Studies On Topology And Control Strategies Of Dual-active-bridge DC/DC Converter For Solid State Transformer

In recent years, the rapid development of new energy power generation is a great help in solving the energy crisis and environmental pollution. But the new energies such as wind and photovoltaic are largely affected by the weather, which will inevitably differ from traditional power plants. Also, with a large number of non-linear loads connected to the grid, the load structure is quietly different from it before. Traditional grid can’t solve these issues well, therefore the smart grid emerges consequently. Currently China has made remarkable achievements in terms of the HVDC, which forms a large grid interconnection as a support for the construction of a strong smart grid. However, it still needs a technical solution which can solve the access to grid issue of a lot of the distributed generation sources and distributed storage devices.As a new type of power electronic devices, Solid State Transformer(SST) can not only achieve the function of voltage conversion and energy transfer, but also add functionalities such as voltage regulation, reactive power compensation, intelligent power management, plug-and-play features for the distribution resources. The cascaded H-bridge converter based SST including AC/DC rectifier, Dual-Active-Bridge (DAB) DC/DC converter and DC/AC inverter is the most widely used because of its high controllability and easy modular implementation.As the isolated stage of a SST, dual-active-bridge DC/DC converter consists of two symmetrical H-bridge and a high-frequency transformer, which can reduce the volume of SST. Using phase-shift control, the output voltage can be controlled at a stable value and the energy flows from the leading leg to the lagging. This paper analyzes the steady operation of DAB in single-phase-shift control and draws a ZVS soft switching operating condition. Selecting the capacitor voltage as the state variables, this paper derives a first-order small-signal equivalent model, on basis of which closed-loop controller is designed. Simulation and experiment results of a scaled down DAB prototype are provided to verify the theoretical analysis. This paper analyzes the cause of the circulation current generated by single phase shift and the mechanism of circulation current elimination by dual-phase-shift, with which a simulation is conducted. In addition, the paper proposes a method of double closed loop control with the voltage outer-loop and the current inner-loop, the simulation of which proves a distinct advantage in the dynamic response speed over the single voltage loop. Finally, the simulation analysis of a hysteretic control is done.Through the analysis of the current sharing method of the converters with a characteristics of the voltage source, this paper pointes out the area in which methods should be improved for the parallel application of multiple DAB module. After analyzing the three typical control methods, the paper summarizes a general idea of current sharing for DAB parallel application. Simulation verifies the theoretical analysis.