Study On Islanding Detection Strategy Of Distributed Generation System

The Distributed Generation(DG) technology has been developed rapidly in recent years. Along with the increasing capacity of DG system, the islanding issue has received more and more attention. Islanding effect can cause the devices to breakdown and may lead to live line work of workers under no protection measures, thus brings potential safety risks. The effective, precise and rapid detection of islanding as well as the adoption of corresponding protection measures to avoid unnecessary loss has attracted much attention to technical staff of the power system. Since each of the existing islanding detection method has its own shortcomings in application, it is of paramount importance in practice to give a deeper investigation into the islanding effect.The thesis introduces the concept of DG system, analyzes the significance of the islanding detection, and describes the international standard of the islanding detection. By analyzing the power flow of the distributed photovoltaic grid-connected generation system, the conditions of islanding effect is given and the mechanism of the islanding is discussed. The fundamental problem of islanding detection is illustrated and all kinds of islanding detection methods are classified and a brief introduction to remote islanding detection method is also included. Besides, the principles and performance of local islanding detection method are analyzed. The effectiveness of islanding detection method is evaluated and the criterion of non-detection zone is analyzed with emphasis on the deduction of the sizes and locations of non-detection zones of various frequency-drift based islanding detection methods. By building a simulation model of photovoltaic grid-connected generation system in MATLAB, the active frequency perturbation scheme is simulated. The consistency between non-detection zones of that kind of methods and the ones deduced by formulas is verified by comparing the detection results and analyzing the simulated non-detection zones.Considering the fact that conventional Active Frequency Drift(AFD) scheme has strong influence on the power quality of the system, a new positive feedback reactive power disturbance islanding detection method is proposed in this thesis. In this method, by introducing the frequency deviation in photovoltaic power generation system into the reactive power disturbance, positive feedback reactive power disturbance can be formed, which will make the voltage frequency shift rapidly to exceed the protection threshold. In this way, precise and fast islanding detection can be achieved.The research shows that the positive feedback reactive power disturbance islanding detection method has good effectiveness and precision, and at the same time has the advantage of little non-detection zone and little influence on the quality of power, it can provide a new reference method for islanding detection.