Analysis Of Nonlinear Dynamics Behavior And Control Strategy Of Quasi-Z-Source PV Inverter In Weak Grid

Due to the need of energy upgrading and energy transformation,new energy generation methods represented by solar energy are attracting much attention.PV inverter is the key interface connecting PV side and grid side,and its reliability is very important.Compared with traditional two-stage boost inverters,quasi-Z-source inverters can be used in PV systems to improve system reliability and transmission efficiency because they can achieve boosting under straight-through conditions and can realize two-stage functions with a single-stage structure.However,the quasi-Z-source inverter has nonlinear characteristics,and in practical applications there are problems such as irregular oscillation behavior,unidentified electromagnetic noise,and waveform distortion,which are also the bifurcation and chaotic nonlinear behavior of power electronic converters.PV power plants are mainly divided into off-grid mode,grid-connected mode,and hybrid grid-connected/off-grid mode according to the power delivery mode.In the mixed mode of grid-connected/off-grid,the off-grid operation mode of PV inverter has certain influence on the stability of grid-connected mode.With the development of photovoltaic power generation technology and the geographical difference of solar energy distribution,more and more grid-connected PV inverters operate far from the load center,and a large number of non-linear converters are connected to the grid,which causes the grid impedance to be non-negligible,making the grid present weak grid operation.Because of the equivalent grid impedance of the weak grid,the grid-connected current is prone to generate a large number of harmonics,or even cause harmonic oscillations and other nonlinear phenomenaThe aforementioned nonlinear behavior generated by the PV inverter will seriously affect the safety and stable operation of the system.This paper focuses on the nonlinear behavior of the quasi-Z-source inverter in off-grid and weak grid modes.First,the nonlinear behavior of the PV quasi-Z-source inverter in off-grid mode is analyzed.The bifurcation and chaotic behaviors present in the system are described by constructing a discrete iterative mapping model of the system and combining Matlab numerical calculations and sampling to obtain the bifurcation,folding,and folding phase diagrams of the system.The stability of the system is analyzed in terms of both Jacobi matrix and Lyapunov exponent,and the nonlinear behavior of the system is verified.The rationality and correctness of the theoretical analysis are further verified by Matlab simulation and the constructed experimental platform,and the parametric stability domain of the system is given according to the stability analysis to provide a theoretical basis for engineering design.Then,the nonlinear behavior of the PV quasi-Z-source inverter in the weak grid mode is analyzed.The discrete iterative mapping model of the system is constructed by considering the bifurcated frequency ripple component possessed by the single-phase inverter under the weak grid.Since the LCL filter has better high-frequency ripple suppression effect,which makes the bifurcation diagram not applicable to the analysis of nonlinear behavior of PV quasi-Z source inverter in weak grid mode,the analysis methods of folding diagram,folding phase diagram,eigenvalue and unit circle are chosen to study the nonlinear behavior of harmonic oscillation existing in the system;and the stability domain of grid equivalent impedance is given for practical application.Finally,in order to suppress the nonlinear behavior existing in the system under the weak grid and improve the stability of the system,an improved delayed feedback control strategy is designed by introducing exponential and intensity coefficients based on the traditional delayed feedback control,which reduces the disturbance of the control signal,thus reducing the harmonic content of the grid-connected current and suppressing the harmonic oscillation of the system;and the effectiveness of the control strategy is verified by stability analysis and simulation.