Research On Control Strategy Combining Repetitive And Quasi-PR Control For LCL-type Grid-connected Inverter

The distributed power generation system based on renewable energy such as geothermal energy,solar energy,and wind energy has become a hot research topic to solve the global energy crisis.In the distributed generation grid-connected system,the grid-connected inverter is an important power electronic device that transmits electrical energy to the public power grid,and its output power must meet the grid-connected power quality requirements.The presence of harmonics can cause loss of power electronics and capacitor heating and reduce efficiency of the motor and transformer,and interference with communication control systems.If the output current of the power generation unit is not filtered and directly connected to the grid,it will cause high-harmonic current to be injected into the grid and cause harmonic pollution.At present,research on grid-connected inverters focuses on current control strategies.In this thesis,a compound control method with both repetitive and quasi-pr control for grid-connected inverter is proposed to achieve zero steady stateerror of reference current,reduce the THD(Total Harmonic Distortion),and improve system dynamics and steady state performance.The steps of this thesis are as follows:(1)A state space model of LCL-type grid-connected inverter is established in the three-phase stationary coordinate system,and the grid-connected current is controlled by the Clark transform in the stationary frame.Topology and mathematical model of LCL grid-connected inverter are established and analyzed it in ?? coordinate system.(2)On the basis of analyzing and comparing different passive damping modes,a passive damping method with good damping effect,less damping loss and simple structure is adopted to eliminate the resonance peak appearing inside the LCL filter for the phenomenon that LCL internal resonance occurs in this topology.Its parameters are designed according to the parameter constraints of the LCL filter.(3)Firstly,the main control technology of single-phase grid-connected inverter is analyzed.Secondly,the characteristics of quasi-proportional resonance control and the influence of its controller parameters on the controller are studied.Finally,the principle of repetitive control and its compensation link are introduced in detail.A compound control method with both repetitive and quasi-pr control for grid-connected inverter is proposed combining the quasi-proportional resonance dynamic performance and repeated control characteristics of suppressing periodic disturbance,which uses the current of the grid-side current feedback.A mathematical model of the inverter is established based on the composite control,the conditions of the system stability is analyzed,and the compensation link parameters that satisfy the stable conditions is designed.(4)The simulation model is built in Matlab to verify the dynamic performance and steady state performance of the composite control strategy.Compared with the traditional control strategy,the results show that the composite control strategy can effectively filter out the harmonics,improve the power quality of the grid,and to a greater extent reduce the harmonic content of the grid current.