Research On Control Technology Of LCL Grid-connected Inverter For Weak Grid

In recent years,our country began to vigorously develop renewable energy,dedicated to a formation of a healthy and environmentally friendly way of using electricity.Grid-connected inverters,as energy conversion interfaces between renewable energy power generation units and the grid,are used to convert DC sources into high quality AC power for feeding into the grid.The quality of the AC power fed into the grid will affect the stability of the entire grid-connected system.Due to the widespread distribution of renewable energy,distributed generation systems need to transmit long distances to feed into the grid.In addition to the existence of transformer leakage inductance,the ideal grid presents the characteristics of a weak grid,it contains more background harmonics and the grid impedance will not be negligible.Therefore,the research on control methods for grid-connected inverters in weak grids plays a key role in the development and utilization of renewable energy sources.LCL filter with its high-frequency harmonic attenuation ability,small size,low cost and other advantages widely used in the grid system.However,due to the presence of a high-gain resonant peak in the LCL filter amplitude-frequency characteristics,it is necessary to suppress the resonance peak.This thesis analyzes the effects of grid impedance on passive damping and active damping,and the influence of grid impedance and grid inductance on system stability.Under the active damping control,according to the performance requirements of the entire system,the PI regulator parameters and the capacitive current feedback coefficient are designed in detail.In the case of a weak grid,the voltage waveform distortion at the grid connection point imposes higher requirements on the phase lock technology.This thesis analyzes the shortcomings of phase-locked technology based on traditional SOGI,and improves the structure of SOGI,which can get more severe grid voltage phase accurately.In this thesis,the full feedforward control strategy of the power grid voltage is adopted to reduce the influence of the grid voltage harmonics on the quality of the grid connected current,and the full feedforward function is analyzed and simplified in detail according to the different harmonic frequency contained in the grid.The output impedance of the grid connected inverter can simultaneously reflect the robustness of the control system to the change of the impedance of the power grid and the suppression of the harmonics of the network current caused by the grid voltage.A series parallel virtual impedance method is used to adjust the output impedance of the inverter,which makes the output impedance of the inverter and the impedance of the power grid satisfy impedance-based stability criteria,and gives the specification of the selection of the size of the series virtual impedance.In this thesis,a grid system model is built in MATLAB/Simulink,and the above theory is verified by simulation.The experimental platform of NPC three-level grid-connected inverter based on LCL filtering is set up.DSP and FPGA are selected as the main control chip of the system digital controller.The above control method is verified by experiments.