As a result of the recent development of the smart grid system, it is inevitable that the future of power generation and distribution will involve a high degree of distributed resources interconnected to the gird via fully-rated power electronic interface. The interface is required to add functionalities, such as voltage regulation and reactive power compensation, intelligent power management and plug-and-play features for the distribution resources. The motivation for this dissertation is to design a new solid state transformer to replace the most commonly used, 100-year old interface --- distribution transformers.
The purpose of this research has been to design a cascaded H-Bridge converter based Solid State Transformer (SST) and to investigate control methods and the hardware implementation. The SST, including AC/DC rectifier, Dual Active Bridge (DAB) and DC/AC inverter is designed to interface with the single phase 7.2kV distribution system.
First, the switching model, average model and small signal model of the SST are developed. The SST controller is designed based on the small signal model bode plot. The average and switching model simulation are implemented to verify the proposed SST features.
Due to the characteristics of cascaded H-bridge converters, the voltage unbalance can appear on DC buses of different H-bridges. Meanwhile, the power unbalance also becomes unavoidable for the parallel DAB modules. In this dissertation, a new voltage balance controller is proposed to balance the cascaded H-Bridge capacitor voltages. The intrinsic constraints of the voltage balance control for the cascaded H-Bridge topology are derived. Meanwhile, a new power balance control method is proposed to balance the parallel DAB power in the SST. The proposed balance method is verified by simulations and experiments.
Furthermore, a new feed-forward power ripple control is proposed and a power synchronization method is designed to minimize the DC bus voltage ripple and the required capacitor size. Finally, the SST rectifier hardware prototype is designed and implemented. The SST features and controller are verified by the hardware experiment.
In order to investigate the feasibility of the SST for future substations, a higher power rating and higher voltage substation solid state transformer is designed based on the 10 kV SiC MOSFET and PIN diodes. A novel Zero Voltage Switching (ZVS) operation for a fivelevel DC/DC converter is proposed to reduce the device switching loss and verified by the simulations.