Research On Islanding Detection Methods For Grid-connected Distributed Generation

Distributed generation system has been more and more widely merged into the grid,which greatly increases the probability of islanding effect.Island effect will not only damage grid-connected inverter and load equipment but also threaten the life of the electric power maintenance personnel when the situation is serious.How to detect the island quickly and accurately is a necessary ability of grid-connected inverter.However,existing islanding detection methods are restrictive in application,and dilution effect will occur in multi-inverter grid-connected system.Therefore,intensive study of the islanding detection methods still has very important reality significance.The thesis introduces and analyzes the basic principle of islanding detection and the commonly used islanding detection methods,and describes the standard of islanding detection,test circuit and non-detection zone based on the causes and harms of the islanding effect.The thesis details the working characteristics of the commonly used islanding detection methods in a single inverter system and the multi-inverter system,simulation model is established by SIMULINK according to the test standard of IEEE Std.929,and the detection performance of the islanding detection method is effectively evaluated in aspect of the non-detection zone,detection speed and power quality.Since active frequency drifting(AFD)method is one-way disturbance,and exist a big non-detection zone due to the properties of the load,this paper proposes a low total harmonic distortion fast active frequency drift islanding detection method.The relationship of non-detection zone and the load is detailed deduced,after obtaining the positive feedback coefficient that satisfy the islanding detection standard,fuzzy dynamic optimization on positive feedback coefficient is imposed.SIMULINK simulation model is established in the situation when islanding detection is most likely to fail.The simulation results show that the proposed method has the advantages of low total harmonic distortion in the output current of the inverter when the grid is normal(the frequency fluctuation of the grid frequency is within the allowable range),the output current of the inverter is the minimum total harmonic distortion,which affect the quality of the electric power grid in the minimum,and the proposed method can quickly and accurately detect the island after power off without non-detection zone.Generally,it is hard to strike a balance between islanding detection speed and power quality of grid-connected system in slip mode frequency shift(SMS)method when selecting the disturbance quantity.Based on the fact above,an improved SMS method was proposed.Through the analysis of relationship between disturbance of SMS method and voltage frequency variation at point of common coupling(PCC),a new disturbance expression is deduced with Taylor formula,and fuzzy optimization of disturbance factor is carried out according to frequency variation and variation rate of PCC voltage.Theoretical analysis and simulation results show that the proposed method can accelerate the process of islanding detection,reduce the non-detection zone and the influence on power quality is reduced when distributed generation system is connected to the grid compared with traditional SMS method.In addition,the proposed method is more robust on the frequency detection error and the line impedance.