Reseach On Resonance Of Grid-connected Inverters In Low-voltage Micro-grid

Distributed power generation(DG) system has been rapidly developed in recent years, but it also has its own disadvantages, such as power quality and system reliability decrease when DG system is connected to the main grid. In order to overcome these shortages and take advantages of DG units, the concept of Micro-grid is proposed. As a very advanced research field, Micro-grid technology will be greatly developed and researched. By now, the development of Micro-grid is still in its infancy and there are some problems we should concern about, such as power quality, the modeling of paralleled grid-connected inverters and the interaction between each inverter. Accordingly, the analysis in this paper is mainly focused on the resonance of single inverter, the modeling of paralleled grid-connected inverters and the resonance of the paralleled system.Firstly, the topology and the mathematical model based on synchronous rotating reference frame of the grid-connected inverter are analyzed, and then make a brief analysis about LCL filter. On this basis, the inner current loop is presented for the further study in the following chapters.Secondly, a depth research about resonance suppression strategy in the LCL based grid-connected inverter is done both in s-domain and z-domain. For the passive damping strategy, impact on the high frequency characteristics and low frequency characteristics of the grid-connected system with damping resistance in the different position is analyzed; moreover, the influence on system stability and filtering performance with different resistance values is studied also. For the active damping strategy, the basic active damping methods of filter capacitor current feedback method and filter capacitor voltage feedback method are studied in s-domain and z-domain with different ratios of resonance frequency and control frequency. Furthermore, the influence of grid impedance is analyzed either.Thirdly, Norton equivalent model of the three-phase grid-connected inverter is presented. On this basis, the same method is used in the modeling of the paralleled grid-connected inverter system in the Micro-grid, thus the Norton equivalent model of paralleled-system is obtained. Then internal resonance of each inverter, parallel resonance among inverters and series resonance among the micro-grid and main grid in the paralleled system is done in z-domain, in addition the effectiveness of active damping implemented in the parallel system is analyzed.Finally, the simulations are performed with the software Matlab/simulink. The models of single grid-connected inverter and paralleled grid-connected inverters have been built. Simulation results are analyzed so as to validate the theoretical study of previous sections, and the result proves that the theoretical analysis is right.