Research On Three-phase Inverter Based On DC Micro-grid

Distributed power and its application are a new development direction of power system, but a mass of distributed power on the verge of grid system’management could lend to grid power system uncontrolled, unsafe and instable. It can affect the quality of the power and make the control of power grid voltage difficult, resulting in a decreased efficiency of power grid. To integrate the advantages of distributed power system, and weaken the negative impact on the grid., the dissertation puts forward the micro grid.Micro power grid is an aggregate of load and small energy sources. They run in networking mode normally, but run in independent mode in an emergency. Micro grid can make full use of environmental resources, reduce cost and improve the reliability of power supply. Inverter technology as the key to the development of new energy technologies transforms solar power or wind power to ac electricity and connects to the grid. Grid inverter as an interface device of solar cell and power grid plays a very important role in the development and utilization of new energy. So, the dissertation discusses about the three-phase inverter in the system of dc micro grid, and verifies the correctness of the research results through the experimental simulations and debugging of hardware circuits and software.Firstly, the mathematical model of three-phase inverter has been discussed. Model of three-phase inverter in dq coordinate axis is established. The dissertation gives the analysis of the Clark transformation and Park transformation and the design of feedforward decoupling control. Simulations experiments are done through PSIM9.0.Secondly, the control algorithm based on SVPWM in this design has the advantages of clear physical meaning, fixed switching frequency and convenient implement etc. The outputting waveform is good and dynamic response is fast.Thirdly, the hardware circuits of the system are designed. The dissertation analyzes the main circuit’s structure of the three phase inverter and operating principle, and gives the parameters’selections of the main energy storage components. At the same time, the parameter measurement circuits and protection circuits are analyzed. The effective controller called TMS320F28335DSP plate is introduced.Fourthly, the dissertation gives the software design based on CCS development platform and realizes the functions of A/D conversion, PWM control and communications between PC and the inverter. The technique of digital phase locked loop and issues of islanding effect in the process of paralleling are discussed. The operating principles of phase-locked loop are analyzed, and the causes of islanding effect and detection methods are discussed. Besides, the advantages and disadvantages of various detection methods are analyzed.Finally, the model machine of5kW is completed, and the inverter works in grid pattern. It can satisfy the stability and high quality of grid. In addition to guarantee the normal work of the local critical load, the system can also convey extra electricity to the grid and the common local load in order to supply energy efficiently.The results of experiments show that the designed hardware circuits and the control strategies can meet the requirements. The system can work safely and steadily. In the end, the work is summarized, and the improvement points are put forward.