Stability Analysis And Improvement Strategy Of PV Grid-connected System Based On Virtual Synchronous Generator

The fast-paced growth of the photovoltaic power generation industry and the surging demand for distributed energy in the power grid have led to a significant rise in the proportion of renewable energy sources,posing unprecedented challenges to the stability of the power grid.And the traditional control method is difficult to improve the characteristics of the distributed power generation system(DPGS)with low damping and low inertia.Currently,virtual synchronous generator(VSG)control is the ideal DPGS control technology to provide voltage and frequency support for the power grid.This thesis focuses on the stability of DPGS in grid connection(GC)mode based on VSG control.This thesis researches the mechanism of synchronous frequency resonance(SFR)phenomenon caused by power grid quality degradation on DPGS and proposes a plan to enhance stability.In addition,combined with the recent widespread application of bifacial PV module,the state space matrix is used to analyze the influence of the controller parameters change in the grid-connected bifacial PV systems(GCBPVS)on the system stability,and the optimization scheme suitable for engineering applications is designed.The main contents studied in this thesis are as follows:(1)This thesis presents the application background and traditional control method of DPGS,and analyzes the history as well as recent progress of domestic and international research on VSG control technology.Finally,an overview is provided of the present research on the stability of PV grid-connected systems using VSG.(2)The influence of power quality degradation on DPGS stability in GC mode is studied.Through the small signal analysis method,the transfer function models between power grid disturbance and output power are established,and then this thesis analyzes the relationship between the system impedance and the poles of established transfer functions.In addition,the frequency response analysis is used to study the influence of the system impedance change on the resonance point.On the premise of considering power coupling,this thesis analyzes the mechanism of SFR caused by voltage sag and imbalanced DC components,and proposes to use virtual resistance(VR)module to improve the deficiency of system resistance,so as to achieve the suppression of SFR phenomenon.(3)Small signal modeling is carried out for each module in GCBPVS to construct a complete system state space matrix.The small disturbance stability and oscillation mode characteristics of the GCBPVS are analyzed by analyzing the influence of the change of control parameters in GCBPVS on the eigenvalues of the state space matrix.The stability of the GCBPVS is also improved by optimizing the position of the eigenvalues using the global optimization design method,which improves the corresponding oscillation frequency and damping ratio.(4)Through MATLAB simulations and hardware in loop(HIL)experiments,it is verified that the accuracy of theoretical analysis in this thesis and the effectiveness of corresponding improvement strategies.