Research On Photovoltaic-storage System Under The Condition Of Weak Grid

With the continuous progress of technology,photovoltaic power generation is becoming more and more widely used.Because of the obvious advantage of solar energy relative to the traditional fossil energy,China and the whole world are vigorously developing the photovoltaic industry.However,photovoltaic power generation system is greatly influenced by light,temperature and other environmental conditions,which makes its output random and unsustainable.This paper mainly focuses on the instability of the photovoltaic system,mainly from conducts the research from three aspects: the maximum power point tracking(MPPT)method of photovoltaic power generation system under partial shadow condition,the tracking plan output of photovoltaic-storage power generation system and the resonance analysis of photovoltaic power generation system connected to weak grid system.(1)When the photovoltaic array is subjected to uneven illumination,the output characteristic curve of the array will change,and the U-P curve is multi peak.In this paper,a photovoltaic simulator is used to simulate the output characteristic curve of the photovoltaic array under the condition of partial shadow,and the traditional MPPT is prone to fall into the local maximum power point,which affects the output efficiency of the photovoltaic array.In this paper,the improved particle swarm optimization(PSO)algorithm is used as the MPPT method.The learning factor and weight coefficient in the algorithm vary with time,which makes the tracking result more accurate and faster.The RT-LAB real-time simulation platform is used to verify the superiority of this method.(2)The PV system needs to follow different planning forces at different time periods,but the conditions of light and temperature change are random.Therefore,the energy storage device is equipped in this paper,which is used to cooperate with the photovoltaic system to carry out peak shaving and valley filling,achieving constant power output,tracking planned output and improving schedulability of photovoltaic system through two-way DC/DC.(3)When the multi PV-storage system is connected in parallel to the weak grid,the interaction between the inverters becomes obvious because the impedance of the grid can not be ignored.These factors will cause the system resonance phenomenon.In this paper,the generation mechanism of system resonance is analyzed,and the resonant frequency of multi PV-storage grid connected system is obtained by modal analysis and transfer function analysis.According to the size of the participation factor,the influence range of the system resonance is determined.Through the modal sensitivity analysis,the participation degree of the system components is further obtained,and the change of the resonance is analyzed with the change of the number of inverters and the change of the transmission distance.The series virtual negative impedance is introduced in the control method to counteract the influence of the impedance of the grid,and the suppression of the system resonance is realized,and the simulation model is verified.