The Research On Islanding Detection For Grid-connected Distributed Generation

Due to the increasingly serious energy crisis and environmental pollution,the distributed power generation technology based on renewable energy development has obtained a rapid development.The islanding detection is an essential function for the grid-connected DG system.On the basis of the information contained in the electrical parameters of the DG system's PCC,this paper did an in-depth study on the passive islanding detection and got some main achievements as following:Firstly,the paper established the mathematical model of the grid-connected inverters and developed the PQ decoupled control strategy.The designed diagram of the whole control system was given in the paper.A simulation model based on three-phase inverter was built using the MATLAB/Simulink.The paper did the islanding simulation in many different situations,and deeply analyzed the variation characteristics of the electrical parameters of the DG system's PCC.The work above provided a research foundation for finding an effective islanding detection method.Secondly,according to the changes' characters of the harmonic signal of PCC before and after the islanding state,this paper proposed an islanding detection method based on Double Tree Complex Wavelet Transform(DTCWT)and an improved Support Vector Machine.A 50 Hz notch filter was designed to extract the harmonic components in the voltages and currents.Multi-scale decomposition was done on the harmonic signals using DTCWT,and characteristic vectors were built from the aspects of energy,singular value and the statistical characteristics.Based on the selection of support vector and genetic algorithm,the paper proposed an improved fuzzy support vector machine algorithm to shorten the training time and improve the classification accuracy.Putting the characteristic vectors before and after the islanding state into the improved SVM for training and recognition,the final simulation results which were done in different situations showed the validity of the new islanding detection method.Thirdly,based on the study of the high frequency harmonics from the inverter outputs,an islanding detection method in multi DGs system containing nonlinear load based on the measurement of characteristic harmonic impedance was proposed.According to the harmonic spectrum of the inverter output voltage and theoretical research,the paper selected the harmonic which is near the switching frequency as the characteristic harmonic.The harmonic impedance model of the grid-connected DG system was established and the changing characteristic of the harmonic impedance before and after the islanding state was measured by simulation in Matlab.The improvement was studied in multi DGs system containing nonlinear load on the strategy which takes the increase multiples of the characteristic harmonic impedance as the islanding detection criterion.The simulation results verified the effectiveness of the new method.Finally,according to the detailed analysis of the performance of the passive islanding detection methods,the paper developed an passive islanding detection device.It takes the 32-bit ARM kernel microcontrollers as the microprocessor,and uses the Goertzel algorithm to calculate the electrical parameters because of its less computational complexity.The device has the function of passive islanding detection,and the communication function with Ethernet,RS232 and RS485.On the basis of the traditional way of human-computer interaction which uses the LCD and buttons,the device increases a new operation method with Wi-Fi and the embedded webs.The corresponding technical realization method is given in the paper.Experimental results showed that the device has good capability of islanding detection,and further verification can be done in the actual islanding scene.