injection mode. The critical load points are recognized within the micro-grid and black start algorithms are developed to provide for uninterrupted power to the critical load points.