Research On Impedance Modeling And Stability Of Photovoltaic Grid Connected Converter System

The stability problem caused by a large number of distributed photovoltaic power generation systems connected to the weak current network has become a research hotspot.In order to ensure the stable operation of photovoltaic grid connected power generation system,this paper introduces the front and back circuit structure of two-stage photovoltaic grid connected converter system from the perspective of impedance,and establishes the impedance model.This paper analyzes the factors that affect the stability of the system,and puts forward the specific scheme to improve the stability of the system.This paper first gives the overall structure diagram of the system.be based on Based on the general small signal model of PWM converter,the working principle of boost converter in different modes is analyzed,and its output impedance is derived.The control block diagram and control strategy of LCL grid connected inverter are given,and the current inner loop and voltage outer loop are modeled.Through equivalent simplification of control block diagram and Mason Gain Formula,the output impedance and input impedance of grid connected inverter are derived.Then,the parameters of the current and voltage loop controller are designed,and the proportional resonance(PR)controller with phase compensation is used in the current inner loop;The outer voltage loop adopts(PI)controller.Secondly,the passivity theory of circuit stability control and the stability criterion of system under small signal model are introduced.The system stability of cascaded boost converter and grid connected inverter is analyzed.Based on the real part of admittance,the passive region of grid current feedback control and capacitor current active damping is analyzed.On this basis,the influence of different grid conditions on the stability of inverter is studied.When the grid impedance increases,the resonant frequency of the system falls in the non passive region of the output admittance,and the stability of the system decreases.Finally,the method of using active damping converter to improve the output impedance of the front stage is proposed to improve the stability of the cascade system.The influence of control parameters and converter parameters on the system output impedance is analyzed.Combined with the influence of filtering parameters on the output admittance,a method of adding leading phase compensation is proposed to improve the system stability.The transfer function and baud diagram of the lead phase compensation link are given,and the relevant parameters are analyzed and calculated.Combined with the curve of the real part of the output admittance,it is proved that the real part of the overall output admittance of the system is positive in the low frequency band,the passive range of the system is greatly expanded,and the stability is significantly improved.The effectiveness of the system design is verified by simulation experiments.