| In order to realize the grid-connected operation of renewable energy,the distributed generation system usually uses phase-locked loop(PLL)structure to lock the voltage fundamental phase at the point of common coupling(PCC)between the grid-connected inverter and the grid.Thus,it can provide a reference for the grid-connected current.Due to the remote location and long lines of distributed new energy power stations,PCC often has a large line impedance,presenting an undesirable weak grid characteristic.At this time,the equivalent impedance of the phase-locked loop presents negative damping characteristics,which easily leads to the stability of the grid-connected inverter system.Therefore,it has great research value to analyze the stability of the "inverter-grid" interaction system in the weak power grid environment to solve its stability problem.Based on impedance modeling and analysis theory,the stability of three-phase grid connected inverter systems is studied in this paper.Based on the impedance model of grid-connected inverter system,the system is divided into two subsystems: "inverter" and "power grid".Aiming at the problem that the parameters of these two subsystems are difficult to obtain accurately,particle swarm optimization(PSO)algorithm is proposed to identify the parameters of grid-connected inverter and power grid impedance,so as to obtain accurate impedance model.For photovoltaic grid-connected inverter,based on the "gray box" modeling theory,the unknown parameters in the model are identified by PSO algorithm to obtain the identification model which is highly fitted with the real model;Based on the principle of passive measurement method,the power network impedance is measured and identified using PSO algorithm.Compared with the traditional passive measurement method,this method has faster identification speed and higher identification accuracy.Reasonably simplify the accurate model of the above subsystems after obtaining them.Then,a small signal model is established considering the dynamic characteristics of the phase-locked loop under the condition of weak grid.Finally,the impedance modeling of the entire "inverter-grid" interaction system is carried out based on the d-q axis modeling method.After modeling,the stability conditions of the system are analyzed based on Nyquist stability criterion and Bode diagram.On this basis,various factors affecting system stability,such as power grid impedance,phase-locked loop parameters and current loop bandwidth,are explored,and the mechanism of system instability in weak grid is analyzed.It can be seen from the analysis that reasonable design of PLL parameters can effectively improve the stability of the system.In order to solve the stability problem of the grid-connected inverter system in weak grids,a phase-locked loop parameter design method based on the modified particle swarm optimization(MPSO)algorithm is proposed based on the above research on system stability.This method effectively improves the dynamic characteristics of the system while ensuring the system stability,and it can be used in a variety of phase-locked loop structures.For the grid-connected inverter system using SRF-PLL,based on the stability margin and dynamic characteristics of the system,considering the influence of short circuit ratio and current loop bandwidth,this paper designs the value range and fitness value function of phase-locked loop parameters.In order to meet the constraints of particles,meet the requirements of multiobjective optimization,and improve the global search ability of the algorithm,MPSO is obtained by improving multiple aspects of PSO.Then,the MPSO algorithm is used to determine the parameter selection of PLL.In order to verify the universality of this method,this paper also applies this method to the PLL parameter design of grid-connected inverter system based on DSOGI-PLL and MRF-PLL respectively.Based on the Matlab simulation platform and low power experimental platform,comparative experiments are carried out to verify the effectiveness and superiority of the above methods.Considering that when the grid impedance changes in a wide range,it may be difficult to ensure that the grid-connected inverter system always operates in a stable state by optimizing the PLL parameters.From the perspective of impedance remodeling,the output impedance of the system is improved by using voltage feedforward strategy to solve the stability problem of the system in weak grids.Based on the stability analysis of the system,a voltage feedforward controller is designed to counteract the adverse effect of PLL on the output impedance of the system,so as to improve the stability margin of the grid connected inverter system in weak grids.Finally,the effectiveness of this method is verified by simulations and experiments. |
