Study On The Impact Of A Wek Grid To A Microgrid Operation

Increased penetration of the distributed generation highlighting the high cost of stand-alone access and control hard issues of their own existence and connecting with the grid only in the form of load limits the ability of distributed generation’s power generation capacity. With the development of power electronic interface technology and modern control theory, the concept of the microgrid emerged. Microgrid integrates the advantages of distributed generation, while addressing its shortcomings. Microgrid has two modes of operation, islanding operation mode and connected grid operation mode, this paper is to study the connected grid operation mode.In this paper, MATLAB / SIMULINK software is used to establish a micro grid model in connected grid operation mode with the photovoltaic cells as source. At present, many such studies are considered the grid as the ideal grid,but in the weak grid conditions, the model and study under this ideal grid have some flaws. Based on the model of microgrid in the connected grid mode under the ideal grid, establish the model of microgrid in the connected grid mode under the weak grid and analysis the effect of grid impedance on the operating characteristics of the microgrid.The key reason for the stable operation of microgrid system is the reasonable control of grid connected inverter. In this paper the inverter model is constructed using LCL filter. Because the core status of inverter in microgrid system, focuses on the analysis of the influence of the grid impedance on the inverter, including the effects for the resonant characteristics of the LCL filter and the effects for the dynamic response and stability of the system. Because of the existence of the grid impedance, the number of parallel inverter will also affect the performance of the microgrid system.The impedance of the power grid has great influence on the microgrid system operating characteristics, and sometimes may cause system instability, so when the grid impedance changes or the number of the parallel grid connected inverter changes, need to modeling the system to analyze the performance again. But frequent and uncertainty of the grid impedance variation make the work has a great workload and very hard to finish. This paper introduces a stability criterion based on impedance which can be used to judge the stability of microgrid system in weak grid conditions, and do not need to repeat modeling.Finally, according to the stability criterion obtained above get the theoretical basis of improving the ability of microgrid to adapt to the weak grid and the method of realization is found, namely virtual impedance method, and then set the corresponding simulation model, the results and the theoretical analysis results have been showed that the method is feasible.