Studies On Solid State Transformer And Its Control Strategies

With the development of science and technology, electrical power system becomes smart, digital, diverse and flexible. More and more mechanical and electromagnetic equipments are replaced by power electronic devices. It is an inevitable trend that transformers, the most basic device of the power transmission and distribution system, will gradually become automatic and intelligent. In recent years, distributed renewable energy resource such as wind power generation system and photovoltaic power generation system grows rapidly. To achieve the good stability after these renewable energy sources are connected to the grid, it must rely on the construction of smart grid. Because the Solid State Transformer (SST) is an important part of smart gird, the research of SST has great practical significance.SST is a new type of intelligent equipment that achieves the function of voltage conversion and power transmission with power electronic conversion technologies. It can not only achieve the function of traditional transformer but also control the voltage and current of transformer and adjust the power factor flexibly; isolate grid and load to reduce the influence of grid voltage sag on load and the influence of harmonic generated by no-linear load on gird; supply AC and DC interfaces to make it easy for distributed power connected to grid.Firstly, the development history of SST is introduced and advantages and disadvantages of several typical topologies are analyzed. Then according to design needs, the topology studied in this thesis, which is a ternary structure with dc link, is determined. The input stage which uses carrier phase modulation to make input SPWM voltage close to sine wave consists of modular cascaded PWM converters. In addition, in order to eliminate the steady state error of ac current, a single phased D-Q vector control is used in the input stage. The isolated stage which uses phase-shift control consists of dual-active-bridge DC/DC converters. Then, the average model and small signal model are developed. Due to the characteristics of SST, the output voltage unbalance of different H-bridges in input stage and the power unbalance for the parallel DAB modules in isolated stage can appear. According to two kinds of ideas of voltage and power balance control, two control strategies are proposed, which are common duty ratio control for input stage with voltage following control for isolated stage and voltage balance control for input stage with power balance control for isolated stage. The process of voltage and power balance of two control strategies and the controller design method of first control strategie are analyzed.A SST experimental platform is designed based on DSP and FPGA. The overall diagram of hardware and design principle of main circuit parameters are introduced in this thesis. Then the implementation of control system is introduced from two aspects which are hardware and software. The communication between DSP and FPGA is realized by XINTF Interface. In order to fully take advantage of the powerful arithmetic function of DSP and the strong logic function of FPGA, information acquisition and complex control algorithm are achieved by DSP, while phase-shifted triangulars, PWM pulses, dead bands and fault handling are produced by FPGA. Finally, simulation and hardware experiment result are given at the end of this thesis. The validity and reliability of two kinds of control strategies proposed in the thesis are confirmed by the result of PSIM simulation and experiment.