Research On Islanding Detection Based On Improved Active Frequency Drift Method

Due to the lack of global fossil energy and the intensification of environmental pollution,solar energy as a renewable green energy has triggered a research boom in countries around the world.People can use solar energy resources to solve the problem of energy shortage while reducing pollution to the environment,so that the global economy will develop towards a friendly development.There are many ways to use solar energy,among which the most effective way is photovoltaic power generation.The islanding effect is a kind of failure phenomenon that occurs when the photovoltaic system and the large power grid are connected to the grid.In severe cases,it will cause staff casualties and damage to electrical equipment.Therefore,islanding detection technology has important research value for the development of photovoltaic power generation systems.Firstly,based on the analysis of the development status of islanding detection technology of photovoltaic grid-connected systems at home and abroad,the detection principles of the grid-side detection method and grid-connected inverter-side detection method are analyzed in this thesis.The non-detection zone of islanding detection method is described in different coordinate systems.The advantages and disadvantages of various islanding detection methods and non-detection zone description methods are compared,and the active detection method is improved,and the non-detection zone of the improved detection method is described by the Q_f0×C_normcoordinate system.Secondly,in-depth study of the Fourier algorithm in the islanding frequency measurement algorithm and the windowed interpolation method is used to solve the problems of spectrum leakage and aliasing of the Fourier algorithm.The algorithm of adding four,five and six cosine window interpolation is analyzed and calculated in detail.According to the results of MATLAB simulation calculations,the interpolation accuracy of the six-cosine window interpolation algorithm is higher,and the calculation process is simple and the amount of calculation is small.Then,the detection principles and non-detection zone of traditional AFD method and AFDPF method are analyzed.Aiming at the traditional AFDPF detection method,the feedback coefficient is added on the basis of the definition of the initial chopping coefficient,which leads to the problems of excessively high total harmonic distortion of the system and the power quality of the power grid after repeated addition of disturbances.The function redefines the chopping coefficient to achieve the improvement of the traditional active frequency drift detection method.The improved method can effectively make up for the deficiencies of the traditional active frequency drift method.Finally,the MATLAB simulation platform is used to build an islanding detection simulation model of the photovoltaic grid-connected system in thesis.The AFDPF method and the improved active frequency drift method are separately analyzed.The results show that the improved active frequency drift detection method can effectively shorten the islanding detection time and reduces the total harmonic distortion rate of the system,reduces the impact on power quality,and reduces the non-detection zone while also improving the detection efficiency.