Research On Voltage Fluctuation And Prevention Strategies Of Distributed Photovoltaic PCC

With the ever-increasing demand for energy and the increasing environmental problems in the world today,renewable energy grid-connected technologies such as wind power generation and solar power generation have received increasing attention and have become an important component of the sustainable energy development strategy.In part,solar energy,as a kind of widely distributed and inexhaustible new energy source,has attracted the attention of all countries in the world.Current solar grid-connected power generation technologies mainly include distributed generation and centralized photovoltaic power generation.With the increasing problem of abandoning solar energy,the development of centralized photovoltaic power plants is being restricted.Distributed photovoltaic power generation has caused more and more attention.In addition to this,in practical projects,distributed photovoltaic grid-connected systems are affected by factors such as the environent and the distribution network structure,resulting in voltage fluctuations at the grid-connected point.The occurrence of fluctuations can further lead to system instability and threaten the safe and stable operation of the system.This paper focuses on the problem of voltage fluctuations in distributed photovoltaic grid-connected systems.First,it establishes a basic.photovoltaic grid-connected model.From a mathematical point of view,it analyzes the causes of grid-connected voltage fluctuations caused by photovoltaic grid-connection.Secondly,the working principle of the three-phase photovoltaic grid-connected inverter is analyzed,and the mathematical model of the inverter in the dq coordinate system is established.On this basis,the decoupling control of the active and reactive power of the inverter output is achieved through the dq transformation.After that,the principle of the reactive power output of the inverter and the constraints of the reactive power output of the photovoltaic system are analyzed.The influence of the change of the output active power of the photovoltaic system on the voltage of the grid-connected point and the supporting role of the grid-connected point voltage from a certain amount of reactive power of the photovoltaic system are discussed.Based on that,a new type of reactive power compensation topology HALFMERS(Half Magnetic Energy Recovering Switch)is introduced.From the topology of HALFMERS,a half-bridge static reactive power compensation device,SVC-HALFMERS(Static Var Compensator-Half Magnetic Energy Recovering Switch),is proposed.That can make up the voltage drop caused by insufficient reactive output of the PV inverter.Finally,a strategy based on photovoltaic inverter and SVC-HALFMERS combined control to suppress voltage fluctuation is proposed.The effectiveness of the strategy is verified by theoretical analysis and RT-LAB experiments.The main research contents include:(l)Aiming at the problem of grid-connected voltage fluctuations in distributed photovoltaic systems,a distributed photovoltaic grid-connected model was established to study the mechanism of voltage fluctuations at grid-connected points caused by distributed photovoltaic grid-connected systems,and based on this,a reactive power system based on photovoltaic inverter was proposed.The control strategy,through the double closed-loop active and reactive decoupling control of the photovoltaic grid-connected inverter,regulates the output reactive power of the photovoltaic grid-connected inverter,and stabilizes the grid-connected point voltage of the photovoltaic system.(2)Aiming at the problem of output reactive power limit of photovoltaic systems,a half-bridge static reactive power compensation device(SVC-HALFMERS)is proposed,which is determined by studying the topology and control strategy of HALFMERS.The SVC-HALFMERS reactive power compensation and reactive power compensation characteristics were finally verified through experiments.(3)Aiming at the difference of PV grid-connected voltage fluctuations,a strategy based on PV inverter and SVC-HALFMERS combined control to suppress voltage fluctuation is proposed.Firstly,analyze the situation of voltage fluctuation in different situations according to different situations,conduct different reactive power compensation strategies for different voltage drop conditions at the grid connection point,and coordinate the reactive output capability of the PV inverter with the reactive output capability of SVC-HALFMERS.Control and jointly stabilize photovoltaic grid point voltage.Through theoretical analysis,simulation verification and RT-LAB experiments,the voltage capability of the photovoltaic grid-connected grid points was verified.