Oscillation Modal Analysis Of Grid-connected Voltage Source Converter Considering Phase-locked Loop Dynamics

With the development and changing of power system,from the power generation to power transmission and distribution,the penetration of electronic power transformation device is getting higher and higher,and it shows multi-timescale control characteristics,including the dynamic timescales of rotor speed control,DC voltage control and AC current control.At present,it is not been taken seriously in the analysis of traditional power system when talking about the dynamic stability of the AC current control timescale,but with the deepening of the power electronicity of power system,the new type oscillation problem of the AC current control timescale caused by control parameters,will cause more and more serious and widespread impact on the stable and safe operation of the power system.Therefore,the small signal stability analysis of grid-connected converters in a weak grid has important research significance and value for the design of controllers and the proposal of fault suppression strategies to ensure the safe operation of the grid.The typical power electronic device,three-phase grid-connected voltage source converters,is taken as the research object in this paper,and modal analysis method combined with simulation verification is used to explore the characteristics of system oscillation mode and stability under the AC current control timescale of the converter.The key parameters that affect the non-linear oscillation behavior of grid-connected converter are analyzed in detail,the control strategys and suggestions for suppressing the non-linear oscillation of converter are proposed finally.The details of this paper are as the following three aspects:(1)The multi-timescale dynamic characteristic of the grid-connected power electronic device is analyzed,and the threats and challenges to the power system stability caused by oscillation problems under different control timescale is described.Taking the grid-connected converter under AC current control timescale considering the dynamic characteristic caused by phase-locked loop as research object,the analysis of the converter control principle is introduced in detail and the original modeling of the system is established,and the orignal model is linearized near the steady-state operating point.Finally,the small signal model of grid-connected voltage source converter under AC current control timescale is obtained.(2)The basic concepts and analysis methods of power system modal analysis is introduced firstly.The characteristic root structure analysis of the small signal model of the grid-connected converter is carried out.The result of system modal analysis is used to determine the stability of the device and the characteristics of the dominant parameters of different oscillation modes.The change laws of the characteristic roots and oscillation modes under the changes of current control parameters and phase-locked control parameters are mainly discussed,and the conclusion of the modal analysis is verified by time-domain simulation.(3)The constant amplitude oscillation phenomenon of the grid-connected converter caused by the dynamics of phase-locked loop control discovered during the debugging of the simulation model is discussed and analyzed with the modal analysis conclusion duiring the initial stage of constant amplitude oscillation and the evolution laws of the simulation waveform after the initial stage,and the threat of constant amplitude oscillation to the power system stability is imminent that cannot be ignored.In terms of the non-linear oscillation phenomenon of the grid-connected converter,the relevant parameters with high participation in the initial state mode analysis of nonlinear oscillation are analyzed by simulation,the intensity of power grid,the feed-forward filter cutoff frequency,the current loop control gain factor,and phase-locked loop control gain factor are found having great impact on constant amplitude oscillation behavior.Then with the results of modal analysis and simulation waveform analysis of the system’s non-linear oscillation behavior,control strategys for suppressing the converter’s nonlinear oscillation behavior are proposed from two aspects,which cotains parameter design and system control strategy design after constant amplitude oscillation,in order to avoid continuous damages and threats to the device and power system.