Research On Ac Current Control Timescale Nonlinear Oscillation Behavior Of Grid-connected Voltage Source Converters

The large-scale development of renewable energy power generation and DC transmission has promoted the electronicization of power system,leading to a significant influence on the modern power system stability mechanism.In recent years,the power system oscillation problem related to power electronics apparatus has appeared several times worldwide,which has seriously threatened the safe and stable operation of modern power system.Many studies have been carried out on the power system oscillation problem,but the change of physical apparatus in modern power system proposes new needs of basic theory and key technologies to look into this problem,especially nonlinear analysis methods.In order to further understand the nonlinear oscillation behavior of power electronized power system,it is urgent to summarize the nonlinear characteristics of power electronics apparatus,to understand system’s nonlinear dynamic response characteristics,and to analyze the excitation-response relationship(original relationship)of power electronics apparatus.Therefore,based on the basic idea “utilizing the linear mode as a bridge to understanding system’s nonlinear behavior”,this dissertation takes the grid-connected voltage source converter(VSC)as the research object and the single-machine infinity system’s oscillation divergence behavior in AC current control timescale as the research scenario.Besides,this dissertation focuses on nonlinear characteristics of the VSC,aims to understand its nonlinear dynamic response characteristics,to study the general evolution rule of dynamic response characteristics,and to explore the original relationship of the VSC.The main research content of this dissertation are as follows,(1)Establish the state-space modal of VSC and clarify its nonlinearity.The topology structure and AC current control timescale control method of the VSC are introduced.According to the circuit equation,phase-locked loop(PLL)equation and current control loop equation,the state-space model of the VSC is established.Moreover,the internal voltage is proposed to describe the dynamic characteristics of the VSC,and the nonlinear links in the VSC are summarized.(2)Analyze the nonlinear dynamic response characteristics and its evolution rule of the VSC.Based on the internal voltage,the system’s nonlinear oscillation divergence behavior from a small disturbance to constant amplitude oscillation is given,and this oscillation process can be roughly divided into three stages,namely early stage,divergence stage and constant amplitude oscillation stage.Modal analysis of the system is carried out and shows that there is an unstable oscillation mode which dominates the oscillation behavior of the system during the early stage.Besides,the signal characteristics of the internal voltage at different stages are concluded as invariant,nonperiodic time-varying and periodic time-varying respectively.Moreover,the evolution rule of the internal voltage during the entire oscillation process is explored and summarized as iterated amplitude-frequency(AF)modulation.(3)Research on the original relationship that determines the nonlinear dynamic behavior of the VSC.Based on the dynamic response characteristics and its evolution rule,the original relationships of key links in the VSC are first studied.Then,by setting two simulation scenarios,the influence of these key links,namely coordinate transformation,PI controller and PLL,on nonlinear dynamic response of the VSC is explored.Finally,the original relationship of the VSC composed of these key links is derived,and the correctness of the original relationship is validated.