Research On Parallel Control For Multi-inverter In Micro-grid

So far, the phenomenon of fossil energy shortage and environmental pollution is becoming more serious day by day. The micro-grid technology with its unique advantages receives extensive attention of scholars both at home and abroad. In the micro-grid, Distributed generation units use inverters as power transformation interface to provide power for loads. The technology of inverter control is one of the most critical technologies in micro grid. Micro-grid inverter is mostly in the form of parallel to improve system capacity and reliability. There are many problems in micro-grid inverters parallel operation. This thesis studies voltage compensation and circulation suppression for parallel control for multi-inverter in low-voltage Micro-grid when the transmission line is longer.Firstly, this thesis introduces the background and the significance of research in micro-grid, elaborates the main functions of the micro-grid inverter, establishes mathematical modeling and analyses modulating methods of micro-grid inverter, then, designs power filter to ensure the quality of the output voltage. Analysis control methods of micro-grid inverter in detail, According to the comparative analysis, quasi PR controller is chosen as the voltage outer loop, proportional controller as the current inner loop to ensure the performance of the system. On this basis, this thesis introduces main existed problems and the solution used about the parallel inverter commonly.Secondly, in the low-voltage micro-grid, the transmission lines impedance is mostly in resistance, which causes the limitations of traditional droop control method, the traditional virtual impedance control method can optimize the ratio of R/L, however, when the transmission line is longer, a large virtual impedance is needed to ensure that traditional droop control method in the application of the micro-grid. Aimed at the shortcoming of traditional virtual impedance, this thesis adopts an improved virtual impedance control method, the configuration of virtual impedance is no longer dependent on line parameters, and can guarantee the quality of inverter output voltage. Micro-grid parallel inverter using droop control to ensure the output power distributed on average, but at the same time, this will make the inverter output voltage frequency and amplitude offset, and the load is sensitive to frequency. Commonly based on this, this thesis adopts a distributed frequency modulation control method, which can ensure the inverter output frequency for its rating. The correctness of the improved virtual impedance method is validated by Matlab simulation.Finally, in this thesis, a single-phase inverter system with hardware and software is designed.The experimental platform is structured in the center of TMS320F2812DSP, utilizing the C language designs software program by the compiler software CCS, and then gets related experimental data and waveforms.