Study On Stability Of Grid-connected Inverter System Based On Impedance Analysis

With the development of social economy,the demand for energy in various countries has become higher and higher.Traditional fossil energy has been increasingly exhausted after long-term exploitation and excavation,and the development of new energy has become more and more important.Distributed power generation is an important form of utilization in new energy development,and its proportion in the power grid is increasing.As an interface device between distributed power generation and power grid,inverters also occupy an increasingly important role at the use of new energy sources.As an energy conversion device in the power grid,the inverter is different from the traditional resistor,capacitor or inductor.Due to its nonlinear characteristics,it generates a series of harmonics during system operation,thus generating the entire inverter system.Important impact,when it is serious,it will cause oscillation,which will make the system unable to run stably.At the same time,in the long-distance transmission line,the introduction of the resistance-impedance of the line also has an important impact on the stable operation of the entire inverter system.Impedance analysis is an important method to study the stability of the system by studying the relationship between the equivalent output impedance of the inverter and the impedance of the grid.In this paper,the single-phase grid-connected inverter is taken as the research object.Firstly,the basic principle of the impedance analysis method is discussed.Then the interface filtering characteristics of the grid-connected inverter and the general control strategy of the single-phase grid-connected inverter are analyzed.The LCL type filter in three typical filtering interfaces is selected as the research object,and the current loop based on PI regulator in the static coordinate system is used as the control strategy.After determining the appropriate filter structure and control strategy,the output impedance model of single-phase grid-connected inverter is established.At the same time,the mathematical model of single-phase phase-locked loop is established.The output impedance model of the phase-locked loop to the inverter is analyzed.Taking the simulation verification of the inverter output impedance model as an example,the paper discusses the basic principle and process of the harmonic injection method to verify the output impedance model of the inverter.The inverter output impedance is calculated by injecting a harmonic disturbance voltage excitation at a Point of Common Coupling(PCC)and then performing a Fourier transform on the incoming current to extract a response current signal corresponding to the frequency.The grid impedance introduced by the long-distance transmission line affects the stable operation of the grid-connected inverter system.A grid impedance detection technique based on harmonic injection is studied.By periodically injecting a voltage signal of a desired frequency and amplitude during a grid voltage zero-crossing,the amplitude of the voltage-current response signal is detected at a common point of the grid connection,and then the system impedance characteristics is quickly calculated based on the obtained signal data at a desired frequency.The article describes in detail the working principle of the impedance detection method,the key parameter design method,the zero-crossing detection method and the corresponding signal data extraction method.The simulation results show the effectiveness of the method.For the stability problem of a single grid-connected inverter system,the influence of grid impedance parameters on the stability of the inverter system is mainly considered.Aiming at the characteristics of the grid impedance being immutable in the actual environment,a method of using the passive lead network to compensate the inverter output impedance phase to improve the system stability is proposed.Taking s set of parameters as an example,the paper introduces in detail the specific principle of the method and the parameter turning in the phase compensation link.For the stability problem of multi-inverter grid-connected system,considering the fact that parallel inverters usually have the same parameters,the influence of the number of parallel inverters on system stability is mainly studied.Finally,based on the existing experimental equipment in the laboratory,a single-phase grid-connected inverter experimental device was built.Through the experimental test and the result analysis,the above problems were analyzed and studied.The above method was verified by experiments.