Adaptive Control Of Grid Connected Inverter Based On Impedance Detection

With the development of clean energy,grid-connected inverters(GCIs)as an important part between distributed power generation system(DPGS)and large power grid.The perturbation of grid impedance not only makes the LCL filter resonance frequency offset,but also affects point of common coupling point(PCC)voltages.The perturbation of grid impedance will lead the system concussion,the output of the phaselocked loop unstable,and the grid current is distorted.Therefore,how to make GCIs maintain stable under the grid impedance perturbation is the key issue at present.In this thesis,by constructing the mathematical model of GCI,the design method of LCL filter parameters is discussed and the control model of inverter is analyzed in ab frame.Based on multi complex filter,the phase-locked loop of grid connected inverter is improved to adapt the situation of weak grid.The parameter influence of cross decoupling complex filter(CCF)is analyzed,and the appropriate parameter values are selected.By studying the method of grid impedance detection,the error of common algorithm is discussed under imbalance of grid impedance.The common algorithm is improved with non characteristic harmonic injection and multi complex filter.In this thesis,by constructing the small signal model of GCI,the grid impedance perturbation effect of GCI is discussed by the impedance ratio.The open loop transfer function of the double current feedforward control strategy is derived.The equivalent impedance of capacitance current feedforward will been changed with frequency when the control strategy add delay link.Based on this,the capacitance current feedforward ratio coefficient is designed.The influence of the inductive and resistive grid impedance on the different control strategies are analyzed,the modified feedforward channel is established and the parameters of the controller is changed based on the grid impedance,which can guarantee the stability of system under the grid impedance perturbation.An experimental platform of GCI is built with NI 9403 controller to finish some deep research,and experimental verification is performed on this experimental platform.The experimental data is analyzed to verify that the above algorithms perform well.