Research On Modeling And Stability Of Three-phase Grid-connected Inverter Based On Harmonic State Space

The rapid development of renewable energy,electric vehicles,and microgrids makes grid-connected inverters account for an increasing proportion of the grid.The harmonic interaction coupling between controllers,inverters and AC grids is becoming more and more complicated.Accurate assessment of power quality and system stability analysis pose challenges.The traditional inverter modeling method averages the switching function,ignoring the harmonic components of the switching action,so that it cannot accurately analyze the harmonic coupling phenomenon inside the inverter.As a new modeling method in the field of power electronics,the harmonic state space(HSS)modeling method converts periodic changes in the time domain into constants in the frequency domain,which can well reflect the various orders in the inverter.The interactive coupling phenomenon of harmonics has high modeling accuracy.In this paper,the HSS theory is used to model the three-phase grid-connected inverter,and the harmonic coupling characteristics and stability of the three-phase grid-connected inverter are studied based on the HSS model.Firstly,the relevant knowledge of HSS theory is introduced.Taking single-phase noload transformer and static var compensator as examples,the corresponding HSS model is derived.The model is used to study the dynamic change characteristics of the amplitude and phase of each harmonic in the system;and the steady-state response of the system is accurately and quickly calculated based on the HSS equation,which verifies the advantages of the HSS model;finally,compares with the simulation results of EMTP and Simulink Be verified.Then,this paper takes the three-phase LCL grid-connected inverter as the research object,and transforms the time-domain switching function model of the grid-connected inverter into an HSS model in the frequency domain.In view of the relatively redundant HSS model of the grid-connected inverter,the HSS model is simplified by constructing a phase shift matrix.Under the premise of ensuring the accuracy of the original HSS model,a reduced-order HSS model of the grid-connected inverter is obtained,which improves The calculation efficiency is improved;in view of the AC and DC harmonic coupling phenomenon in the grid-connected inverter,the corresponding harmonic transfer function matrix is derived through the HSS model,the AC and DC harmonic coupling characteristics are analyzed,and the coupling admittance amplitude is drawn through the harmonic transfer function matrix Value graph,intuitively analyze the strength of the harmonic coupling relationship,and carry out simulation verification.Finally,this paper uses HSS theory to model the grid-connected inverter in the closed-loop state,gives a detailed modeling process,and establishes the HSS model of the grid-connected inverter in the closed-loop state;Aiming at the strong nonlinear characteristics of grid-connected inverters in closed-loop mode,a small-signal HSS modeling method for three-phase grid-connected inverters suitable for closed-loop control systems is proposed,and the accuracy of the small-signal HSS model is verified by simulation.;Judge the stability of the system by calculating the eigenvalues of the small-signal HSS model,and analyze the influence of the controller parameter changes on the stability of the system;when the system is unstable,the eigenvalues of the smallsignal HSS model can be used to accurately judge the oscillation of the system Frequency provides guidance for system stability evaluation and controller parameter optimization.Comparing with Simulink simulation results,it verifies the accuracy of the small signal HSS model and the correctness of the system stability analysis.