Research On Unplanned Islanding Detection Method For Distributed Grid-connected Power Generation System

The distributed grid-connected power generation system using solar energy is changing the architecture of the power system.How to deal with the unplanned island phenomenon in distributed grid-connected power generation system becomes a problem that must be considered when the distributed generation connected to the grid.An unplanned island is defined as a self-sufficient system between the distributed generation system and the local load due to the failure of the power grid.The generation of unplanned islanding can cause many hazards.Therefore,the detection of unplanned islanding is necessary when distributed power generation is connected to the grid.In view of this,the research on islanding detection method is meaningful.In this thesis,the active frequency drift islanding detection method and wavelet packet transform are studied in detail.Firstly,the basic principle of active frequency drift islanding detection method is described,and the relationship between chopping factor and performance evaluation parameters such as non-detection zone(NDZ)and total harmonic distortion(THD)in active frequency-shifted islanding detection is calculated,and the relationship between chopping factor and non-detected zone and total harmonic distortion rate is calculated.An improved active frequency-shift island detection algorithm is proposed.Secondly,aiming at the negative effect of active frequency drift islanding detection on power quality,wavelet packet transform that usually use in signal processing is applied to island detection,and the wavelet basis function suitable for island detection is selected according to the criterion of minimum description length.The key parameter of wavelet packet is defined,and its threshold is determined by analysis,which is used to judge the validity of island detection.A hybrid island detection method is proposed by combining wavelet packet transform with active frequency shift method.In this way,the reliability of grid-connected power generation and the rapidity of island detection are improved.In this thesis,the MATLAB/Simulink platform is used for simulation.Firstly,the photovoltaic power grid-connected system model is built,the control mode of photovoltaic inverter is determined,and the system simulation diagram of the active frequency-shift island detection and the islanding detection method with wavelet packet transform is given.Finally,theoretical verification is carried out.