Study On MPPT And Islanding Detection Method Of Grid-connected PV Generation System

The grid-connected photovoltaic(PV)generation system is taken as the studied object in this dissertation,and the general structure of the proposed PV generation system is given.The mathematical model is established and the stability performance is analyzed for the PV generation system.Furthermore,the islanding phenomenon detection and maximal power point tracking(MPPT)control of the PV generation system are studied deeply.The study contents of this dissertation are mainly as follows;The characteristic under different daily sunshine and surface temperature of the PV cells are summarized and analyzed,and the linearized model is established.The mathematical models of three kinds of converter system(Buck type,Boost type and Tri-state Boost type)with linearized PV cells model are established.The effects of different power converters on the output voltage stability of PV cells are systemically analyzed and compared at different work points in frequency domain.For the characteristic of the traditional Boost converter being difficult to maintain stable output in the wide voltage output range of PV cells,the Tri-state Boost converter is adopted to replace the traditional Boost converter,in order to improve the stability of the PV generation system and to widen the stable output voltage range of PV cells.Based on the causes and damages of the islanding phenomenon and the advantages and disadvantages of traditional islanding phenomenon detection methods,an islanding detection method on basis of automatic phase shift is proposed.This method only needs to change the initial phase angle of the output current and doesn't need to change its frequency to detect whether the grid occurs fault,and adds the judgment programs of the frequency change direction and the phase balance point.Comparing with traditional islanding detection methods,this method solves the detection failure problem caused by not jumping out of the phase balance point,and enhances the accuracy of grid fault detection.Based on the advantages and disadvantages of conventional maximal power tracking control methods of the solar power generation system,two kinds of maximal power tracking control methods on basis of single-variable current method,that is single-variable current perturbation tracking method and single-variable quadratic-curve extreme-value tracking method.The two methods only need to detect the output current variable of the solar PV panels,which greatly simplify the control process of maximal power tracking.Furthermore,the boundary range of duty cycle is limited,and the control mode of combining coarse adjustment with fine adjustment can accelerate the tracking speed,reduce the power loss caused by environmental change,and greatly improve the problem that the disturbance quantity of traditional perturbation methods oscillates round the maximal power point.The validity and feasibility of single-variable current perturbation tracking method and single-variable quadratic-curve extreme-value tracking method are verified by simulation and experiment respectively.The experiment platform of the solar power generation maximal power tracking control system is built,taking FPGA chip as the control core,adopting NI Compact RIO module,and programming through LabView,and the experiment studying and validating are carried out under the environmental conditions of daily sunshine changing and stabilizing.