Research On Islanding Detection Methods Of Single-phase Grid Connected Photovoltaic Systems

Grid-connected photovoltaic systems are increasingly applied due to the limitation of fossil fuels in terms of sustainability and pollution. For the interconnection of PV systems and utility grids, islanding detection is an indispensable feature for PV-systems and many algorithms have been developed in the past decade. These methods may be divided into two basic categories: passive methods and active ones. Among them frequency-shift techniques, phase-shift techniques and voltage-shift techniques are good choice because of high effectiveness and low cost. The thesis discussed some key issues of these three techniques:(1) None-detection zones (NDZ) are important index to evaluate the performance of anti-islanding methods. A proper mapping method is important as it set up relationship between NDZ and load characters, which are complex in real world. A new mapping method is proposed which can fully depict the performance of anti-islanding in a plain plot, which is compliance with IEEE standard certification.(2) The NDZs of frequency-shift techniques and phase-shift techniques are analysed in the thesis. Based on analysis the relationship between NDZ and algorithm parameters are decuced, which can be expressed as a mathematics inequation and provide range for algorithm parameters to gurantee anti-islanding protection. The inequation is simple and accurate, thus can be taken as a good guideline in algorithm parameters setting.(3) The influence of anti-islanding algorithm on inverter output current distortion is discussed. The current total harmonics distortion increased by anti-islanding is discussed in conjunction with anti-islanding algorithm optimization. It provides comparison between phase-shift techniques and frequency-shift techniques on anti-islanding efficiency as well as the adverse impact on power quality.(4) A mathematics model of voltage-shift technology is proposed. Based on it, the characteristics of voltage-shift technology are discussed. Key factor that lead to anti-islanding failure is sugguested and a mathematic inequation to gurantee anti-islanding protection is provided.(5) Islanding detection assessment of multi-inverter systems are developed, especially some new issues result from parallel operation. Possible solutions are suggested at the end of discussion.