Stability Analysis And Control Strategies Of Large-Scale Photovoltaic Grid-connected Systems Under The Influence Of Grid Impedance

Under the dual pressures of energy shortage and environmental pollution,the photovoltaic(PV)power generation industry has developed rapidly in recent years.With continuous reduction of the cost and maturity of the technology,the PV power generation system presents a trend of large-scale development.Compared with smallcapacity PV power generation systems,large-scale PV grid-connected systems can achieve higher efficiency and participate in the adjustment of power system,such as load clipping,reactive voltage control,and grid frequency adjustment.However,the equivalent grid impedance formed by the step-up transformer and transmission line may cause serious harmonic oscillation and even lead to system instability,which seriously restricts the consumption of PV power generation systems.To address this issue and based on the research contents of the National Natural Science Foundation Project“Research on resonance mechanism and suppression strategy of large-scale PV gridconnected system(51477021)”,this dissertation researches on the large-scale PV gridconnected system.By studying the system stability under the influence of grid impedance,the mechanism of harmonic amplification of the grid current is revealed.Then,control strategies that can effectively improve the system stability are proposed,so that the large-scale PV grid-connected system can work stably and the quality of output grid current is improved.(i)The equivalent model of a large-scale PV grid-connected system is established.The large-scale PV grid-connected system is essentially a multi inverter parallel connected system.Firstly,this dissertation introduces the topology and character of the typical distributed PV grid-connected inverter.Next,the main circuit model and control system of the grid-connected inverter are studied.Then,the main circuit parameters and control parameters of the inverter are designed in detail,and simulation verifications are carried out.In order to facilitate the stability analysis of large-scale PV grid-connected system,the equivalent impedance models of the PV grid-connected inverter,three-phase double-splitting transformer,and long-distance transmission line are established.So the equivalent impedance model of large-scale PV grid-connected system can be obtained.(ii)The stability of large-scale PV grid-connected system under the influence of grid impedance is studied,and the mechanism of harmonic amplification of the grid current is revealed.Firstly,the stability condition of large-scale PV grid-connected system is derived,considering the existence of grid impedance and the normal distributions of inductance and capacitance parameters of LCL filter.Then,to quantify the harmonic amplification in the system,the coefficient of harmonic amplification is defined in the dissertation,on the basis of the minimum harmonic impedance.Further,by considering the grid voltage background harmonics and the inverter output voltage harmonics in the system,the mechanism of harmonic amplification of the grid current is studied.It is found that the harmonic amplification of the grid current is mainly caused by the decrease of the system stability,and the grid current can be high distorted.Therefore,in order to ensure the quality of grid current and avoid serious harmonic amplification,the stability of large-scale PV grid-connected system should always be high.However,to effectively suppress the grid voltage background harmonics and damp the resonance of LCL filter,the PV grid-connected inverter usually adopts multi parallel harmonic compensation controllers and the active damping strategy with capacitor current feedback.The stability of the system will gradually decrease and even be instability with the increase of grid impedance.(iii)Aiming at the problem of system stability reduction caused by the harmonic compensation controller,an improved phase compensated resonant controller for PV grid-connected inverters is proposed in the dissertation.Firstly,the instability mechanism of the system is analyzed under the harmonic compensation controller.It is found that the phase delay of harmonic compensation controller can decrease the system stability.To improve the phase delay of the harmonic controller,the frequency characteristic of phase compensated harmonic controller is analyzed.It is found that although the traditional phase compensation method can maximize the system phase margin,it reduces the current tracking performance and the ability to suppress the grid voltage background harmonics.Therefore,this dissertation proposes an improved phase compensation method,which can obtain sufficient phase margin and improve the system stability without affecting the control performance of the inverter.(iv)Aiming at the problem of system stability reduction caused by the capacitor current feedback active damping strategy of the LCL filter,the digital H? controller for PV grid-connected inverters is proposed in the dissertation.Firstly,the active damping characteristic of LCL filter is studied,and the system stability under the disturbance of grid impedance is studied through the minimum phase system and non-minimum phase system,respectively.It is found that,due to the existence of digital control delay,when the capacitor current feedback active damping provides negative damping for the resonance of LCL filter,the system has open-loop unstable poles,that is,the system is a non-minimum phase system.Consequently,the grid-connected inverter system is difficult to be stable under the disturbance of grid impedance.In order to improve the active damping characteristic of LCL filter and the robustness of grid-connected inverters to grid impedance,the traditional control model of the grid-connected inverter is converted into a standard H? control problem,on the basis of the S/KS/T mixed sensitivity problem in H? control theory.The digital H? controller of the grid-connected inverter is designed directly in the discrete domain by establishing a discrete generalized control model and selecting weighting functions appropriately.To verify the theoretical analysis and the proposed control strategies in this dissertation,the corresponding simulation and experimental tests are carried out.The results have proved the correctness of theoretical analysis and the effectiveness of proposed control strategies.