Research On State Evaluation Method Of Inverter Filter Circuit Based On Harmonic State Space

With the rapid development of renewable energy,new energy vehicles and microgrids,which make grid-connected inverters more and more important in power grids,and the harmonic coupling interaction between controllers,inverters and AC grids becomes more and more complex.The traditional inverter modeling method ignores the harmonic component of the switching frequency and only averages the switching devices,thus the coupling effect between the harmonics of each unit cannot be obtained.In contrast,the Harmonic State Space(HSS)modeling method converts the period variation quantity in the time domain into a constant quantity in the frequency domain,which can effectively express the coupling relationship between each harmonic,and its accuracy can be infinitely high in theory.Therefore,this paper adopts the HSS theory to model the three-phase grid-connected inverter,analyzes the harmonic coupling characteristics of the three-phase grid-connected inverter through the HSS model,and investigates the states of the filter capacitor and filter inductor in the filter circuit.First,introducing how to obtain the analytical model of the switching function with harmonic content by using the exponential form of the bivariate Fourier transform,and then briefly introduces and compares three single-carrier pulse widths,and gives the analytical model of the switching function of space vector pulse width modulation.Next,the knowledge of HSS theory is introduced,and the corresponding HSS model is derived from the time domain model by Fourier series and convolution operation.Finally,the advantages of harmonic state space method and time domain in finding the steady state value are given.Then,the three-phase with LCL type inverter as the studyobject,the fitted function of SVPWM obtained earlier is combined with the timedomain function model of the inverter and transformed into the HSS model with Toplitz matrix in the frequency domain.In order to reduce the computation time of the HSS model of the grid-connected inverter,the HSS model is decoupled by the phase shift characteristic to reduce the size of the model in the frequency domain,so as to obtain a reduced-order HSS model of the inverter and reduce the computation time of MATLAB.Meanwhile,the state equations of the HSS model are transformed into the corresponding transfer function matrices,for the coupling phenomenon of input and output in the inverter,and then the transfer functions between output and input are obtained.The harmonic coupling characteristics of the inverter are analyzed between the transfer function.The values of the input and output transfer function matrices are used to analyze the strength of the coupling between the input and output harmonics,and the effect on the coupling strength when the filter circuit components fail,and the simulation is verified.Then a certain disturbance is injected into the DC side to study the transformation of the output side current with the component values in a certain interval,which is fitted to a linear curve,and the reliability and accuracy of the fitted curve are verified.Finally,a small-signal HSS modeling method of three-phase inverter that applicable to closed-loop control systems was proposed,and the accuracy of the inverter small-signal closed-loop HSS model through simulation was verified.it is verified through simulation that the state of the filter circuit components can be judged to have changed in the closedloop case as well when the state of the filter circuit components has changed.a prototype inverter is designed to prove the correctness of harmonic coupling characteristics and filter circuit state diagnosis method.