Research On Power Quality Control Of Photovoltaic Grid-connected System

Affected by fossil energy consumption and environmental problems caused by the use of it,the energy structure is gradually transferred from traditional primary energy to renewable clean energy.As one of the main components of renewable energy,photovoltaic has the characteristics of clean and pollution-free,abundant energy storage,and has attracted the attention of China and the rest of the world.The development scale of photovoltaic grid-connected power generation has increased significantly.During the working process of the system,the PV array is affected by the illumination and temperature changes,and the inverter is realized by the power device itself,so that the power quality of the output of the system has DC bus voltage fluctuation and high output harmonic content of the grid-connected current.These problems will have a serious impact on the life and reliability of distribution networks and electrical equipment.Therefore,this thesis takes 10kW distributed photovoltaic grid-connected inverter system as the research object,and conducts specific research and realizes optimal control for its existing power quality problems.(1)The basic working principle of photovoltaic grid-connected systems and the standards for photovoltaic grid-connected at home and abroad are being studied.Analyze the standard requirements of national standards for power quality problems and the mechanism of voltage fluctuation and harmonic problems,and clarify the root causes and main solutions.(2)In order to solve the DC bus voltage fluctuation caused by the external environnent and the traditional maximum power point optimization algorithm,a maximum power point tracking method capable of suppressing steady-state oscillation is proposed.By suppressing the fluctuation of the DC bus voltage,the influence of the bus voltage on the power quality of the subsequent inverter connected to the grid is reduced.(3)Aiming at the problem of high harmonic content of grid-connected current caused by power device's own factors,a mathematical model of three-phase LCL-type PV inverter grid-connected system is established in two-phase stationary coordinate system.By analyzing the third-order under-damping characteristics of the LCL filter in the latter stage,an active damping suppression method based on capacitive current proportional feedback is designed,and a composite control strategy combining multiple-resonance quasi-PR control and repetitive control is used.The control strategy can effectively reduce the harmonic content of the grid output current of the system,improve the overall stability of the system,and optimize the current quality of the photovoltaic grid-connected system.(4)Established a lOkW photovoltaic grid-connected system experimental verification platform.The main hardware circuit and software program of the system were designed.The experimental method of the overall power quality control method was verified on the laboratory equipment.Compared with the output voltage and current under the traditional inverter control strategy,the accuracy of the power quality control method proposed in this paper and the optimization of the output power quality of the grid are proved.