Studies On Application Of Dual-active-bridge DC/DC Converter In Solid State Transformer

In recent years, in order to solve the two challenges, i.e., energy crisis and environmental pollution, the new energy and renewable energy power generation has been developed rapidly. At the same time, with the continuous expansion of the scale of the modern power grid, more and more distributed genetation systems and distributed energy storage devices need to be connected to the power grid. However, there are many difficulties exist in the interface devices among the distributed genetation systems and distributed energy storage devices and the power grid, such as different voltage grades, different forms of the energy and so on. Due to the limited capabilities of the traditional transformer, it fails to solve the above issues. A modular cascaded solid state transformer (SST) is introduced as the interface device for the distributed genetation systems and distributed energy storage devices in this paper.SST combines high-frequency conversion technology and power electronics technology, so it can not only achieve the basic functions of voltage conversion, electrical isolation and energy transfer, but also add functionalities such as reactive power compensation, voltage regulation and energy management, as well as provide AC and DC interfaces. Due to the limilation of the voltage and current grades of power electronic devices, the modular cascaded SST is introduced when used in high power applications. The modular cascaded SST includes three parts, i.e., AC/DC rectifier, DC/DC converter and DC/AC inverter. So this paper focuses on the control strategy for the dual-active-bridge (DAB) DC/DC converter and the voltage and power balance strategy for AC/DC rectifier and DC/DC converter.Firstly, this paper analyzes the background and significance of SST, reviews the development history and status of SST, and lists several typical topologies of SST. Then, this paper analyzes the operating principle of the DAB converter under the sigle-phase-shift (SPS) control and the dual-phase-shift (DPS) control, derives the mathematical model of the transmission power. And it gives the soft switching operating condition under SPS control, and studies the optimal current control strategy under bilateral DPS control. Finally, the simulation and experimental results verify the correctness of the theoretical analysis of the phase shift control.This paper studies two kinds of control strategies used in the AC/DC rectifier to maintain voltage balance, i.e., a common duty ratio control based on single-phase D-Q conversion and voltage balance control, then studies three kinds of control strategies for the DAB converter, i.e., input-output voltage following control, current sensor power balance control and current sensorless power balance control. Furthermore, this paper analyzes the control block diagrams of several control strategies, discusses their advantages and disadvantages. Finally, the simulation results verify the feasibility of the control strategies to maintain voltage and power balance.In this paper, an experimental prototype is designed and built. It introduces the selection principle of the devices used in the rectifier and the converter, analyzes the calculation process of the input and output resistances used in the sample circuit, studies the design principle of the protection circuit, and designs the DSP software to control the hardware circuit. The correctness and feasibility of two kinds of control strategies, i.e., a common duty ratio control based on single-phase D-Q conversion used in the AC/DC rectifier and input-output voltage following control used in the DAB converter, voltage balance control used in the AC/DC rectifier and current sensorless power balance control used in the DAB converter, are finally verified by prototype experiments.