Control Strategy Analysis And Improvement Of Pv Grid-connected Inverter Based On LCL Filter

As the application of solar photovoltaic power generation is the most extensive,its research technology has developed rapidly,and the related photovoltaic grid-connected control technology has become a research hotspot in this field.The photovoltaic power supply is connected to a large grid through a grid-connected inverter to form a photovoltaic grid-connected power generation system,and the quality of the grid-connected current in the system is mainly related to the output filter of the inverter and the corresponding control strategy.In order to improve power quality,the most common application is the use of LCL grid-connected inverters.The advantage is that it can greatly suppress the high-frequency harmonics of the grid-connected current.However,because the LCL filter is a third-order system,there are high frequencies.Problems such as resonance and difficulty in stable control.The subject takes the LCL inverter used in the three-phase photovoltaic grid-connected power generation system as the research object,takes the system stability,dynamic and high efficiency as the research purpose,and focuses on the improvement of the power conversion efficiency and output power quality of the entire system.The main research and discussion The content is as follows:(1)Establish mathematical models of L-type,LC-type and LCL-type grid-connected inverters separately,use MATLAB/simulink simulation platform to draw amplitude-frequency characteristic curves,and analyze their respective filtering performance through comparative analysis.The high-frequency harmoni cs can reduce the cost of the LCL inverter to a greater extent.(2)In order to solve the resonance phenomenon generated by the LCL inverter during the filtering process,on the basis of the resonance damping strategy,the passive damping increases the load and causes a large loss.Therefore,the capacitive current active damping strategy is adopted.While suppressing the resonance peak generated by the LCL filter,no additional active power loss has been added.(3)In order to improve the dynamic and steady-state performance of the system,an additional grid-connected current loop is added on the basis of the capacitive current active damping control based on the LCL inverter to form the capacitor current as the inner loop and the grid-connected current as Outer ring double ring grid connection system.Analyze the three commonly used fundamental current tracking control methods,introduce a quasi-PR controller that can obtain a very high gain in a certain frequency range,and calculate the quasi-PR according to the three indicators of amplitude gain,phase margin,and amplitude margin The parameters of the controller.Considering that the existing grid voltage feedforward strategy is not sufficient to meet the complexity of the system,a method of full feedforward of the grid voltage is proposed,and the effects of different terms are analyzed according to the derived function.The error curve diagram discusses the effectiveness of the grid voltage full feedforward control strategy to suppress grid background harmonics.(4)The long-distance transmission causes the line impedance to be inductive,which makes the power grid in a weak power grid state,thereby reducing the stability of the photovoltaic grid-connected system.Based on the existing research of the subject,the grid impedance model is added to improve the traditional weighted average current calculation method,and an improved control strategy is proposed.With the help of pole-zero cancellation,a new weighting coefficient composed of filter parameters is derived,so that grid-connected current is not affected by grid impedance.Comparing the simulation results before and after the improvement,the correctness and feasibility of the improved weighted average current control strategy are verified again.