Research On Low Voltage Ride Through Method Of Photovoltaic Grid-connected

With the development of high penetration rate of new energy power generation,the grid-connected capacity of photovoltaic power generation is increasing.Once the large-capacity grid-connected photovoltaic power generation system is offline,it will have a great impact on the grid and affect the quality of power.Will lead to the collapse of the power grid.Therefore,the current domestic and foreign photovoltaic grid-connected standards require that the photovoltaic power generation system must have a certain low voltage ride through capability.Therefore,the thesis has carried out research on the low voltage ride through control method of photovoltaic grid-connected systems.The specific research contents are as follows:Firstly,the composition of the photovoltaic grid-connected system and the topological structure of the main circuit are determined,and the mathematical model of the photovoltaic array is established.Based on the determined main circuit topological structure,the working principle of the pre-stage booster circuit and the variable step disturbance observation method to achieve MPPT are studied.The method,the working principle of the back-stage inverter circuit and its mathematical model provide a theoretical basis for building an overall photovoltaic power generation system.Secondly,the cause of voltage sag and its influence on the photovoltaic system are analyzed,and the detection method of voltage sag and the discrimination of voltage sag depth are studied,and a second-order generalized integrator is used to realize voltage sag detection and realize the separation and detection of positive and negative sequence components.Thirdly,it analyzes the vector-oriented control strategy based on the grid voltage,and studies the voltage-oriented double closed-loop control method based on fuzzy PI control.Fuzzy PI control is used in the current inner loop and the voltage outer loop,respectively,and the photovoltaic grid-connected low voltage ride-through control is designed.The solution includes reactive power control during the low voltage ride-through process and active power control during the recovery process.When a voltage drop is detected,a given current command is adopted to adjust the active and reactive power separately,reducing the input of active power to prevent inversion When the inverter is over-current and disconnected from the grid,the input of reactive power is increased to support the restoration of the grid voltage.At the same time,considering the frequency stability problem caused by the low voltage ride-through process,a method is designed to adjust the active current recovery rate during the low voltage recovery period through the deviation signal of the grid frequency.Finally,in the simulation software,an overall model of the two-stage photovoltaic power generation system was built to simulate the voltage drop in different degrees.It is verified that the method of improving the inverter control strategy in this paper can ensure that the inverter does not run off the grid when the voltage drops,and can provide a certain amount of reactive power to the system to support the recovery of the voltage.In the active power recovery period,the grid frequency is introduced into the low voltage ride-through control,which improves the problem of the frequency drop caused by the low voltage ride-through.