Research On Low Voltage Ride Throug And Island Eetection Fechnology Of Photovoltaic Grid-Connected System

With the development of social economy,the contradiction between the decrease of traditional fossil energy and people’s energy demand has become increasingly prominent.In order to alleviate this contradiction,the development and use of clean energy has gradually become a research hotspot.Photovoltaic power generation is one of them.In recent years,photovoltaic power generation has been rapidly developed due to its simple power generation process and inexhaustible.With the continuous improvement of photovoltaic penetration rate,the impact of photovoltaic grid-connected on the power grid can not be ignored.This paper mainly studies Low Voltage Ride-Through(LVRT)technology and islanding detection technology in photovoltaic grid-connected systems to improve the reliability and safety of system operation.The main research contents of this paper are as follows:1.Research on modeling and grid-connected control strategy of photovoltaic power generation system.The working mechanism of photovoltaic cells and common maximum power tracking control methods are analyzed.Starting from the topological structure of photovoltaic power generation system,the mathematical model of photovoltaic inverter was established under the abc,ap and dq coordinate systems.The vector control strategy based on grid voltage orientation was studied,and designed the voltage outer loop and current inner loop control parameters.2.Set up a simulation model of photovoltaic grid-connected power generation system.A simulation model of two-stage photovoltaic grid-connected power generation system was built in MATLAB/simulink software.The front stage uses the disturbance observation method,and the latter stage adopts the voltage orientation vector control strategy.The simulation results show that the model meets the requirements of grid-connected technology and lays a foundation for the further research.3.Research on low voltage ride-through control strategy.By analyzing the influence of grid voltage drop on photovoltaic grid-connected system,an LVRT control strategy based on DC side Crowbar circuit and inverter reactive compensation was designed.The simulation results show that during the voltage drop,the control strategy stabilizes the DC side voltage while limiting the inverter overcurrent,and outputs reactive power to support the grid voltage recovery,which improves the low voltage ride-through capability of the PV grid-connected system.4.Research on island detection technology.The mechanism of the islanding effect in photovoltaic grid-connected power generation system and the testing standards at home and abroad are expounded.The common active/passive island detection methods and their advantages and disadvantages were studied,and the Non-Detection Zone of active island detection method was analyzed in Qfox Cnorm coordinate system.5.A new low distortion active island detection method was proposed.Aiming at the problem that the harmonics generated by the active frequency offset method(AFD)have great influence on the power quality of the grid-connected power,a low-distortion islanding detection method different from the traditional AFD scheme was proposed.The simulation results show that the method can effectively reduce the harmonic distortion rate and improve the detection speed,which significantly improves the output power quality of the photovoltaic system.