Study On The Oscillation Stability Of Grid-connected Inverter System

Grid-connected inverter is the key device utilized to connect the renewable power generation and the power grid.With the rapid development of renewable power generation technology and the wide application of power electronic device,large-scale diversified grid-connected inverters are connected to traditional power systems.Unlike traditional power generation devices,the inverter has a small inertia and fast response speed,which causes complex stability problems after being connected to the traditional power system.In this paper,the grid-connected inverter system is taken as the research object,and the stability analysis method based on the impedance model is used to study the influence of the system parameters on the oscillation characteristics and the generation mechanism of the oscillation under weak grid conditions.Relevant research has been carried out on the oscillation stability of the grid-connected inverter system.Firstly,according to the characteristics of photovoltaic,wind power and other new energy power generation,the grid-connected inverter is equivalent to a controllable current source.The impedance model of the grid-connected inverter is established in the three-phase static coordinate system based on the harmonic linearization method.The grid-connected inverter can be equivalent to the Norton model in which the current source and the inverter impedance are connected in parallel.In the process of establishing the impedance model of the inverter,the effects of the current control loop and the phase-locked loop are considered comprehensively.The current loop and the phase-locked loop of the grid-connected inverter are faster than the DC voltage loop.In order to simplify the research,the dynamics of the DC link are not considered in inverter circuit and the control loops,and are replaced the DC link by an ideal DC voltage source.After the inverter impedance model is established,the frequency sweep method is used to verify the accuracy of the established model.Then,based on the established output impedance model of the grid-connected inverter,the effects of short-circuit ratio,inverter output level and controller parameters on the grid-connected inverter impedance and grid impedance characteristics are analyzed,as well as the stability of the interactive system.A simulation model of grid-connected inverter with rated power of 1.5MW is built in MATLAB/Simulink,and the theoretical analysis are verified by simulation.Simulation and theoretical analysis results show that as the short-circuit ratio decreases and the inverter output power increases,the interaction between the inverter impedance and the power grid increases,which can easily cause system instability.In addition,within a certain range of frequency,an increase in the bandwidths of the phase-locked loop and of the current inner loop will increase the negative damping frequency range of the output impedance,causing voltage and current resonance at the grid connection point.Finally,the oscillation stability of multi-inverter parallel grid-connected system is studied,and the influence and mechanism of static VAR generators on system stability are studied.Finally,a dual-inverter parallel experiment platform established to verify the validity of the theoretical analysis in laboratory.