VSG Stability Analysis And Control Strategy Research Under Weak Grid

As the utilization rate of new energy continues to rise,the proportion of distributed generation units in the power grid continues to expand,and the power grid gradually presents weak grid characteristics.System stability analysis and operation strategy optimization under weak power grids have become the hotspots of grid-connected inverter research.Based on the VSG control strategy,this paper studies the influence of power grid strength and two typical phase-locked loop parameters on the stability of weak power grids,improves a pre-synchronization control strategy based on virtual power,and designs a control strategy without voltage sensors.Firstly,the mathematical model of the active power-frequency controller is established according to the rotor equation of the synchronous generator on the basis of primary frequency regulation,and the mathematical model of the reactive powervoltage controller is established according to the excitation control of the synchronous generator on the basis of primary voltage regulation.Design the main parameters of the VSG ontology model.Mathematical models of synchronous coordinate system phase-locked loop and dual generalized second-order integrator phase-locked loop are established.Secondly,the influence of the short-circuit ratio on the stability of the VSG control system is analyzed,and a small disturbance modeling method based on the impedance model is studied.The analysis shows that the short-circuit ratio is inversely proportional to the stability of the system.Analyze the influence of the parameters of the synchronous coordinate system phase-locked loop and the biquad generalized integrator phase-locked loop on the stability of the VSG system,and study a small disturbance modeling method based on impedance modeling in the control dq coordinate system.Analysis shows that the larger the bandwidth of the phase-locked loop,the worse the system stability.Under the same parameters,the stability of the dual generalized second-order integrator phase-locked loop is better than that of the synchronous rotating coordinate system phase-locked loop.In the presence of high-order harmonics in weak power grids,the biquad-order generalized integrator has stronger anti-interference ability,and can accurately lock the phase in the case of frequency and amplitude fluctuations.A pre-synchronization control strategy based on virtual power is improved.A virtual reactance is introduced on both sides of the grid-connected switch.The voltage obtained by the VSG internal algorithm can be used to directly calculate the power transmission between the inverter and the grid.By controlling the transmitted power When it is 0,the pre-synchronization of VSG control is realized.Design a control strategy without voltage sensor.The high frequency signal model of the inverter is established,and the state space equation is obtained.According to the basic theory of rotation vector,in the zero vector state,the voltage of the capacitor is approximately equal to the grid voltage,and the voltage on the capacitor is the grid voltage calculated according to the state space equation.A simulation platform is built based on MATLAB/Simulink,and the analysis results show that there is no inrush current when the improved pre-synchronization control strategy is connected to the grid,and the fluctuation is small.The voltage sensorless control strategy can accurately calculate the grid voltage and quickly detect grid faults.