| State grid Shenyang power electric supply company is accelerating the construction of user rooftop PV project,in order to create convenient conditions for distributed power access to the power grid,shorten the grid connection time,and improve the efficiency and efficiency of distributed power construction.Ensure the full utilization of distributed power supply and promote the harmonious unification of distributed power supply and grid development.Therefore,the structure of Distributed Photovoltaic(PV)generation system,which includes the former stage boost converter and the later stage three-phase inverter,is studied in this thesis.Moreover,the photovoltaic cell,Boost converter and three-phase inverter are modeled mathematically.In this thesis,the optimization of maximum power point tracking(MPPT)algorithm,the control strategy of three-phase grid connected inverter and control strategy of grid-connected and island operation Switching are deeply studied.First,this thesis studies three kinds of traditional MPPT algorithms:constant voltage method,perturbation observation method and conductance increment method.The traditional MPPT algorithm of variable step size has a poor applicability in special environment because the base disturbance step size and disturbance factor are greatly affected by environmental factors,which leads to the poor tracking effect of maximum power.In this thesis,an improved MPPT algorithm with variable step length is adopted.The advantage of MPPT algorithm with variable step length is that large disturbance step length can be set at the initial stage of the photovoltaic system tracking to accelerate the dynamic response speed of the system,so that the photovoltaic system can reach the maximum power point quickly.When the system reaches the maximum power point,the disturbance step of the system is reduced to avoid the shock process of the photovoltaic system at the maximum power point,reduce the unnecessary power loss of the system,and improve the tracking accuracy of the system.Secondly,the traditional double-loop grid-connected control algorithm of voltage and current is studied in this thesis.However,the dynamic process of transient regulation of dc bus voltage is slow,and large oscillation will affect the stability of the system.For this reason,an inverter control algorithm based on power feed forward is adopted in this thesis and the stability of the system and the dynamic response speed are improved.Finally,Off-grid inverter adopts droop control based three-loop control,which can effectively control the voltage and frequency characteristics of the load.In this section,the principle of droop control,p-f droop curve and q-u droop curve are improved to ensure the stable voltage and frequency range of the inverter.In addition,in order to realize the inverter in the network operation mode and the parallel operation mode smooth switch,design the voltage regulator and frequency regulator,to ensure that the start time of interconnection,quickly adjust the inverter output voltage and frequency,make its keep synchronous with the grid voltage and frequency,thus reducing the grid instantaneous for the impact of the inverter. |
