Research On Grid-connected Current Optimization Of LCL Grid-connected Inverter In Weak Grid

In recent years,with the rapid development of renewable energy such as solar energy and wind energy,new energy grid-connected power generation technology has received more and more attention.The grid-connected inverter is the core of the grid-connected power system,and its performance directly affects the quality of the grid-connected current.When the grid-connected inverter is connected to the grid,the current control mode is essentially the voltage source input and current source output of the inverter.At present,there are two major problems with grid-connected inverters: one is that when the power grid is performing long-distance transmission,the inductive reactance in the power grid cannot be ignored,which will make the power grid change from a strong grid to a weak grid;the second is grid-connected inverters.The device is sensitive to the environment and prone to fluctuations.Based on the above problems,this paper takes the LCL grid-connected inverter system under the weak grid as the research object,analyzes and studies three factors that affect the quality of the grid-connected current,and proposes improvement measures for the three influencing factors respectively.The LCL grid-connected inverter system is theoretically analyzed and verified by simulation.The LCL grid-connected inverter system under digital control will generate inherent delay.In order to eliminate the influence of the delay under digital control on the system,a control strategy based on the combination of double-current closed-loop real-time sampling and zero-pole compensation is proposed for the LCL grid-connected inverter model under weak grid.Firstly,the LCL grid-connected inverter model under weak grid is established,and the influence of the capacitive current delay generated by the LCL grid-connected inverter under the traditional capacitive current feedback control on the system robustness is analyzed.Secondly,the influence of the delay of the grid-connected current loop on the dynamic performance of the grid-connected system is analyzed.After that,real-time sampling is applied to the capacitive current loop and grid-connected current loop and zero-pole compensation is added to the forward path.Finally,the robustness and stability of the improved LCL grid-connected inverter system are analyzed.The phase locked loop also plays an important role in the LCL grid-connected inverter system.Because the PI controller in the second-order generalized integrator phase-locked loop(SOGI-PLL)cannot follow the AC component without static error,it will cause a large system steady-state error.In addition,the fixed parameters of the controller will greatly reduce the dynamic performance of the system.Aiming at the above problems,a phase-locked loop design combining fuzzy controller and PDFI controller is proposed.First,the PDFI controller is used to replace the PI controller,and the fuzzy adaptive control is used to adjust the control parameters of the PDFI to adapt to the changes of the system parameters.In addition,in order to make SOGI track the fundamental frequency well,the resonant frequency of SOGI is locked to the fundamental frequency.In addition to the defects and system delay of the traditional phase-locked loop,the harmonics of the grid-connected current will also have a greater impact on the grid-connected current quality of the LCL grid-connected inverter system.In order to reduce the low-order harmonics of the LCL grid-connected inverter system,a control strategy combining resonant feedforward and repetitive proportional controller is proposed.Firstly,impedance analysis method is used to analyze and compare the stability of LCL grid-connected inverter system under traditional proportional feedforward and resonant feedforward control.Then,the stable conditions of the repetitive proportional control system are analyzed,and the parameters of the repetitive proportional controller are designed according to the stable conditions.Based on the above three improvement measures,the overall model of the LCL grid-connected inverter system is established,and the small-signal model of the phase-locked loop is established.After that,the stability analysis of the overall LCL grid-connected inverter system is carried out using the impedance analysis method.Finally,the improved system is simulated and verified using MATLAB/SIMULINK.The simulation results show the effectiveness of the proposed control strategy.