Study On Grid Impedance Online Measuremen Method For Single-phase Grid-Connected Inverters

With the rapid development of the distributed generation systems,the application of grid-connected inverter is extremely widespread.The existence of the grid impedance will affect the power quality and system stability of the grid-connected inverter,which makes the harmonic resonance between the grid-connected inverter and the grid become more and more frequent.Therefore with the complex and changing grid conditions,accurate grid impedance estimation is one of the key technologies to achieve the high performance adaptive control of the inverter.This paper focuses on how to use three broadband methods(single pulse?MLBS?DIBS)to realize the online impedance measurement for the grid-connected inverter.Fristly this paper gives the design considerations of the single pulse response analysis and describes the online implementation of FFT algorithm to obtain the grid impedance.Then the maximum length binary sequence(MLBS)which is one of the most commonly used signals of pseudo random binary sequence(PRBS)is used to measure the grid impedance.This paper analyses the theory behind MLBS and synthesis of the sequence to evaluate the possibility of application of MLBS in grid impedance measurement.Finally,discrete-interval binary sequence(DIBS)which's spectrum can be designed by users is used to measure the grid impedance.On the basis of the preliminary measurement of the grid impedance,DIBS is re-designed to avoid the resonance frequency and reduce the influence of excitation on the system.Finally,simulations and experiments verify the validity of three kinds of broadband pulses(single pulse,MLBS,DIBS)to measure the grid impedance.Furthermore,the simulation model of the single-phase grid-connected inverter is built by the Matlab/Simulink software to verify the feasibility of the three broadband excitations in this thesis.Finally,a digital control based experimental prototype of single-phase grid-connected inverter is built and the software and hardware design of the inverter is depicted.The experimental results are presented to demonstrat the feasibility and effectiveness of online measurement of grid impedance based on the three broadband excitations.