Research On Voltage Quality Of Photovoltaic Grid-connected System Based On Virtual Synchronous Generator

With the depletion of fossil energy and the aggravation of environmental pollution,new energy power generation has become the focus of the world.Photovoltaic power generation as a "upstart" in the new energy generation,its power generation in the total proportion of power generation is increasing day by day.However,the large-scale use of photovoltaic energy has led to the transformation of grid scale from single form and centralized form of power supply to multiple forms and distributed forms of power supply,further influencing the stability,security and reliability of the grid.Above all,photovoltaic energy has its own characteristics of intermittence,volatility and randomness,which makes the grid voltage fluctuate and greatly affects the voltage quality of distribution network.Then,the grid connected photovoltaic energy needs to borrow power electronic equipment such as inverters,which enables the grid voltage quality to show new characteristics.Finally,because of the diversity and complexity of the load types in the distribution network,it tends to affect the power quality of photovoltaic grid connection.In view of the above problems,this thesis studies and analyzes the voltage quality of photovoltaic energy incorporated into the active distribution network based on the virtual synchronous generator(VSG).The main research work is as follows:First of all,this thesis studies the photovoltaic grid-connected inverter and analyzes the traditional inverter control mode.It is found that the traditional control mode can not provide sufficient inertia and damping for the grid.Therefore,the second-order model of synchronous generator(SG)is introduced into the control mode of inverter to establish the power frequency,excitation module and overall control model of VSG.Secondly,the photovoltaic grid-connected system is established based on virtual synchronous generator.This thesis analyzes the influence of external temperature and external solar intensity on the photovoltaic cells by using the method of variable control,and designs the DC / DC variable circuit,the maximum tracking system and the inverter mathematical model.In addition,this thesis aims to discern the voltage deviation and impulse current caused by the photovoltaic direct grid-connected system based on virtual synchronous generator,and put forward the smooth grid-connected control strategy of virtual synchronous generator based on virtual impedance.The effectiveness of the control strategy is verified by simulation.Finally,the voltage quality problem caused by the failure of photovoltaic grid connected system is solved,and a voltage adaptive compensation controller based on virtual synchronous generator is proposed.The voltage and current separation controller is designed,and the active and reactive power amplifiers of the converter are controlled independently according to the controller.The voltage and current inner loop controller is established to track the output voltage sequence and the instantaneous value of the filter inductor current accurately.The thesis elaborates the influence of three-phase asymmetric fault and three-phase symmetrical fault on the parallel network,and establishes the reactive power compensation control strategy according to the mathematical model.The simulation results show that the voltage quality can be improved by the proposed reactive compensation control strategy.