| With the development of new energy sources such as photovoltaics and wind power,grid-connected inverters have brought great challenges to the stability of distribution networks and power quality.In practical applications,a large number of distributed inverter power sources and non-linear loads are connected to the public power grid.The actual power grid becomes a non-ideal voltage source,but presents a weak grid characteristic,which is mainly manifested by the presence of non-negligible time-varying line impedance and a large number of background Harmonics,affecting the power quality of the grid-connected inverter’s grid-connected current.In the weak grid environment,this paper takes three-phase LCL grid-connected inverter as the research object,mainly studies the influence of grid impedance change on the resonance suppression of LCL filter and PCC voltage(Point of Common Coupling)feedforward,and proposes corresponding solutions.In the weak grid environment,this paper reconstructs the mathematical model of the LCL grid-connected inverter,and derives its mathematical model in different coordinate systems.Since the LCL filter itself is a third-order system,the presence of resonance spikes at its resonance frequency will cause system instability.In response to this situation,the resonance peak can be suppressed by passive damping or active damping.By comparing and analyzing the advantages and disadvantages of commonly used passive damping methods and active damping methods,it is pointed out that the damping method based on notch filter has failed in weak power grid.In the weak grid environment,the resonance frequency of the LCL filter will change due to the change of the grid impedance,which will affect the stability of the grid-connected system.To solve this problem,this paper proposes a grid-side current delay feedback control method for LCL grid-connected inverter.This method uses a single-loop control structure on the grid side.By properly designing the relevant parameters of the LCL filter,an appropriate delay compensati on link is introduced on the grid-side current feedback branch,thereby expanding the stable bandwidth of the LCL grid-connected inverter,so that the resonance peaks formed by the LCL filter itself will not affect system stability.Under weak grid conditions,the grid voltage has time-varying line impedance and a large number of background harmonics.The traditional PCC(Point of Common Coupling)voltage feedforward will additionally introduce grid-side current,which will cause system stability to decrease.In view of this situation,this paper proposes a method of combined filtering feedforward,which introduces a specific harmonic order of notch groups to suppress background harmonics,selects a low-pass filter with a larger cut-off frequency,and improves system stability while considering dynamic response.Based on the combined filtering feed-forward control strategy,the single-loop control of the grid-side current feedback is adopted,the specific parameters of the LCL filter are reasonably designed,and the combined filtering is digitally designed to effectively ensure the stability of the system.In Matlab / Simulink environment,a simulation model is built,and then the system hardware and software are designed in detail,and an experimental platform for the grid-connected inverter using the TMS320F28335 digital signal processor as the control chip is established.On this platform,experiments such as start-up and variable load are performed.The experimental results show that the grid-side single-loop time-delayed feedback control enables the stable operation of the LCL grid-connected inverter;under weak grid conditions,the traditional method has failed,and the combined filter feed-forward control strategy is used to ensure stable system operation Improving the stability of the system also takes into account the dynamic response,verifying the effectiveness of the combined filter feedforward control strategy in this paper. |
PDF Full Text Request